2009 summary, deep seismic swarm at Mammoth Mountain

This report on Long Valley caldera, California, summarizes USGS reports for 2009. The volcano remained non-eruptive. Long Valley Observatory (LVO) is now part of the California Volcano Observatory (CalVO). A tectonic earthquake sequence during 2011 in nearby Hawthorne, Nevada, is also discussed.

Long Valley caldera entered relative quiescence in the spring of 1999 (BGVN 26:07) following unrest that began in 1980 (SEAN 07:05); this relative quiescence continued through 2009.

Seismicity during 2009 was characterized by a low level of seismicity within the caldera, and a typical higher level of seismicity in the surrounding Sierra Nevada range (figure 41). Three recorded earthquakes were larger than M 3.0, yet none of them occurred within the region of Long Valley caldera as delimited by LVO. The largest earthquakes within Long Valley caldera were an M 2.7 on 9 January in the S moat, and a pair of M 2.3 earthquakes on 10 December that were located beneath the resurgent dome.

Deep seismic swarm at Mammoth Mountain.At Mammoth Mountain, increased seismicity began in late May, and a deep seismic swarm occurred on 29 September. The 29 September seismic swarm included over 50 M ≥0.5 high-frequency earthquakes that occurred at depths of 20-25 km, depths inferred to be in the mafic lower crust (figure 42). The high frequencies of these earthquakes indicated brittle-rock failure similar to shallow earthquakes that typically occur at <10 km depth, and were distinctly different than the long-period earthquakes that occur within the silicic upper crust, at depths of 10-25 km. The increased seismicity at Mammoth Mountain during 2009 produced more earthquakes there than occurred within Long Valley caldera (figures 41, 42, and 43).

Slow inflation of the caldera's resurgent dome. Deformation trends during 2007-2009 highlighted slow inflation of the resurgent dome. At the end of 2009, the height of the resurgent dome remained ~75 cm higher than prior to the onset of unrest in 1980. Measurements since 2007 indicated horizontal displacement rates of ~5 mm/year, mostly in a pattern radiating away from the resurgent dome (figure 44).

During 2009, soil CO₂ emission measurements revealed variations typical of most previous years. The increase in seismicity at Mammoth Mountain on 29 September did not produce a corresponding increase in CO₂ emissions.

2011 Hawthorne, Nevada, earthquake sequence. In March 2011, an earthquake sequence (mentioned in LVO weekly activity updates) began in Hawthorne, Nevada (~100 km NNE of the center of Long Valley caldera) that, according to Smith and others (2011), initially sparked brief concerns of unrest at Mud Springs volcano (figure 45). Mud Springs volcano is a probable Pleistocene volcano of the Aurora-Bodie volcanic field, Nevada (Wood and Kienle, 1992). The Hawthorne earthquakes did not show volcanic signatures in near-source seismograms (Smith and others, 2011), and the sequence was quickly identified as tectonic in origin.

According to Smith and others (2011), "An additional concern, as the sequence . . . proceeded, was a clear progression eastward toward the Wassuk Range front fault. The east dipping range bounding fault is capable of M 7+ events, and poses a significant hazard to the community of Hawthorne and local military facilities. The Hawthorne Army Depot is an ordinance storage facility and the nation's storage site for surplus mercury."

Earthquakes of the March 2011 sequence were as strong as M 4.6 (figure 46); the largest earthquakes may have been felt in Bridgeport, CA (~60 km SW of Hawthorne, and ~70 km NNW from the center of Long Valley caldera), according to LVO. The earthquakes occurred along at least two shallow faults, originating at 2-6 km depth (Smith and others, 2011). The earthquake sequence "slowly decreased in intensity through mid-2011" (Smith and others, 2011).

References. Smith, K.D., Johnson, C., Davies, J.A., Agbaje, T., Antonijevic, S.K., and Kent, G., 2011. The 2011 Hawthorne, Nevada, Earthquake Sequence; Shallow Normal Faulting. American Geophysical Union, Fall Meeting 2011, Abstract ##S53B-2284.

Wood, C.A. and Kienle, J., 1992. Volcanoes of North America: United States and Canada, Cambridge University Press, 354 p., pgs. 256-262.

Information Contacts: Dave Hill, California Volcano Observatory (CalVO), formerly theLong Valley Observatory (LVO), U.S. Geological Survey, Menlo Park, CA (URL: http://volcanoes.usgs.gov/observatories/calvo/); Nevada Seismological Laboratory, Laxalt Mineral Engineering Building, Room 322, University of Nevada-Reno, Reno, NV 89557 (URL: http://www.seismo.unr.edu/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

May 1982 (SEAN 07:05) Cite this Report

Seismic swarms; new fumaroles; uplift

Since the four M 5.5-6.1 earthquakes of 25-27 May, 1980, eight seismic swarms have occurred in the S part of the caldera (table 1). Most lasted roughly 1-2 hours and consisted of several hundred events, including some B-type shocks. All of the swarms included periods of spasmodic tremor, produced by a succession of earthquakes too closely spaced to allow clear separation into discrete events on seismograph records. Some earthquakes were also recorded between swarms. In January 1982, a new group of fumaroles was discovered in the vicinity of Casa Diablo Hot Springs, about 2.5 km E of the earthquake epicenters, and existing fumaroles in the area had become more vigorous. The most recent swarm began 7 May at 0517 with a 40-minute burst of seismicity. Occasional 5-10-minute periods of spasmodic tremor were recorded during the next 24 hours. The swarm's strongest earthquake was a M 4.3 event at 2047. Depths of the 7-8 May events ranged from 3.8 to 8.5 km below the surface, comparable to the 9-10 July 1981 swarm, but substantially shallower than others since May 1980. Epicenters were about a half km N of those from previous swarms.

Table 1. Data from seven seismic swarms at Long Valley Caldera, June 1980-August 1981, courtesy of Alan Ryall. Analysis of data from the 7-8 May 1982 swarm is in progress.

Date	Average Depth (km)	Standard Error (km)	Largest Event Magnitude	Number of Events Located
07 Jun 1980	8.4	1.2	3.5	6
02-03 Jul 1980	8.0	1.0	4.2	14
03 Aug 1980	8.2	1.3	3.4	7
25 Nov 1980	7.8	1.3	3.4	5
21 Apr 1981	8.5	3.0	2.8	6
09-10 Jul 1981	5.9	1.3	3.2	30
09 Aug 1981	9.0	2.9	3.4	5

In October 1980, a survey along the highway that passes through the caldera showed as much as 25 cm of uplift of the caldera's resurgent dome, possibly within the previous 2 years. A 10 km-long geodimeter line over the dome, remeasured in May 1982, showed 50 cm of lateral spreading since 1978. Dry tilt stations, installed 8-9 May over the earthquake swarm epicentral area, showed no changes when reoccupied about three weeks later.

Miller and others (1982) noted that "A preliminary interpretation of this evidence is that magma at depth in the Long Valley Caldera moved upward at about the time of the May 1980 swarm of earthquakes. This caused bulging of the resurgent dome and opened fractures at depth in the S part of the caldera (R.A. Bailey and R. Cockerham, written communication, 1982), thereby allowing a tongue of magma to move toward the surface beneath the epicentral site . . . ." On 25 May 1982, the USGS issued a notice of potential volcanic hazard for the Long Valley area, adding to the earthquake hazard watch in effect since 27 May 1980.

Reference. Miller, C.D., Crandell, D.R., Mullineaux, D.R., Hoblitt, R.P., and Bailey, R.A., 1982, Preliminary Assessment of Potential Volcanic Hazards in the Long Valley-Mono Lake Area, East-Central California and Southwestern Nevada; USGS Open File Report 82-583.

Information Contacts: R. Bailey, USGS, Reston, VA; C.D. Miller, D. Mullineaux, USGS, Denver, CO; A. Ryall, University of Nevada, Reno.

June 1982 (SEAN 07:06) Cite this Report

Minor earthquake swarm; additional uplift of resurgent dome since 1980

Since the seismic swarm of 7-8 May, only 1 minor swarm of microearthquakes has been recorded in the epicentral area under the S side of the caldera. This swarm occurred 13 July at 1200 and consisted of six minor shocks of M 1.5 or less during a span of 1.5 hours, at a depth of about 3.5-5.5 km. The swarm was not accompanied by spasmodic tremor. Seismicity otherwise has been at background levels.

Relevelling of U.S. route 395 across the resurgent dome in June 1982 revealed additional uplift since September 1980—a maximum of 8.2 cm at the crest of the dome, suggesting an average rate of uplift of about 0.5 cm/month during the 18-month time span.

Gas analyses and H₂ probe work that began in May have shown no systematic changes at Casa Diablo Hot Springs (2.5 km E of the epicentral area), where increased fumarolic activity was discovered in January. The USGS, University of Nevada, California Division of Mines and Geology, and USFS are continuing a coordinated geophysical and geochemical monitoring program at Long Valley.

Information Contacts: R. Bailey, USGS, Reston, VA; T. Casadevall, D. Dzurisin, USGS CVO, Vancouver, WA; A. Ryall, Univ. of Nevada, Reno; W. Duffield, R. Cockerham, USGS, Menlo Park, CA.

July 1982 (SEAN 07:07) Cite this Report

No new activity

Seismicity remained at background levels through mid-August. No changes occurred in fumarole activity at Casa Diablo Hot Springs.

Information Contacts: B. Dalrymple, USGS, Menlo Park, CA.

August 1982 (SEAN 07:08) Cite this Report

Minor seismicity; deformation data summarized

Seismic activity in the Long Valley area remained minor in August and early September. No spasmodic tremor was recorded, and since the six minor shocks of 13 July only occasional individual earthquakes have occurred in the epicentral area of previous swarms, on the S side of the caldera.

Francis Riley provided the following report.

"Preliminary review of spirit levels and electronic distance measurements obtained May-August 1982 in Long Valley Caldera suggests that uplift and horizontal extension in the W-central part of the caldera are continuing. Maximum values of uplift as defined by presently available data occur along the W side of the resurgent dome and amount to 33 cm since 1975. Maximum values of extensional strain since 1978 exceed 45 microstrain (change in length divided by original length x 10⁶) and are associated with the resurgent dome and S caldera rim."

Information Contacts: A. Ryall, University. of Nevada, Reno; F. Riley, USGS, Denver, CO; R. Bailey, USGS, Reston, VA.

October 1982 (SEAN 07:10) Cite this Report

More earthquake swarms

Occasional earthquake swarms have been located in the S side of the caldera, but the region that had been most active seismically has remained quiet since the swarm of 7-8 May. The earthquake swarms since 7-8 May have not included spasmodic tremor. Hypocenters for the swarms have typically been restricted to pipe-like zones. The typical swarm pattern has begun with a burst of deep events, followed by a shallow burst, then a series of events beneath the two.

On 19 September, 40-50 events occurred at depths of 3-6.5 km along an intracaldera branch of the Hilton Creek Fault, on the edge of the resurgent dome about 4 km NE of Casa Diablo Hot Springs (figure 1). The largest event , M 3.2, occurred well after the beginning of the swarm. Roughly 9 km to the WSW, a swarm of more than 100 shocks started 18 October. Most of the events occurred the first day, but a few continued through 24 October. All ranged from 3.5-6.5 km depth (most 5-6 km) and the maximum magnitude was 2.5. On 3 November at 0945, a swarm of more than 100 events began near a thermal area about 2.5 km N of the 19 September epicenters. Depths ranged from roughly 3.5-10 km and the maximum magnitude was 2.9. Only very small events were recorded after 2113.

Information Contacts: R. Bailey, USGS, Reston, VA; R. Cockerham, USGS, Menlo Park, CA; A. Ryall, University of Nevada, Reno.

November 1982 (SEAN 07:11) Cite this Report

No new earthquake swarms or deformation changes

As of early December, no earthquake swarms had been reported in the caldera since the 3 November events. A network of nine dry tilt stations in the S part of the caldera has been reoccupied about six times since May. The sensitivity limit of this network, estimated at about 10 µrad, is too large to allow monitoring of the continuing uplift of the resurgent dome at present (1980-1982) rates. However, these tilt stations would be sensitive to substantial increases in the overall rate of resurgence, or significant local deformation, neither of which has been detected. Borehole tiltmeters installed near two of the dry tilt sites 1 November are expected to be able to measure tilt changes several times as small and will telemeter data to the USGS.

Information Contacts: D. Dzurisin, USGS CVO, Vancouver, WA; R. Cockerham, USGS, Menlo Park, CA.

December 1982 (SEAN 07:12) Cite this Report

Earthquake swarms and increased thermal activity

The following is from the USGS.

"Earthquake swarms in the Long Valley area resumed in mid-December, after quiescence that lasted through most of November. On 14 December between 0056 and 0200, 200-300 small events were recorded, of which only about ten could be located. These were centered at 2-3 km depth in the S part of the caldera, in the Casa Diablo epicentral area of many previous swarms. Spasmodic tremor was recorded for the first time since the 7-8 May swarm. Increased thermal activity was noted along Hot Creek and near the epicentral area a few days before this swarm. Geyser-like activity at one Hot Creek vent occasionally ejected hot water to about 10 m height and water from another vent surged intermittently to about 1.5 m height.

"On 21 December at 1428, two M 3.3 earthquakes occurred at 6 km depth in the same epicentral area and were followed by a series of aftershocks. On 22 December between 2140 and 2200 about 100 events were recorded outside the caldera near Red Cones, two basaltic cinder cones about 9 km SW of the 14 and 21 December epicenters. Spasmodic tremor also accompanied this brief swarm.

"On 6 January at 1623, the most intense and prolonged swarm of earthquakes since May 1980 began in the S moat of the caldera. During the first 12 hours, more than 1,000 events were recorded, most in the Casa Diablo epicentral area, but with a secondary concentration near the caldera wall at Convict Creek (about 10 km ESE) and with many distributed between. Strong spasmodic tremor was nearly continuous during the first 12 hours. Two particularly strong shocks, M 5.5 and 5.6 at 1738 and 1924, caused minor damage in Mammoth Lakes and disrupted electrical and telephone service for about an hour.

"During the first 36 hours, earthquakes of magnitude greater than or equal to 1 were occurring at a rate of 80-100/hr, those of magnitude greater than or equal to 3 at 1-5/hr. During the succeeding 36 hours, the number of earthquakes gradually declined to about 15/hr. Sporadic events of M 3-3.5 continued through 1200 on 10 January. As of 12 January, recorded events were continuing at a rate of 4-5/hr, still above the normal background of about 50/day. Hypocenters during the swarm ranged from 10 km to less than or equal to 3 km depth, with most between 4 and 7 km.

"Deformation (borehole tiltmeter, dry tilt, and geodimeter) measurements made during the swarm on 10-11 January suggest that uplift of the resurgent dome accompanied the swarm, but the exact amount awaits completion of remeasurement of selected parts of the leveling network. This, together with the concentration of seismicity in the S moat and the absence of significant seismicity in the Sierra block S of the caldera during this swarm, strongly suggests that the swarm was associated with magma movement at depth. Reoccupation of the geodimeter network in early December had shown no apparent change in deformation since the previous measurements in August."

Information Contacts: R. Bailey, USGS, Reston, VA; R. Cockerham, USGS, Menlo Park, CA; F. Riley, USGS, Denver, CO.

January 1983 (SEAN 08:01) Cite this Report

Seismicity declines; epicentral area deforms

"The level of earthquake activity in the caldera continued to abate following the intense swarm that began 6 January. In mid-February, earthquakes of magnitude greater than or equal to 1 have occurred in the caldera at a rate of roughly 30/day, compared to rates of about 100/day toward the end of January and 1,000/day on 7 January. Background activity for several months prior to the January swarm averaged 8-10 earthquakes per day of magnitude greater than or equal to 1 in the caldera. No events with magnitudes greater than 3 have been recorded in Long Valley since the brief flurry that included two M 4 and one M 3.5 earthquakes on 3-4 February.

"In the days immediately following the earthquake activity of 6 January, various deformation networks in the epicentral region were resurveyed. Preliminary analysis of laser-ranging measurements and precise leveling showed that strains of 3-4 ppm and uplift of up to 7 cm accompanied the earthquake activity. The deformation pattern generally resembled that observed since mid-1980, although recently determined changes appeared to be most pronounced in the epicentral region, 2-5 km E of the town of Mammoth Lakes.

"While no definitive statement can be made on the basis of available information, it appears that the deformation pattern can be explained by movement within the source region of the earthquakes. One preliminary model suggests up to 20 cm of right-lateral slip on the seismically defined fault zone, accompanied by 80 cm of opening within that zone. The right slip is consistent with seismically determined focal mechanisms for the earthquakes. Other evidence for extension at depth in the region comes from the re-analysis of the two largest May 1980 earthquakes, which can most simply be explained as the rapid opening of a tensile crack that is filled with fluid as it grows."

Information Contacts: D. Hill, USGS, Menlo Park, CA.

February 1983 (SEAN 08:02) Cite this Report

Seismicity remains elevated; but no new swarms

As of early March, an average of 10-30 events per day of magnitude greater than or equal to 1 continued to occur in the epicentral area of the major January earthquake swarm. Few larger events were recorded in February, but five shocks with M>3 occurred 18-19 February and a M 4 earthquake was recorded 24 February in the January epicentral region. Heavy snows have severely limited deformation monitoring, but available data suggest that no major changes have occurred since January.

Information Contacts: D. Hill, USGS, Menlo Park, CA.

March 1983 (SEAN 08:03) Cite this Report

Seismicity declines to near background

Seismicity continued to decline in the epicentral area of the major January earthquake swarm. Brief bursts of small shocks were occasionally recorded, but by early April, daily earthquake counts were approaching the pre-January background levels of 2-5 events of magnitude greater than or equal to 1 per day.

Information Contacts: D. Hill, USGS, Menlo Park, CA.

June 1983 (SEAN 08:06) Cite this Report

Earthquake swarms but no deformation changes

Occasional periods of increased seismicity have continued in the Long Valley area. On 4 June a series of 100-150 small (maximum M 2.1) events occurred in the S part of the caldera, about 1 km N of the January epicentral zone. Focal depths of the June earthquakes ranged from about 3.8-4.9 km. Several km S of the caldera rim, in the aftershock zone of the four M 5.5-6.1 earthquakes of 25-27 May 1980, a M 4 event occurred on 3 July at 0950, followed by a M 5 shock in the same area at 1140. About 150 aftershocks have been recorded at depths of 8-10 km, the largest a M 3.5.

Little measurable deformation has been detected during the past several months, but large areas of the caldera are inaccessible until summer because of heavy snow cover. Considerable uplift and horizontal extension had occurred between 1975 and mid-1982 in the W-central part of the caldera (SEAN 07:08), but data from summer field work will be necessary before geophysicists can determine if this activity has continued.

Information Contacts: R. Cockerham, D. Hill, USGS, Menlo Park, CA; D. Dzurisin, USGS CVO, Vancouver, WA.

July 1983 (SEAN 08:07) Cite this Report

New collapse pits and fumarole

As of early August, no significant earthquake swarms had been detected since 4 June. An average of 2-4 events per day larger than M 1 were recorded in July. Completion of deformation measurements in the caldera is expected at the end of the summer.

The following is from Dartmouth College geologists:

"While conducting an extensive geochemical study of Rn and Hg° concentration at Long Valley, Dartmouth College geologists found evidence of activity in two previously unreported places. Very recent ground breakage (one large collapse pit 3 m long, 2.5 m wide, and 1.5 m deep; another pit 0.5 m in diameter and 0.5 m deep) was found along a fault sag approximately 0.5 km long bearing 340°. The fault is approximately 350 m W of Deer Mountain, the southernmost of the Inyo Domes in the NW part of the caldera (at UTM coordinates 321450 E and 4175800 N). A fumarole located in the Casa Diablo area (just S of the resurgent dome in the S part of the caldera) and associated with older alteration also appears to be previously unreported. New ground breakage may have allowed its formation (at UTM coordinates 333190 E and 4169370 N)."

Information Contacts: D. Hill, R. Cockerham, USGS, Menlo Park, CA; S. Williams, K. Hudnut, E. Lawrence, J. Lytle, Dartmouth College.

November 1983 (SEAN 08:11) Cite this Report

Brief earthquake swarm in S part of the caldera

A small earthquake swarm occurred in the S part of the caldera on 14 November. Between the start of the swarm at 1909 and its end about 40 minutes later, 40-50 events larger than M 0.8 were detected. The strongest shocks had magnitudes of 3.2 (at 1929) and 3.0 (at 1936). The 14 November events (centered about 6.5 km SE of the town of Mammoth Lakes) were within the epicentral zone of the major January swarm (SEAN 07:12 and 08:01), but about 3 km SE of the initial January swarm seismicity. Focal depths of the November earthquakes ranged from 2.5-4 km, making it the first swarm for which it was possible to confirm that no events were deeper than 4 km; depths of past swarms were typically 3-7 km. Some seismicity continued after the end of the main swarm at 1950; all events during the next 4 hours were at depths of 2-3.5 km.

Roughly 8 km to the W (under the E flank of Mammoth Mountain) a swarm of about 50 events had occurred 24 September between 0525 and 0630. The largest magnitude was 2.8. Seismic stations were too distant for precise depth determinations, but the events were roughly between 4 and 6.5 km depth. A slight increase in seismicity was noted in the same area at the end of November.

Information Contacts: R. Cockerham, USGS, Menlo Park, CA.

September 1984 (SEAN 09:08) Cite this Report

Seismicity at relatively low level and inflation

The following is from a USGS report.

"Following decay of the January 1983 earthquake swarm activity to background levels in the spring of 1983 (SEAN 07:02, 08:01, 08:02, and 08:03), seismic activity has persisted at a relatively low level. Within the caldera, this background level typically involves several earthquakes (magnitude greater than or equal to 1) per day and an occasional locally felt M 3 event. A M 4.2 earthquake on 28 April 1984 and a M 3.8 event in a swarm that began 16 July 1984 are the largest events to occur in the caldera since the January 1983 swarm. Deformation data show evidence for only modest inflation of the resurgent dome following the January 1983 swarm. Frequently repeated trilateration measurements, however, show continued extensional deformation within the caldera at rates of several microstrain units per year.

"With the reduced rates of seismic activity and ground deformation in the caldera during the last year, the possibility of an imminent eruption appears more remote than during the previous 2 years. In a letter dated 11 July 1984, the Director of the USGS advised California officials that a volcanic eruption does not pose an immediate threat to public safety in the Long Valley region and that the Long Valley region does not satisfy the imminent threat criteria for an official Hazard Warning for volcanic activity.

"The Long Valley region had been under a Notice of Potential Volcanic Hazard since May 1982. Prior to September 1983, this was the lowest of three levels used by the USGS to express the relative urgency of a potential geologic hazard when formally notifying the public and responsible officials. In September 1983, the 3-level terminology was changed to a single-level system termed a Hazard Warning. The USGS is continuing its intensive seismic, geophysical, and geodetic monitoring of Long Valley Caldera that was initiated in mid-1982."

Information Contacts: D. Hill, USGS, Menlo Park, CA.

October 1985 (SEAN 10:10) Cite this Report

Continued deformation and low-level seismicity

The quoted material is from the USGS.

"Since the spring of 1983, seismicity has persisted at relatively low levels and regional extensional deformation and uplift centered over the resurgent dome have continued.

"Seismicity and geodetic measurements through August 1985 indicated that broad-scale deformation across the entire caldera was continuing. However, decreases have occurred in both seismicity within the caldera and the rate of deformation within the S moat, the region of the caldera between the resurgent dome and the S rim (figure1).

"No detectable earthquakes occurred within the caldera 1-23 July, and only 4 small ones (M <2.2) were recorded during the last 8 days of June. On 24 July, however, the low-level seismicity characteristic of the previous several months (an average of 0.7 events with magnitude greater than or equal to 1 per day) resumed and continued through August. The largest July-August earthquake in the caldera was a M 2.1 event on 12 August. Preliminary examination of more recent data suggests that this pattern of low-level seismicity continued through September and October.

"Recently completed annual measurements of the regional trilateration network and several of the major level lines indicated that regional extensional deformation and uplift centered over the resurgent dome continued at rates little changed from those measured from mid-1980 to mid-1984. Summer 1985 results for the level line along Highway 395, for example, showed that cumulative uplift for bench marks near Casa Diablo (at the S side of the resurgent dome) was 47 cm with respect to elevations measured in 1975 (figure 2).

"Data from the frequently measured 2-color geodimeter network showed that local deformation within the S moat, including the epicentral region of the major January 1983 earthquake swarm, continued to slow and had virtually stopped along certain lines. July-August data from the telemetered tiltmeters are consistent with minimal local deformation within the caldera. Data from a continuously recording dilational strainmeter in a borehole about 3 km outside the caldera (at Devil's Postpile) showed that the strain rate apparently decreased somewhat abruptly on about 20 June, and remained flat until a small change about 28 August. Measurements of magnetic field variations at two locations in the SE and NW parts of the caldera (Hot Creek and Smokey Bear Flat) showed no changes through August.

"The geodetic evidence that broad-scale deformation across the entire caldera is continuing at a more or less uniform rate points to continuing inflation of the deeper section of the magma chamber (depths of 10-12 km). In contrast, the marked decrease in both the seismicity rate within the caldera and the rate of local deformation across the S moat indicate that stress differences in the upper crust associated with the January 1983 earthquake swarm (and possible related intrusive event) have substantially relaxed. However, as long as we see evidence for continuing inflation of the deeper magma chamber, we must regard the recurrence of another episode of unrest somewhere within the caldera as a distinct possibility."

In cooperation with the USGS, the California Division of Mines and Geology installed a system for monitoring seismicity within the caldera, with particular emphasis on low-frequency events. Stephen McNutt reported that no low-frequency events were recorded in the caldera between July 1984, when the system became operational, and October 1985.

Further References. Hill, D.P., Bailey, R.A., and Ryall, A.S., 1984, Proceedings of Workshop XIX: Active Tectonic and Magmatic Processes Beneath Long Valley Caldera, Eastern California; USGS Open File Report 84-939, 942 p. (32 papers).

Savage, J.C., Cockerham, R.S., and Estrem, J.E., 1987, Deformation Near the Long Valley Caldera, Eastern California, 1982-1986; JGR, v. 92, no. B3, p. 2721-2746.

Special Section — Long Valley Caldera, California; JGR, 1985, v. 90, no. B13, p. 11111-11252 (14 papers).

Information Contacts: D. Hill, J. Savage, USGS, Menlo Park, CA; S. McNutt, California Div. of Mines & Geology.

July 1986 (SEAN 11:07) Cite this Report

Nearby earthquakes cause no pronounced seimicity and deformation change in caldera

As of the end of July, no pronounced changes in seismicity and deformation had been observed ...after a series of moderate earthquakes began mid-July in the Chalfant Valley, ~20 km to the SE. [The largest event, 6.2 Ms/6.0 mb, occurred 21 July at 2142 (37.54°N, 118.45°W, 9 km depth). Small foreshocks were recorded two days earlier, and several moderate-sized aftershocks occurred the following week.] Some co-seismic deformation across the caldera was observed. The 2-color trilateration network showed uniaxial contraction in an E-W direction and the borehole dilatometer showed a compressional signal of comparable magnitude. Most of the borehole tiltmeters in the caldera showed a co-seismic offset coincident with the Chalfont Valley earthquakes. Stephen McNutt reported that no low-frequency earthquakes were recorded within the caldera by the California Division of Mines and Geology network.

Information Contacts: D. Hill, USGS Menlo Park; Stephen McNutt, California Division of Mines and Geology, 630 Bercutt, Sacramento, California 95814 USA.

February 1987 (SEAN 12:02) Cite this Report

Tectonic seismicity and slow inflation in 1986

The following is from a summary of 1986 activity by David Hill. "Earthquake activity ... persisted at a low level throughout 1986. The average seismicity rate was 0.5 events (M >1)/day with only 1 event reaching M 3.0, on 14 March. That event was centered in the S moat ~ 1 km S of the resurgent dome. Activity in the Sierra Nevada block to the S persisted at a somewhat higher level (figure3). Two M 3.0 events occurred in January and another in mid-March.

"The seismically active region expanded E (figure 3) to include the area near the W front of the White Mountains (Chalfant Valley), ~20 km E of Long Valley Caldera; this area had very little earthquake activity in previous years. Low-level foreshock activity on 3 July culminated in a M 5.8 foreshock on 20 July. The M [6.2] mainshock occurred on 21 July and was followed by a vigorous aftershock sequence that included three M >5 events and several thousand smaller events, many of which were locally felt. Focal mechanisms for three of the larger events in the sequence showed predominantly right-lateral slip with a small component of normal slip on NNW-striking planes dipping 55-90° W. A series of preliminary reports on this earthquake sequence were published in Bulletin of the Seismological Society of America (v. 77, no. 1, p. 280-313).

"The dominant deformation event during 1986 was associated with the Chalfant Valley mainshock. This earthquake resulted in a 0.5 x 10^-6 co-seismic strain change across the caldera as recorded by the two-color geodimeter network and the borehole dilatometer at Devils Postpile. These observations were consistent with the deformation field expected from roughly 1.5 m of right-oblique slip in a 15-km-long buried fault beneath Chalfant Valley striking N55°W, dipping 55°W, and with the seismic 25 dyne-cm based on teleseismic data. Deformation measurements from 1986 otherwise showed little change from what appeared to be a gradual long-term slowing trend throughout the region. Two-color laser data showed the same slow extension (roughly 1 microstrain/year) across the SW section of the caldera that has occurred for the last several years. Annual measurements of the regional trilateration and levelling networks completed this summer showed only marginally significant changes in line length and elevation. The telemetered data from the borehole tiltmeters and the borehole dilatometer showed no unusual excursions during the year.

"The hydrologic system in the caldera also remained relatively stable throughout 1986, showing normal seasonal changes. The most pronounced short-term variations involved co-seismic changes in the water level in monitored wells and in the flow rates of some hot springs accompanying the Chalfant Valley mainshock."

Further Reference. Savage, J., Cockerham R., Estrem, J., and Moore, L., 1987, Deformation Near the Long Valley Caldera, Eastern California, 1982-1986; Journal of Geophysical Research, v. 92, p. 2721-2746.

Information Contacts: D. Hill, USGS Menlo Park.

November 1988 (SEAN 13:11) Cite this Report

Several hundred earthquakes in E caldera moat

One of Long Valley caldera's more significant earthquake swarms of the last several years occurred 20-22 November (figure 4). Of the 393 recorded events (to M 2.0), most were located in the E moat of the caldera (near the E margin of the resurgent dome) with minor activity in the S moat (figure 5). The swarm trended N-S at depths of about 6 km. As the number of events/hour decreased, average magnitude increased. First order levelling surveys were completed across the epicentral area in 1986; about two weeks before the swarm; and the day after it began. The data suggested slight uplift of the area W of the epicentral zone relative to the E side, consistent with normal faulting, but the measured changes were within error limits (figure 6).

Further Reference. Langbein, J., 1989, Deformation of the Long Valley Caldera, Eastern California, from mid-1983 to mid-1988; Measurements Using a Two-Color Geodimeter; Journal of Geophysical Research, v. 94, p. 3833-3849.

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento; D. Hill, John Estrem, and James Savage, USGS Menlo Park.

June 1989 (SEAN 14:06) Cite this Report

Earthquake swarm near caldera rim

An earthquake swarm began 4 May under the SSW flank of Mammoth Mountain, just outside the SW caldera rim (figure 7). The number of events increased through early June, with 44 recorded on the 11th. Seismicity was continuing as of 10 July, and totaled 712 recorded events (magnitude greater than or equal to 0.3) (figure 8). Most were small (M <1); the largest, M 3.1, occurred on 21 June at 0058. As the swarm continued, most of the events remained centered beneath the SW flank of Mammoth Mountain, on strike with the Inyo chain, at depths ranging from 2 to 9 km. Focal depths during previous swarms have generally been around 6 km. Most of the shallower earthquakes showed less high-frequency energy in their spectra, probably because of attenuation effects, but had clear S-waves and were therefore not considered low-frequency events. However, seven low-frequency events were recorded on 11 June. Several mixed-frequency events had high-frequency P and S-waves superimposed on 1-2-Hz waves, suggesting possible resonance of a fluid-filled cavity. Possible spasmodic tremor was recorded for 2-3 minutes on 2 and 26 June, and 6 July.

The Devils Postpile dilatometer, near the W foot of Mammoth Mountain, recorded 0.05 microstrain of deformation during the swarm's most active day, 11 June. No significant changes to existing trends were reported from other instruments a few kilometers away.

The May-July swarm is the largest near Mammoth Mountain in 3.5 years; a small swarm occurred there in January 1987. During the past 4 years, virtually all of the other seismic swarms in the Mammoth Lakes area have lasted only a few days. The largest recent swarm, 393 recorded events in the caldera's E moat, began 22 November 1988 and ended after 3 days.

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento.

July 1989 (SEAN 14:07) Cite this Report

Seismic swarm continues

The seismic swarm under the SW flank of Mammoth Mountain continued in July, and by the 31st, 960 events had been recorded by the California Division of Mines and Geology NEWT system. Most of July's 342 recorded events were M <1.0, with the largest on 20 June at 1758 (M 3.3) and 1 August at 1917 (M 3.4). Forty earthquakes were recorded 27 July (figure 8), the second most active day since the swarm began; 44 were recorded 11 June (14:6). Hypocenters remained beneath the SW flank of Mammoth Mountain at depths mostly <10 km, but two low-frequency events on 27 July were located by the USGS at 15-18 km depth. The USGS seismic station at Mammoth pass (MMP), in the epicentral area (figure 9), recorded several hundred events/day, too small to be detected by other stations.

Seismic activity remained relatively constant in July but the number of spasmodic tremor episodes increased. Bursts of spasmodic tremor with 7-13 subevents over 1-3-minute periods were detected 2 and 26 June, and 6, 13, 19, and 27 July. Seven events on 11 June, and one on 30 July had low frequencies of 1-3 Hz (probably due to attenuation effects) and poorly constrained depths of ~2 km (because no S-waves were detected). Mixed-frequency events, suggesting resonance of a fluid-filled cavity, had clear high-frequency P- and S-waves superimposed on 1-2-Hz waves, with locations similar to the high-frequency events. Stephen McNutt noted that the seismicity suggests the presence of a fluid phase (meteoric water or a volatile phase), possibly associated with magma.

During mid-late June and July, the USGS Devil's Postpile dilatometer, near the W foot of Mammoth Mountain, showed several short-term (1-10-day) fluctuations, with amplitudes of ~0.6 microstrain, superimposed on long-term extension. The regional 2-color laser network has recently shown some E-W compression in contrast to the extension of the last several years. USGS L-shaped leveling arrays on Mammoth Mountain and in the caldera's S moat showed small anomalies (possibly related to the earthquake swarm) consistent with intrusion under Mammoth Mountain. Two weak fumaroles had slightly increased temperatures and were sampled for He³/He4 analysis.

The swarm represents the highest level of seismicity near Mammoth Mountain since 1979 and has had an unusually long duration (three months); 80% of swarms analyzed by McNutt in volcanic areas worldwide have lasted less than two months. Many small events were still being recorded as of 7 August.

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento; D. Hill, USGS Menlo Park.

August 1989 (SEAN 14:08) Cite this Report

Shallower seismicity; minor deformation

The earthquake swarm was continuing in early September (figure 8). Most of the events have been small (M <1), with the largest, M 3.3, on 21 June. By 30 August, the state of California's NEWT system had recorded 1,150 swarm events, 193 in August. The USGS MMP station, in the epicentral area, continued to record several hundred shocks daily that were too weak (M <0.5) to be detected by other nearby instruments [but see SEAN BGVN 14:09]. A series of 37 earthquakes was recorded on 29 August, the 3rd largest daily total of the 4-month swarm. The epicenters for this day were still on the SW flank of Mammoth Mountain, but focal depths were only 2-4 km, compared to the 4-9 km typical of most previous swarm events.

The USGS Devil's Postpile dilatometer, roughly 4 km WNW of the epicentral area, has shown small changes that may be correlated with the Mammoth Mountain seismicity. Since its installation several years ago, it has recorded a slow compressional trend that typically accelerates in the fall and flattens in the spring, yielding a cumulative change of up to 5 ppm. The dilatometer has also measured occasional superimposed wavelets lasting two to a few days with amplitudes of roughly 0.5 microstrain. One such deviation occurred about three weeks before the onset of the swarm, and several others were apparently associated with bursts of seismicity during the swarm. The slow dilatation that had been occurring since late July began to level off ~1.5 days before the onset of the shallow 29 August events. At about the time they started, the dilatometer detected a distinct 1-2-hour perturbation that was a fraction of a tidal amplitude. Although the changes were small, their polarity is consistent with an intrusion beneath Mammoth Mountain.

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento; D. Hill, USGS Menlo Park.

September 1989 (SEAN 14:09) Cite this Report

Frequent seismicity continues

Frequent seismicity continued through September beneath the SW flank of Mammoth Mountain (see figure 9). As of 30 September, 1,375 events had been recorded by the California Division of Mines and Geology NEWT system since the swarm began on 4 May. The September total rose slightly to 220 shocks, from 193 in August.

Strong bursts of seismicity occurred on 9 and 19 September (see figure 8). Most of the 42 events recorded on 9 September occurred during several strong bursts of spasmodic tremor and were centered at 4-9 km depth. On 19 September, the bulk of the 35 events were isolated and had focal depths of 2-12 km, nearly the full depth range of the May-September swarm. Other than the several shocks at 11-12 km on 19 September, there was no indication of systematic depth migration during the month. Low-frequency earthquakes were detected on 9, 19, and 21 September. Locations were similar to those of other swarm events, but they had emergent P-waves, lacked clear S-waves, and were of lower overall frequency content than other shocks of the same magnitude. In the epicentral area, the USGS MMP station generally recorded only 10-20 small (M < 0.5) events/day that were not detected by more distant instruments. Previous totals as high as several hundred events/day from this instrument (SEAN 14:07 and 14:08) have been recorded only during periods of high seismic activity. Although precise data from MMP were not available, the typical September daily values represented a severalfold decline from earthquake counts during analagous periods of relative seismic quiet in June and July.

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento; D. Hill, USGS Menlo Park.

November 1989 (SEAN 14:11) Cite this Report

Seismic swarm gradually declines; minor inflation

The seismic swarm...continued in October and November (figure 10). The number of earthquakes has slowly declined, from 220 in September to 137 in October and 52 in the first half of November. By 15 November, 1,564 swarm events had been recorded by the California Division of Mines and Geology NEWT system. There was no indication of any systematic depth migration during October or November. A USGS seismic station (MMP) in the epicentral area continued to record about a dozen to several dozen small (M<0.5) events/day through mid-December. Ten low-frequency events occurred in Octoer, for a total of 31 since the swarm began. Locations were similar to those of other swarm events, but the low-frequency shocks had emergent P-waves, lacked clear S-waves, and were of lower overall frequency than others of the same magnitude. Spasmodic tremor has occasionally been recorded, but only one episode occurred in October.

Relevelling of a line (along Rt. 203) a few kilometers N of the main epicentral area in late September yielded ~1.5 cm of relative uplift at its W end (Minaret summit) compared to summer 1988 values. New 2-color geodimeter sites on the E and N flanks of Mammoth Mountain that were surveyed in late August, late September, and about 10 November showed about four microstrain of NW-SE extension, parallel to the T axes of swarm focal mechanisms. These data were consistent with extension and uplift of the epicentral region. Preliminary data also suggest increasing dilatation across the resurgent dome in recent months, and seismic instruments have detected a few small (magnitude less than or equal to 2) earthquakes in the caldera's W moat.

Information Contacts: S. McNutt, California Division of Mines and Geology, Sacramento; D. Hill, USGS Menlo Park.

December 1989 (SEAN 14:12) Cite this Report

Seismicity increases in caldera's S moat

The seismic swarm ... declined significantly from October through December. Meanwhile, a few earthquakes began in October around the S margin of the resurgent dome, which has been relatively inactive for several years. A distinct increase in seismicity began there in November. A burst of 40 events was recorded 5 January in the S moat area, followed by fluctuating activity; some days were seismically quiet. Seismicity appeared similar in many ways to that of the early 1980's, although more of the current activity occurred on the W side of the resurgent dome than previously. Focal depths, generally ~6-10 km, also resembled those of the early 1980's, with the deeper events on the E side of the epicentral zone. Some shocks on the W side of the resurgent dome were <4 km deep. The rate of extension across the resurgent dome began to increase in mid-1989, reaching ~5 microstrain/year by December. This is ~5 times the rate seen in recent years.

Information Contacts: D. Hill, USGS Menlo Park.

January 1990 (BGVN 15:01) Cite this Report

Continued swarm-like activity on S margin of resurgent dome and S moat; brief swarm at Mammoth Mountain

"Swarm-like activity of magnitude greater than or equal to 2 earthquakes along the S margin of the resurgent dome and S moat continued to increase through January and early February (figure 11). During the same period, Mammoth Mountain, on the SW rim of the caldera, remained relatively quiet except for a brief burst of activity beneath the mountain's N flank on 19 January that included a locally felt earthquake of about M 3." A preliminary report on the Mammoth Mountain swarm will appear in the April issue of the Bulletin of the Seismological Society of America (see Reference, below).

"Activity within the caldera continues to occur in much the same area as the recurring swarms of the early 1980's, although the current activity shows a tendency to be concentrated somewhat farther N beneath the resurgent dome and along its W and E margins. Focal depths range from roughly 8 to <3 km as during the earlier activity. This increased swarm activity within the caldera no doubt reflects the 5-fold increase in extensional strain rate across the resurgent dome, detected by repeated measurements of the 2-color geodimeter network that spans the central section of the caldera. The strain rate increase began in September and has continued through January at a rate of ~5 microstrain/year."

Reference. Hill, D.P., Ellsworth, W.L., Johnston, M.J.S., Langbein, J.O., Oppenheimer, D.H., Pitt, A.M., Reasenberg, P.A., Sorey, M.L., and McNutt, S.R., 1990, The 1989 Earthquake Swarm Beneath Mammoth Mountain, California: An Initial Look at the 4 May through 30 Sep Activity: Bulletin of the Seismological Society of America, v. 80, p. 325-339.

Information Contacts: D. Hill, USGS Menlo Park.

February 1990 (BGVN 15:02) Cite this Report

Seismicity continues to increase along S margin of resurgent dome

Seismicity has continued a systematic increase along the S margin of the resurgent dome. Swarms occurred on 4 and 14 Febuary, the latter containing >100 located events, one reaching M 3. Another swarm on 27-28 February contained several of M >2, and on 3 March more than 200 shocks were recorded, the largest at M 2.8. In the two weeks following the 3 March swarm, seismic activity remained relatively quiet, with a few days having as many as ten recorded events. Depths remained between 8 and <3 km. The 2-color geodimeter system has continued to detect enhanced strain rates of ~5 microstrain/year since September. On the SW rim of the caldera, Mammoth Mountain was generally quiet, although a series of events occurred there on 6 March, the largest reaching M 1.7.

Information Contacts: D. Hill, USGS Menlo Park.

March 1990 (BGVN 15:03) Cite this Report

Continued earthquake swarm in caldera's S moat

Earthquake swarm activity in the caldera's S moat continued through March. A swarm of >300 events of magnitude greater than or equal to 2.8 occurred 3 March, followed by smaller swarms on 9, 18, 28, and 30 March. The swarm on the 30th included more than 100 events, all of which were smaller than M 2. Only a few isolated events occurred beneath Mammoth Mountain. Two-color geodimeter measurements indicate that extension across the S moat and resurgent dome continued through March at the 5 ppm/year rate that began in late September.

Information Contacts: D. Hill, USGS Menlo Park.

April 1990 (BGVN 15:04) Cite this Report

S moat earthquake swarms of 6-7 May most intense since 1983-84

Earthquake swarms continued in the S moat through early May, with bursts of activity (magnitude greater than or equal to 2.5) on 28 and 30 March, 18-20 April, and 6-7 May. The 6-7 May swarm was the most intense activity within the caldera since 1983-84. It began with an earthquake of about M 2.7 on 6 May at 2,238 and produced more than 300 of M >0.5 over the next 24 hours. The earthquakes included nearly 20 of magnitude greater than or equal to 2.5 and three of magnitude greater than or equal to 3, the largest of which was about M 3.5 at 0241. This swarm, as with most of the others that have occurred since the beginning of the year, was centered in the S moat, ~4 km E of the town of Mammoth Lakes (figure 12). Focal depths ranged from <3 km to as deep as 10 km, with most concentrated between 5 and 8 km (figure 13). Earthquakes in the S moat area as small as about M 2.5 are felt in Mammoth Lakes, and residents reported feeling some 15-20 events during a 5 1/2-hour period starting 6 May at 2238.

An approximately 5-fold increase in extensional strain across the S moat and resurgent dome began to be recorded by the caldera's 2-color geodimeter network in September 1989. The extension rate continued to average 4-5 ppm/year through early May, although recent measurements indicated that the rate may be slowing somewhat. Small strain changes, apparently associated with the 6-7 May swarm, were detected by borehole dilatometers at distances of 6 and 10 km. The changes were generally ~0.03-0.05 microstrain.

Information Contacts: D. Hill, USGS Menlo Park.

May 1990 (BGVN 15:05) Cite this Report

S moat seismicity declines and deformation slows

Earthquakes continued to occur in the S moat, but at a declining rate following the 6-7 May swarm. About 80 earthquakes of magnitude greater than or equal to 2 occurred 6-13 May, but the number of events of that magnitude declined to nine for the week of 14-21 May, then seismicity fluctuated between three and six magnitude greater than or equal to 2 shocks/week through mid-June. The only M 3 event in the region through 11 June occurred in the Chalfant Valley aftershock zone (40 km SE of the caldera) on 21 May. Repeated trilateration measurements, spanning the caldera with the 2-color geodimeter, showed a gradual slowing of the episode of accelerated extension across the S moat and resurgent dome that began in September 1989.

Information Contacts: D. Hill, USGS Menlo Park.

March 1991 (BGVN 16:03) Cite this Report

Earthquake swarm in S moat is most intense inside the caldera since 1983-4

All times are UTC. The following is a report from David Hill.

"Strong earthquake swarm activity resumed ... during the last week in March, after nearly nine months of sporadic seismic activity at low-to-moderate levels. The swarm began at 2212 on 23 March with a M 3 earthquake and several M >2 events. The rate increased significantly beginning about 0500 on 24 March with the occurrence of 5 additional M >3 events accompanied by more than 40 M >1 events/hour over the next five hours. Activity gradually slowed through the remaining hours of the 24th and the first half of the 25th with occasional M 3 events, only to surge to an even higher level beginning about 1630 on the 25th and continuing through the 26th. This last phase of the swarm included more than 15 M >3 events (the largest of which approached M 3.6) together with hundreds of smaller events. Altogether, the swarm included 22-25 M >3 earthquakes and >1,000 smaller events, with a cumulative seismic moment comparable to that of a single M 4 earthquake.

"The swarm was located in the S moat of the caldera ~4 km E of Mammoth Lakes (figure 14). Preliminary hypocentral locations indicate that most of the earthquakes were confined within a relatively tight cluster roughly 2 km in diameter with most of the focal depths between 4 and 6 km. This is essentially the same volume of crust that has produced numerous swarms since the early 1980 onset of unrest in the caldera; it contained the epicenter of one of the four M 6 earthquakes that occurred during the May 1980 sequence as well as the two M 5.3 earthquakes during the January 1983 S moat swarm. This is the most intense swarm within the caldera since the 1983-84 activity in terms of the total number of M >3 earthquakes in less than a week.

"We saw little evidence of deformation associated with this swarm. The borehole dilatometers showed no significant changes during the swarm, and measurements of the two-color geodimeter lines before and after the swarm showed only minor extension (a few mm at most). Over the longer term, however, the two-color data indicate that the resurgent dome continues to inflate at a rate of ~2 microstrain/year. This is roughly twice the rate that persisted through the last half of the 1980's. The current rate represents the gradually slowing tail of a renewed episode of inflation that began in early Oct 1989 with an abrupt acceleration in rate to 9 microstrain/year."

Information Contacts: D. Hill, USGS Menlo Park.

June 1992 (BGVN 17:06) Cite this Report

Abrupt increase in seismicity triggered by M 7.5 earthquake hundreds of kilometers away

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers. No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Long Valley Report. Within eight minutes of the major earthquake's origin time, seismic activity within Long Valley caldera (400 km NNW of the epicenter) increased abruptly (figure 15). Of the >260 events located by the RTP system during the next three days, three were of M 3 or greater. The first event within the caldera located by the RTP system was a M 1.4 earthquake at 1207, but develocorder film from caldera stations provides evidence of local earthquakes beginning at least a minute earlier within the strong coda waves from the M 7.5 event. The P-wave travel-time from the epicenter is just over 1 minute, and the S-wave travel-time just under two minutes, so it appears that local earthquake activity began no later than six minutes after the S-wave arrival.

Earthquake activity within Long Valley caldera had persisted, but at relatively low levels, through the first half of 1992, averaging

Information Contacts: D. Hill, USGS Menlo Park.

April 1994 (BGVN 19:04) Cite this Report

Summary of 1992-93 seismicity and deformation

The following report summarizes caldera seismicity and deformation during 1992-93.

Earthquake activity within the caldera gradually increased through 1993, with the level of activity significantly higher than in 1992 (figure 16). More than 30 earthquakes of M >3 occurred within the caldera during 1993; six were associated with a swarm on 27 November. By comparison, only four events in 1992 had M >3. The level of activity in the Sierra Nevada block, S of the caldera, was also higher in 1993, with over 30 M >3 earthquakes, compared to 14 in 1992. During 10-15 August 1993, the Red Slate Mountain swarm was the most intense earthquake sequence of the year in the region. It included the largest event (M 4.5) and the largest number of events (~400 M >1) for any one swarm.

An interesting episode of seismic activity in the Long Valley region followed the 28 June 1992 Landers earthquake (M 7.4) in southern California, >400 km S. Local increases in seismicity triggered by the Landers earthquake were also recognized in the Geysers geothermal area and in Yellowstone National Park, over 1,200 km from the epicenter. These were clear examples of seismic "remote-triggering."

A series of >60 long-period earthquakes began beneath the SW flank of Mammoth Mountain during a mid-1989 earthquake swarm, and continued through 1993. These events lacked significant energy at frequencies above 5 Hz, distinguishing them from the much more common volcano-tectonic earthquakes. Reliable hypocenter locations were determined for 30 long-period events in 1992. Focal depths ranged from 10 to 28 km, distinctly deeper than the 2-10 km depth range for local tectonic earthquakes. Most of the long-period events were located between Mammoth Mountain and Red Cones (a pair of Holocene basaltic cinder cones 4 km SW of Mammoth Mountain), although two were located 5-7 km W of Mono Craters and one was within the caldera beneath the W moat. Intervals between events ranged from weeks to months, with individual occurrences typically consisting of several events within a few minutes. Magnitudes were in the 0.7-2.2 range. In 1993, long-period events beneath the SW flank of Mammoth Mountain were detected at depths of 10-25 km. Instruments detected eleven episodes of long-period activity from March through December 1993. Approximately half were solitary events with the rest consisting of bursts of several events over time intervals of 1-2 minutes. All of the long-period events in 1993 were small; the largest had a magnitude of ~1. Similar sporadic long-period events have been recognized beneath Lassen Peak, Medicine Lake caldera, and young volcanic areas in Japan. Evidence suggests that occasional long-period events at mid- to lower-crustal depths are common beneath areas of Holocene volcanism and are not indicative of imminent volcanic activity.

Deformation measurements from late 1989 through 1992, using a two-color geodetic distance-meter (geodimeter), revealed elevated deformation rates across the caldera compared to the modest rate that characterized the last half of the 1980's. Deformation remained steady in 1993 at 2-3 ppm/year. The increased rate of seismicity, however, is still not adequate to account for the deformation that has accumulated since 1990. Geodimeter data and annual GPS surveys of the regional Long Valley-Mono Craters geodetic network indicated that caldera deformation was dominated by inflation of the resurgent dome, driven by a pressure source at a depth of 5-8 km. However, this single source cannot fully account for either the regional deformation pattern or the geodimeter and 1988-92 leveling data. These data sets require a second, relatively deep (>15 km) inflation source somewhere beneath the SW margin of the caldera and Mammoth Mountain, although the location is poorly constrained. Analysis of the combined data sets indicated that the pressure source beneath the dome may have a flattened ellipsoidal shape, and that normal slip on the medial graben faults may have accompanied dome inflation.

Further References. Hill, D.P., Reasenberg, P.A., Michael, A., and 28 others, 1993, Seismicity remotely triggered by the magnitude 7.3 Landers, California, earthquake: Science, v. 260, p. 1617-1623.

Langbein, J., Hill, D.P., Parker, T.N., and Wilkinson, S.K., 1993, An episode of reinflation of the Long Valley caldera, eastern California: 1989-1991: Journal of Geophysical Research, v. 98, p. 15,851-15,870.

Pitt, A.M., and Hill, D.P., 1994, Long period earthquakes in the Long Valley caldera region, eastern California: Geophysical Research Letters (in press).

Information Contacts: D. Hill, USGS Menlo Park.

March 1995 (BGVN 20:03) Cite this Report

Summary of 1994 seismicity, deformation, and CO₂ discharge

The following summarizes more detailed reports (Hill, 1995; Johnson and others, 1995; and Sorey and others, 1995) on caldera seismicity, deformation, and CO₂ discharge at Mammoth Mountain during 1994.

Earthquake activity within the caldera gradually decreased through the first months of 1994, and activity thereafter remained moderate with a few exceptions. During the entire year there were only ten-twelve M ~3 earthquakes in the caldera, in comparison with 30 in 1993. The earthquakes continued to cluster in the caldera's S moat, and gradually moved northward. During 1994, earthquakes with M <2 took place beneath Mammoth Mountain at depths of 4-20 km.

Seismicity in the Sierra Nevada block, S of the caldera, persisted at a moderate level throughout the year and was concentrated in a broad band extending S from Mount Morrison to Red Slate Mountain. In the Chalfant Valley, E of the Long Valley Caldera and W of the White Mountains, over 20 M ~3 earthquakes occurred throughout 1994, with many smaller late M <2 aftershocks associated with the M 6.4 Chalfant Valley earthquake of 1986.

Swelling of Long Valley's resurgent dome continued at a steady rate of 2-3 ppm/year, resembling 1993 activity. Deformation measurements, using a two-color geodetic distance-meter (geodimeter), revealed steady extension rates to the N and E of a central survey site (CASA, figure 17) from mid-1991 through the end of 1994. To the W and SW of CASA, extension rates gradually decelerated beginning in mid-to-late 1993 and continuing through 1994.

Dead mature pine trees were found in four separate areas on the flanks of Mammoth Mountain during 1994. Reports of asphyxia among workers entering poorly ventilated parts of the tree kill areas and an area near the top of the Chair 3 ski lift were also recorded during 1994, and were correlated with high (10-90%) CO₂ concentrations in the soils (Sorey and others, 1995). The area of tree mortality has expanded since 1989, when the first tree death was reported. Several explanations have been put forward, including: 1) dike intrusion during the intense earthquake swarm below Mammoth Mountain of April-December 1989; 2) ongoing shallow silicic magma intrusion; 3) ongoing input of basaltic magma from a deeper source associated with the long-period earthquakes that began in 1989; and 4) gas release from a volatile-rich vapor zone surrounding areas of previously emplaced igneous rocks.

References: Hill, David P., 1995, Long Valley Caldera Monitoring Report (Oct - Dec 1994): U.S. Geological Survey, Office of Earthquakes, Volcanoes, and Engineering, 345 Middlefield Rd. Menlo Park, CA 94025, 16 p.

Sorey, Mike, Evans, Bill, and Farrar, Chris, 1994, Gas composition and discharge rate at Mammoth Mountain, in Hill, 1995, Long Valley Caldera Monitoring Report (Oct - Dec 1994): U.S. Geological Survey, 2 p.

Information Contacts: D. Hill, USGS Menlo Park.

April 1996 (BGVN 21:04) Cite this Report

Summary of 1995 activity; March-April 1996 earthquake swarm

The 17 x 32 km Long Valley caldera (figure 18) lies E of the central Sierra Nevada, ~320 km E of San Francisco. The caldera formed about 730,000 years ago as a result of the Bishop Tuff eruption. Resurgent doming was followed by eruptions of rhyolite from the caldera moat and rhyodacite from the outer ring-fracture vents until ~50,000 years ago. Since then the caldera has remained thermally active, and in recent years has undergone significant deformation. Although distinct from Long Valley Caldera, both Inyo Craters and Mammoth Mountain sit adjacent to it. The following report summarizes a more detailed report on caldera seismicity, deformation, and CO₂ discharge at Mammoth Mountain during 1995 (Hill, 1996).

Two earthquakes on 2 and 4 January 1995 (M 3.2 and 2.5, respectively), occurred in the area just W of the Highway 203-395 junction. After these events, the epicentral area, a locality with frequent earthquake swarms, turned relatively quiet for the remainder of 1995. Then, after a M 3.3 earthquake on 14 January centered in the S moat, the activity in the caldera shifted to the E. Seismicity through the rest of 1995 in the caldera and adjacent areas was largely confined to a N-S corridor extending from the SE margin of the resurgent dome to the wall of the caldera and beyond into the Sierra Nevada block (figure 18).

On 4 March, M 4.4 and 4.3 earthquakes occurred near the southern caldera boundary: these were the largest events to occur in the region during 1995. A swarm on 19-20 March in the S moat included more than 150 M > 1 earthquakes and three M > 3 events. Activity slowed down through mid-June both within the caldera and in the Sierra Nevada block. Activity within the caldera picked up briefly on 23 and 27 June, with swarms at the S margin of the resurgent dome. Each included a M > 3 earthquake accompanied by more than 20 smaller events.

Beginning with the last two days of June, activity shifted S to the Sierra Nevada block. A brief pause near the end of July was followed by a stronger surge in the number of earthquakes through August and September. This swarm-like surge included more than 20 M > 3 earthquakes with individual clusters, commonly producing 20-30 events. The largest cluster occurred on 17 September and included a M 3.7 earthquake and over 50 smaller events. Seismic activity also increased along the SW stretch of the caldera and at the S outlet of Crowley lake, where earthquakes clustered at a depth of 10 km. Both areas previously had low seismicity. After September seismicity gradually slowed through the end of the year.

Mammoth Mountain continued to produce small (M <2) earthquakes in the upper 10 km of the crust whereas long-period events took place at depths of 10-30 km beneath the SW flank. These long-period earthquakes continued at the steady rate of 20-25 events/year; their epicenters were distributed along a belt extending S of Mammoth Mountain well into the Sierra Nevada block.

The resurgent dome continued to inflate at a strain rate of 2-3 ppm/year, a value that corresponds to an uplift rate of 2-3 cm/year based on past comparisons with results from leveling data. This rate may be gradually slowing with time, as suggested by a number of geodimeter baselines. In mid-1995, most baselines showed a brief pause in extension followed by a period of increased extension rate. The timing of this pause with respect to the onset of the seismicity surge in the Sierra Nevada to the S is intriguing. Similar, but less pronounced variations in extension rate occurred at fairly regular intervals since 1991 (BGVN 19:04 and 20:03). No systematic relation to seismicity variations either within the caldera or the Sierra Nevada block were ever recorded.

The areas of dead pine trees on the flanks of Mammoth Mountain expanded during 1995 and new areas formed in the vicinity of Reds Creek on the W flank and on the N flank above the main ski lodge. In all of these areas, high concentrations of CO₂ and small amounts of helium were measured. In general the soil-gas He/CO₂ ratio was similar to that in the fumarole just S of the Chair 3 lift on the E flank.

Earthquake swarm, March-April 1996. A M 3.9 earthquake on 29 March 1996 triggered an earthquake swarm in the S moat of Long Valley crater. By 2 April more than 1,000 aftershocks (M > 0.5) were detected and located; 18 of these events had M > 3, and the two largest events reached M 4. The highest rate, up to 40 events/hour, was recorded during the night of 30-31 March. The decline in activity was accompanied by four M > 3.3 earthquakes during the rest of 31 March. Epicenters were clustered 10-11 km ESE of Mammoth Lakes at depths of 7-11 km. No ground deformation was associated with this swarm.

References. Hill, David P., 1996, Long Valley Caldera Monitoring Report (October-December 1995): U.S. Geological Survey, Office of Earthquakes, Volcanoes, and Engineering, 31 p.

Information Contacts: David Hill, U.S. Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025 (URL: https://volcanoes.usgs.gov/observatories/calvo/).

June 1996 (BGVN 21:06) Cite this Report

Two S-moat earthquake swarms during June

Moderate earthquake swarms on 14 and 19-21 June triggered a "D" alert status (moderate unrest) for the caldera. The D-status for the second swarm expired at 0600 on 28 June. The activity associated with these swarms is common in the caldera and poses little or no threat. Focal depths of both swarms centered around 5-6 km, and there was no significant ground deformation.

The first swarm, which began on the morning of 14 June, was centered at the SW margin of the resurgent dome (near the Highway 203/395 junction 5 km E of Mammoth Lakes). It included 17 M > 2 earthquakes and more than 150 events large enough to be located (M >0.5). The two largest events in this swarm were M 3.0 and 3.3 earthquakes at 0943 and 2154, respectively. This swarm gradually died out early the next morning.

The second swarm, centered 5 km to the E along the SE margin of the resurgent dome, began late in the evening of 18 June and gradually tailed off through the early morning of 21 June. This swarm included a M 3.3 earthquake at 0513 on 19 June, and over 33 events of M >2. The number of M >0.5 events detected and located through the morning of 21 June exceeded 400. This swarm was centered 2-3 km N of the 29 March-10 April S-moat swarm, which included three M > 4 earthquakes (BGVN 21:04).

This 17 x 32 km caldera formed about 730,000 years ago as a result of the Bishop Tuff eruption. Resurgent doming was followed by eruptions until ~50,000 years ago. Since then the caldera has remained thermally active, and in recent years, has undergone significant deformation. Although distinct from Long Valley Caldera, both Inyo Craters and Mammoth Mountain sit adjacent to it.

Information Contacts: David Hill, U.S. Geological Survey, MS 977, 345 Middlefield Rd., Menlo Park, CA 94025 USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

November 1997 (BGVN 22:11) Cite this Report

Summary of 1996 activity

This report summarizes 1996 activity (Hill, 1996). More recent activity will be presented in subsequent reports.

During early 1996, a series of small earthquake swarms occurred in the S moat of the caldera between Convict Lake moraine and the SE margin of the resurgent dome. Swarm activity in the area gradually increased in intensity during February-March 1996, culminating with an earthquake swarm during 29 March-10 April, the most energetic in the caldera since January 1983 (SEAN 07:12); the swarm included 24 earthquakes of M 3 or greater. On 30 March two M 4.0 events occurred; on 1 April there was a M 4.1 event, the largest in the sequence. Altogether the swarm included over 1,600 locatable earthquakes (M >0.5) and had a cumulative seismic moment of ~5 x 1022 dyne-cm, the equivalent of a single M 4.8 earthquake. Instruments showed no unusual ground deformation associated with the swarm.

Earthquake activity within the caldera gradually slowed following the 29 March-10 April swarm through the remainder of April and May. Activity increased again in June with four bursts of seismicity at 5-day intervals during 9-25 June. Swarms on 9, 14, and 25 June were located near the SW margin of the resurgent dome (figure 19), near the junction of Highways 203 and 395; the swarm on 19-20 June was located at the SE margin of the resurgent dome (~2 km N of the airport). The largest earthquakes in these swarms were a M 2.6 event on 9 June, M 3.2 and M 3.5 events on 14 June, and a M 3.3 event on 19 June. The long-base tiltmeter, centered 1 km SE of the 19-20 June swarm, showed a 0.3 µrad tilt down to the NW coincident with that swarm.

Small earthquake swarms on 30 July, and 7 and 9 August, were the last to occur within the caldera for the remainder of 1996; all were located near the SW margin of the resurgent dome. The caldera was relatively quiet during the last half of 1996 (figure 20), producing only occasional small earthquakes, all less than M 3.

Occasional long-period volcanic earthquakes continued to occur during 1996 at depths of 10-20 km beneath the Devils Postpile area SW of Mammoth Mountain. These events have become more frequent since their 1989 onset during a swarm beneath Mammoth Mountain (SEAN14:06).  Minor volcano- tectonic earthquake activity in the shallow crust (<10 km depth) beneath Mammoth Mountain showed no significant change in rate or spatial distribution since 1989.

Long-term uplift and extensional deformation of the resurgent dome gradually slowed through the last half of 1996; this was defined by 2-color geodimeter measurements. The decrease in the resurgent dome's deformation rate and intra-caldera earthquake activity during the last half of 1996 was similar to the relative seismic quiescence and low deformation rates during 1984 to mid-1989. Continuous deformation monitoring showed no significant changes during 1996, with the exception of the 0.3 µrad tilt accompanying the 19-20 June earthquake swarm.

Dominant variations in carbon dioxide soil-gas concentrations in the tree-kill areas around Mammoth Mountain reflected the blanketing effect of snow during the winter months. Continuous CO₂ monitors at Horseshoe Lake showed increased concentrations from early February through the end of April. Concentrations gradually returned to minimum values by mid-summer. The areas showing evidence of high CO₂ soil-gas concentrations around the flank of Mammoth Mountain changed relatively slowly since 1991. In the late summer of 1995, there were seven areas of CO₂-induced tree-kill scattered around the S, W, and N flanks of the mountain covering ~150 acres. A series of small collapse pits extending from the S-most tree-kill area at Horseshoe Lake merged with a crack in the bottom of Horseshoe Lake that was first detected in late September. Whether this system of shallow fractures is related to the anomalously high CO₂ soil-gas concentration in the adjacent Horseshoe Lake tree-kill area has not been determined; however, the fracture system explained Horseshoe Lake's tendency to drain internally. A survey around Horseshoe Lake was planned in order to determine if the fracture system was associated with local deformation.

The 17 x 32 km Long Valley caldera lies E of the central Sierra Nevada, ~320 km E of San Francisco. The caldera formed ~730,000 years ago as a result of the Bishop Tuff eruption. Resurgent doming was followed by eruptions of rhyolite from the caldera moat and rhyodacite from the outer ring-fracture vents until ~50,000 years ago. Since then the caldera has remained thermally active, and in recent years has undergone significant deformation. Although distinct from Long Valley Caldera, both Inyo Craters and Mammoth Mountain are adjacent to it.

Reference. Hill, David P., 1996, Long Valley Caldera monitoring report (October- December 1996): U.S. Geological Survey, Volcano Hazards Program.

Information Contacts: David Hill, U.S. Geological Survey, MS 977, 345 Middlefield Rd., Menlo Park, CA 94025 USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

December 1997 (BGVN 22:12) Cite this Report

Seismic quiescence continues during January-June 1997

The following summarizes USGS quarterly Long Valley Caldera Monitoring Reports from January-June 1997. Activity during 1996 was summarized in BGVN 22:11. During December 1997 an episode of increased deformation occurred at Long Valley; information on this event will be included in a future Bulletin.

Seismicity. Earthquake activity within the caldera remained low during the first three months of 1997; activity included occasional small (M <3) events in the S half of the caldera but no swarms. A M 2.9 event on 22 February at the resurgent dome's SE margin was the largest during January-March 1997.

Outside the caldera, two M 4 earthquakes S of Convict Lake (4 km S of the caldera) attracted considerable media attention due to their timing with respect to the release of disaster movies featuring erupting volcanoes. The first, a M 4.2 earthquake on 10 February, occurred three days after the opening of "Dante's Peak," a movie depicting a destructive eruption in the Cascade Range. The second, a M 4.1 earthquake on 24 February, occurred the day after the television movie "Volcano: Fire on the Mountain" aired about a fictitious volcano at a ski resort in California. The level of earthquake activity in the Convict Lake vicinity showed a modest increase after 10 February but gradually slowed through mid-March. All events were M <3 except for three aftershocks of the 22 February earthquake.

Although the frequency of small earthquakes increased in early May, seismicity remained low during April- June. On 1 April, a cluster of earthquakes near Laurel Springs along the caldera's S margin (8 km ESE of Mammoth Lakes) included M 2.7 and 3.3 events. The largest event within the caldera during April-June was M 2.9 on 5 May inside the S margin ~2 km NE of Convict Lake. A small cluster of earthquakes including M 2.5 and 2.6 events occurred E of the geothermal plant in the S section of the dome on 13-14 May. Low-level seismicity beneath Mammoth Mountain included M 2.2 and 2.0 events on 27 May.

Deformation. Two-color geodimeter data indicated that extension across the resurgent dome continued at a reduced rate during January-June 1997 (~1 cm/year compared to 2 cm/year during much of 1991 through mid-1996). The reduced deformation rate generally coincided with the decrease in earthquake activity since mid-1996. Differential magnetometer data showed a similar slowing. The most striking signal from continuous deformation instruments (borehole dilatometers or tiltmeters) was a ~2 microstrain compressional step on a dilatometer associated with massive runoff and flooding after a 1-3 January storm.

CO₂ beneath Mammoth Mountain. During January-March 1997, continuous CO₂ monitoring sites in the Horseshoe Lake tree-kill area reflected the annual buildup in soil gas due to the blanketing effect of snow. During April-June, no new observations were made.

Regional activity. The only M 3 earthquake in the Sierra Nevada block (other than on 10 and 24 February) during January-March was a M 3.3 event beneath Red Slate Mountain (17 km SSE of Mammoth Lakes) on 18 March. Minor earthquake activity near Tungson Hill (10 km W of Bishop) included a M 3.0 event on 11 March and a M 3.2 event on 25 March. During April-June, the Sierra Nevada block produced occasional M ~3 and smaller earthquakes. A cluster of earthquakes on 10-12 April included two M 3 events beneath Laurel Canyon (~8 km SE of Mammoth Lakes). On 25 May, a M 3.3 earthquake occurred beneath the N flank of Red Slate Mountain. On 26 June, a M 3.0 event occurred near Rock Creek Lake (24 km SE of Mammoth Lakes).

Notification and response terminology. Because the system of public notification of activity used at Long Valley since 1991 was often misinterpreted, new terminology to describe conditions was adopted on 12 June 1997. The old system assigned letters to the levels of volcanic unrest; however, the alphabetic terms were not meaningful to the public and were often reported as "alerts" or "watches" that overstated the level of unrest. The new terminology uses color codes and more descriptive phrases. In this system, condition green indicates no immediate threat and typical behavior; condition yellow indicates a watch due to intense unrest, for example earthquake swarms including events of M 5 or greater; condition orange indicates that an eruption is likely and evidence of magma movement at shallow depth; and condition red indicates an eruption underway.

The 17 x 32 km Long Valley caldera lies E of the central Sierra Nevada, ~320 km E of San Francisco. The caldera formed ~760,000 years ago as a result of the Bishop Tuff eruption. Resurgent doming was followed by eruptions of rhyolite from the caldera moat and rhyodacite from the outer ring-fracture vents until ~50,000 years ago. Since then the caldera has remained thermally active, and in recent years has undergone significant deformation. Since 1980, typical behavior at Long Valley has included as a many as 10-20 earthquakes/day of M <2, occasional small to moderate earthquake swarms, and steady uplift of the resurgent dome at a rate of ~2-3 cm/year. Although distinct from Long Valley Caldera, the N-S trending Inyo Craters volcanic chain partially extends into the caldera.

Reference. Hill, David P., 1997, Long Valley Caldera Monitoring Report (January-March and April-June 1997): U.S. Geological Survey, Volcano Hazards Program.

Information Contacts: David Hill, U.S. Geological Survey, MS 977, 345 Middlefield Rd., Menlo Park, CA 94025 USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

June 1999 (BGVN 24:06) Cite this Report

Continued dome inflation and persistent earthquake swarms through 1998

The following summarizes activity at Long Valley during the second half of 1997 and all of 1998 (Hill, 1998). A summary of activity during 1996 and the first half of 1997 can be found in BGVN 22:11 and 22:12.

Summary of activity during July-December, 1997.After nearly a year of relative seismic quiescence within the caldera, earthquake swarm activity returned in early July. The deeper focus (>10 km) , long-period (LP) earthquakes centered beneath the SW flank of Mammoth Mountain and the Devil's Postpile area, which had increased in early 1997, continued at an elevated rate through the end of the year. Altogether over 200 of these events were detected during 1997, all M <2.0 (figure 21). This exceeded the total number of deep LP events detected from their onset in 1989 through 1996. Earthquake activity in the shallow crust beneath Mammoth Mountain (depths <10 km) at the SW margin of the caldera remained low throughout 1997.

The gradual increase in the inflation rate of the resurgent dome begun in late May 1997 marked the onset of an episode of unrest in the caldera that intensified at an accelerating rate through the summer and fall, culminating in a series of strong earthquake swarms from mid-November through early January 1998. The extension rate of an 8-km baseline spanning the resurgent dome peaked at over 20 cm/year during the second week of November. Following a strong earthquake swarm on 22 November that included three M >4.5 earthquakes, the deformation pattern briefly changed to one dominated by right-oblique slip along the WNW-striking "S-moat fault zone." In early December and through the rest of 1997, the deformation resumed the earlier pattern of dome inflation with a relatively steady 12-15 cm/year extension rate. By the end of 1997, the 8-km baseline was 7 cm longer than in late May.

The earthquake activity associated with the swarms begun in early July developed over a broad 15-km zone spanning the S-moat and southern margin of the resurgent dome. Typically, several areas within this zone were active simultaneously. This swarm activity, which included more than 12,000 M >1.2 and 120 M >3.0 earthquakes over a 7-month period through mid-January 1998, had a cumulative seismic moment of 3.3 x 10²4 dyne-cm, equivalent to a single earthquake of M 5.4. The peak in seismicity from mid-November through early January included eight M >4.0 earthquakes. Focal mechanism data for the larger earthquakes indicated a dominantly right-lateral slip along a WNW-trending fault zone within the S-moat, although broadband seismograms admit the possibility of a dilatational component (volume increase) in the source mechanism for some of the M >4 earthquakes. The great majority of earthquakes had the broadband character of brittle, double-couple events (tectonic or volcano-tectonic earthquakes). A few shallow (<3 km) events beneath the southern half of the resurgent dome had energy concentrated in the 1-3 Hz band typical of shallow LP earthquakes. Whether the unusual appearance of the seismograms can be attributed to the earthquake source or wave propagation effects remains to be determined.

Monitoring of the gases around Mammoth Mountain showed two noteworthy changes in 1997, both more likely related to the increased deep LP earthquake activity beneath the SW flank of the mountain than to the much stronger activity within the caldera. The helium isotope ratio, ³He/4He, in samples collected in May and October from the MMF fumarole on the NE flank of Mammoth Mountain showed an increase with respect to the gradually declining values measured in late 1995 through early 1997. Continuous CO₂ monitors in tree-kill areas on opposite sides of Mammoth Mountain detected an abrupt increase in CO₂ soil-gas concentrations beginning in late September and ending in early December before significant snow accumulations.

The episode of persistent unrest during the second half of 1997 is the third most energetic activity in the caldera since the intense earthquake swarms of May 1980 (which included four M 6 earthquakes) and January 1983 (which included two M 5.3 earthquakes). The strongest seismic activity in the 1980 and 1983 episodes occurred within the first few days of the swarm sequences. The 1997 activity level accelerated gradually over a 4-month period prior to the strongest seismicity, which then spread out over nearly an additional 3 months. This activity provided the first test of the color-code notification system for ranking activity levels within the caldera. "Condition Green" (no immediate risk) remained throughout the year but the activity peaks on 22 and 30 November came close to meeting the guidelines for "Condition Yellow" (watch).

Summary of activity during 1998. Three events dominated activity during 1998. First, a decline in the acceleration of the resurgent dome uplift and the persistent earthquake swarm activity in the S-moat, which began in June-July 1997 and peaked in November-December 1998. Second and third, M 5.1 earthquakes on 8 June and 14 July 1998, centered W of the Hilton Creek fault and 2-4 km S of the caldera, together with their aftershock sequences.

In the S-moat of the caldera, the final phase of the strong earthquake swarms that dominated activity through the second half of 1997 extended into January 1998. An earthquake (M 4.8) on 31 December marked a shift in the center of the most intense activity from beneath the western part to the eastern-central section. A strong swarm burst occurred during 1-5 January in which >2,000 earthquakes were detected and located, including more than a dozen with M >3. On 6 January, a M 4.1 earthquake was the last of nine earthquakes with magnitudes of 4.0-4.9 in the S-moat since 13 November 1997.

Activity in the S-moat area declined to background levels by midsummer, interrupted by occasional M >3 earthquakes and brief swarms, particularly in February and March. The latter period included several days with 80-180 small swarm events. A M 3.2 earthquake occurred on 13 February, and swarm events on 2, 6-7, and 15-16 March included earthquakes of M >2.5. The last six months of 1998 included five M >3.0 caldera earthquakes. The two largest, on 14 July and 7 December, had magnitudes of 3.7 (the 14 July event occurred 2 hours after the M 5.1 earthquake).

Geodimeter measurements confirmed that the inflation rate of the resurgent dome gradually slowed from a peak of 30 cm/year in November 1997 to ~15 cm/year in early 1998 and by mid-May had dropped to 1-2 cm/year, a rate that persisted through the end of 1998.

Long-period earthquake activity 10-25 km beneath Devil's Postpile and the SW flank of Mammoth Mountain continued through 1998 (figure 22). Some 140 of the LP events were detected in 1998, just over half the 1997 number (250 events). That rate is still higher than any time since the mid-1989 onset of LP activity; during 1989-96 a total of only 165 LP events were detected. In 1998, many of the LP earthquakes were preceded by several tens of seconds of a tremor-like signal with a dominant frequency around 1 Hz. This was a change from the character of the LP activity in earlier years, when the tremor-like signals were generally of shorter duration and the dominant frequency was 2-3 Hz.

An event of M 1.8 at a depth of 12 km on 11 November was the largest LP event recorded since the initial activity in 1989; it was followed by a week of ~25 smaller events. Occasional LP earthquakes continued to occur through 1998 at depths between 15 and 30 km centered beneath an area roughly 7 km W of Mono Craters. One of the largest in this area, M ~2.4, occurred at a depth of 24 km on 26 September.

Over the summer months, field studies around the flanks of Mammoth Mountain suggested that the CO₂ flux rate had been slowly decreasing over the past 3 years. Airborne measurements in September and November detected a CO₂ plume downwind, consistent with the multiple sources around the flanks of the mountain.

The first of the two M 5.1 earthquakes occurred on 8 June just 1.5 km S of the caldera at a depth of 6.7 km. The second occurred on 14 July and was centered 3 km to the SSE at 6.2 km depth. Both earthquakes were located within the footwall of the E-dipping Hilton Creek fault; neither appeared to have involved slip on the fault itself. Both earthquakes were followed by rich aftershock sequences that tailed off though the end of the year and, together, included nine earthquakes with magnitudes between 4.0 and 4.5. The aftershock epicenters defined a nearly orthogonal pattern in map view. Those of the 8 June event were confined to a WNW lineation through the mainshock epicenter, whereas those of the 14 July event were confined to a more diffuse SSW lineation through the mainshock epicenter. Both lineations cut across the Hilton Creek fault and intersect just E of the 8 June epicenter.

Earthquake activity elsewhere in the region showed no significant variation from background activity over the past several years.

References. Hill, David P., 1997, Long Valley Caldera monitoring report (July-December 1997): U.S. Geological Survey, Volcano Hazards Program.

Hill, David P., 1998, Long Valley Caldera monitoring report (October- December 1998): U.S. Geological Survey, Volcano Hazards Program.

Information Contacts: David Hill, U.S. Geological Survey, MS 977, 345 Middlefield Rd., Menlo Park, CA 94025 USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

July 2001 (BGVN 26:07) Cite this Report

Decreased seismicity during 1999-2000

The following summarizes activity at Long Valley during 1999 (Hill, 1999) and 2000 (Hill, 2000). Summaries of activity during 1996, 1997 and 1998 can be found in BGVN 22:11, 22:12, and 24:06.

Summary of activity during 1999. The lowest level of activity within Long Valley since the onset of unrest in 1979-80 occurred in 1999. Earthquake activity and ground deformation were subdued throughout the year. The two largest earthquakes within the caldera were M 2.9 and 3.1 events that occurred on 1 January beneath the S margin of the caldera (5 km ESE of Mammoth Lakes), and on 27 March beneath the S margin of the resurgent dome (9 km E of Mammoth Lakes), respectively. On 24-25 February, a swarm of ~42 small earthquakes was centered just outside the caldera 1-2 km E of Lake Mary (5 km WSW of Mammoth Lakes); the largest in this sequence were M 3.2 and 2.9 events.

Two aspects of caldera seismicity during 1999 were noteworthy. One was the abrupt decrease in seismicity rate within the caldera on 15 May coincident with a M 5.6 earthquake S of the caldera in the Sierra Nevada. The second was a brief swarm of small earthquakes beneath the N flank of Mammoth Mountain within the hour following the M 7.1 Hector Mine earthquake of 16 October, the epicenter of which was in the Mojave Desert ~430 km SE of the caldera. This latter set of events appear to be a subtle example of remote triggering similar to events in the Long Valley caldera and elsewhere following the M 7.3 Landers earthquake in June 1992. Aside from a transient response to the 16 October earthquake, deformation within the caldera remained stationary through 1999.

The rate of deep long-period (LP) "volcanic" earthquake activity beneath the W flank of Mammoth Mountain tapered off following the elevated rate that persisted through the end of 1998. Deep LP earthquakes in 1999 included ~30 events, compared to an average of ~200 events/year during 1997-98. Initial results from the analysis of data collected during a 1997 seismic experiment indicates that these LP events occurred within a N-striking planar distribution that dips steeply (roughly 80°) W at depths of 10-20 km.

Carbon dioxide (CO₂) soil-gas concentrations measured at fixed depths in the Horseshoe Lake (HSL) tree-kill area continued to show annual variation with snow depth and occasional fluctuations during the snow-free months. The only notable fluctuation in CO₂ concentrations during 1999 involved a three-week increase at the SKI monitoring site (near Chair 19 in the Mammoth Mountain Ski Area) that began four days after the 15 May earthquake; whether these two events are related is unclear. With respect to the cold CO₂ emissions from the soils, radioactive carbon measurements on cores from trees at the margin of the HSL tree-kill area indicated that the CO₂ discharge in that area has been relatively constant since about 1995. Analyses of helium isotopic composition on the N side of Mammoth Mountain showed that the trend of decreasing ³He/4He at the MMF steam vent since 1997 was interrupted by a rise in May 1999 following a period of increased LP activity in the fall of 1998.

Summary of activity during 2000. Continuing the trend set in 1999, activity levels in the Long Valley caldera and vicinity remained low throughout 2000 (figure 23). Low-level earthquake activity within the caldera was scattered beneath the S moat, the S and E margins of the resurgent dome, and Mammoth Mountain. The largest of these intra-caldera earthquakes was a M 2.3 event that occurred as part of a cluster of half a dozen small earthquakes beneath Mammoth Mountain on 27 April. Activity in the Sierra Nevada immediately S of the caldera was largely concentrated in the aftershock zone of the 8 June 1998, 14 July 1998, and 15 May 1999 earthquakes. The largest earthquake of the year in the region was a M 3.8 earthquake on 20 January located in the Sierra Nevada midway between Convict Lake and Mt. Morrison.

The rate of deep LP earthquakes beneath the W flank of Mammoth Mountain, which began in 1989-90, accelerated significantly in 1997 through early 1998, tapered off in early 1999, and increased again in mid-2000 (figure 24). The increased rate began with a burst of some 15 events in July and included several additional bursts of 5-10 events each in December. Altogether, about 50 deep LP earthquakes were recorded at depths of 10-25 km beneath Mammoth Mountain during 2000.

Two very-long-period (VLP) earthquakes were detected with hypocenters roughly 4 km beneath the summit of Mammoth Mountain; one on 6 July (0356 UTC) and the other on 13 August (0007 UTC). These two events, together with a similar event on 12 October 1996, are the only VLP earthquakes that have been detected beneath Mammoth Mountain since instrumental capability for detecting seismic events in this frequency band was acquired sometime in the 1990's. The fact that both the 6 July and 13 August VLP events were accompanied by spasmodic bursts of brittle failure earthquakes, opens the possibility that the 1989 Mammoth Mountain earthquake swarm, which included multiple episodes of spasmodic bursts, may have also included significant VLP activity. These Mammoth Mountain VLP events are similar to those beneath Kīlauea, which Bernard Chouet and colleagues interpret as the result of small slugs of magma, magmatic brine, or magmatic gas moving through a crack-like restriction. At this low rate, these VLP events do not indicate impending volcanic activity.

No significant deformation episodes were recorded during 2000. The two-color EDM data show small fluctuations about a slight contraction (subsidence) of the resurgent dome of 0.5-1.0 cm for the year. The center of the resurgent dome remains roughly 80 cm higher than in the late 1970's prior to the last two decades of caldera unrest. In contrast to Yellowstone and Campi Flegrei calderas, which showed pronounced uplift through the early 1980's followed by partial subsidence, Long Valley caldera has yet to show any significant subsidence. Rabaul, the other large caldera with well-documented deformation over the last couple of decades, showed sustained uplift at varying rates through the 1980's and early 1990's with no evidence of subsidence until the onset of eruptive activity in September 1994.

Hydrological monitoring in the caldera revealed no significant changes in water wells or stream flow that might be attributable to caldera unrest. Short-term CO₂-flux variations during the snow-free months in the HSL tree-kill area appeared to be primarily related to local meteorological conditions. These measurements also show that the total CO₂ flux has remained relatively steady over the past several years with no indication of a systematic decline with time. Soil-gas CO₂ measured at fixed depths in the HSL tree-kill area continue to show an annual variation with snow depth and occasional temporary fluctuations during the snow-free months. The only notable fluctuation in CO₂ concentrations during 2000 occurred at the Laurel Springs station (LSP), which showed a spike in late April and a number of spikes from mid-June through September. The process leading to these spikes remains to be determined. At this point, however, these spikes do not represent a hazard of the sort associated with the sustained high CO₂ flux in the Mammoth Mountain tree-kill areas.

References: Hill, David P., 1999, Review of Long Valley Caldera activity for 1999: Long Valley Observatory, U.S. Geological Survey.

Hill, David P., 2000, Long Valley Observatory quarterly report October-December 2000 and annual summary for 2000: Long Valley Observatory, Volcano Hazards Program, U.S. Geological Survey.

Information Contacts: David Hill, Long Valley Observatory, U.S. Geological Survey, Volcano Hazards Program, MS 910, 345 Middlefield Rd., Menlo Park, CA 94025 USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

March 2003 (BGVN 28:03) Cite this Report

Summary of 2001-2002 activity; renewed inflation of the resurgent dome

The following are summaries from the U.S. Geological Survey (USGS) of activity at Long Valley during 2001 (Hill, 2001) and 2002 (Hill, 2002). Summaries of activity during 1996, 1997, and 1998 are found in BGVN 22:11-22:12 and 24:06; activities during 1999 through 2000 are found in BGVN 26:07. Figure 25 shows some of the locations mentioned in this report.

Summary of activity during 2001. Activity levels in Long Valley caldera and vicinity were incrementally lower in 2001 than in 2000, thus continuing the trend of extended quiescence that began toward the end of 1999. Low-level seismic activity within the caldera typically included five or fewer earthquakes per day large enough to be located by the online computer system. Most were smaller than M 2.0, and none were as large as M 3.0; the largest was a M 2.8 earthquake beneath the southern margin of the caldera 800 m N of Convict Lake on 21 May. Seismic activity in the Sierra Nevada S of the caldera continued to be concentrated within the aftershock zone of the 1998-99 sequence of three M 5 earthquakes. The 2001 activity (figure 26) included eight earthquakes of M 3.0 or larger. The largest was the M 3.4 earthquake of 2 December located near the epicenter of the M 5.6 earthquake of 15 May 1999.

Mid-crustal long-period (LP) volcanic earthquakes continued to occur at depths of 10-25 km beneath the W flank of Mammoth Mountain (figure 27), although at a much reduced rate compared with the peak in activity in 1997-98. Some 60 LP earthquakes were detected during 2001, with over 15 of these occurring in a cluster on 10 February.

Deformation within the caldera was limited to continuing slow subsidence of the resurgent dome at a rate of roughly 1 cm/year. All together, the center of the resurgent dome has lost some 2 cm in elevation since inflation stopped in late 1998, leaving the center of the resurgent dome roughly 75 cm or so higher at the end of 2001 than in the late 1970's. The continuous strain and deformation monitoring networks detected no short-term deformation transients during the year. The same is true for the magnetometer networks.

The diffuse carbon dioxide (CO₂) degassing at the Horseshoe Lake tree-kill area (BGVN 22:11) and other sites around the flanks of Mammoth Mountain has shown no significant change over the past several years. The total CO₂ flux continued to fluctuate ~200 tons per day, with the Horseshoe Lake area contributing roughly 90 tons per day.

The lull in caldera unrest over the past couple of years has provided the Long Valley Observatory (LVO) an opportunity to look back over the wealth of data collected during the previous two decades of activity and to investigate the nature and significance of the processes driving the unrest, toward the goal of assessing future unrest episodes and their significance in terms of potential volcanic hazards. Data from the intense unrest during the 1997-98 episode in the S moat, for example, indicate that fluids (magmatic brine or perhaps magma) played a central role in this activity. This underscores the value of a closely integrating the seismic, deformation, and hydrologic monitoring efforts.

Summary of activity during 2002. Activity in 2002 was dominated by the onset of renewed inflation of the resurgent dome following nearly three years of gradual subsidence. Earthquake activity within the caldera, which remained low through the first half of the year, showed a slight increase through the second half. Of particular note was the response of the caldera to the shear and surface waves generated by the M 7.9 Denali Fault earthquake of 3 November 2002 in the form of a burst of some 60 small earthquakes beneath the S flank of Mammoth Mountain, a coincident strain transient consistent with aseismic slip on a normal fault beneath the E flank of the mountain, and an earthquake swarm the following day in the S moat that included the first M 3.0 earthquake since 1999. This is the third time Long Valley has shown a well-documented response to large, distant earthquakes, the first two being with the M 7.4 Landers earthquake of 28 June 1992 and the M 7.2 Hector Mine earthquake of 16 October 1999. No other significant changes occurred within the caldera during the year. Both the carbon dioxide flux from the flanks of Mammoth Mountain and the rate of deep long-period (LP) volcanic earthquakes beneath Mammoth Mountain showed little change from previous years. The LVO detected no very-long-period (VLP) earthquakes during 2002.

Beginning around the first of the year, both the 2-color EDM and continuous GPS data for the baselines radiating from the CASA monument turned from gradual contraction to renewed extension that persisted through the year at rate of 2.5-3.0 cm/year. This rate is comparable to extension rates that prevailed through the mid-1990's. Cumulative uplift of the center of the resurgent dome associated with this extension has returned to its 1999 value of roughly 80 cm with respect to the late 1970's.

Earthquake activity within the caldera remained low through the first half of the year averaging fewer than five earthquakes per day, most with M 2.0 (figures 27 and 28). The largest event within the caldera during this period was a M 2.8 earthquake on 15 March located in the W lobe of the S moat seismic zone, 1.6 km S of the 203-395 Highway junction. Activity increased slightly in mid-June beginning with a cluster of small earthquakes beneath the W flank of Mammoth Mountain on 26 June that included four events of about M 2. A number of small (M 2) events with the appearance of LP earthquakes occurred at shallow depths (less than 2 km) beneath the southern section of the resurgent dome during the last half of August.

The most notable activity began with a burst of over 60 small earthquakes of M 1 beneath the S flank of Mammoth Mountain as the surface waves generated by the M 7.9 Denali Fault, Alaska, earthquake of 3 November 2002 passed through just 17 minutes after the mainshock rupture. At the same time, the borehole dilatometers detected a 0.1-microstrain strain transient that is consistent with slow (aseismic) slip on a normal fault at a depth of about 7 km beneath the W flank of Mammoth Mountain. As with the caldera activity remotely triggered by the M 7.4 Landers earthquake of 28 June 1992 and the M 7.2 Hector Mine earthquake of 16 October 1999, this strain transient is much larger than can be explained by cumulative slip for the 60 or so earthquakes of M 1 triggered by the Denali Fault earthquake. The following day, 4 November, the largest earthquake swarm in the S moat of the caldera since 1998 developed as a sequence that included six earthquakes of M 2 and one of M 3.0. This S-moat swarm was unusual in that it occurred in a relatively aseismic section of the S moat, focal depths of the swarm earthquakes were unusually shallow (4 km), and the NNW lineations of the swarm epicenters cuts across the prevailing WNW-trend of the usual S-moat swarm activity. The latter was also true for the swarm activity triggered by the M 7.4 Landers earthquake of 1992. This S-moat earthquake swarm was not accompanied by detectable strain changes. Mid-crustal long-period (LP) earthquakes have continued at depths of 10-25 km beneath Mammoth Mountain at a fairly steady rate over the past three years. Occasional bursts of activity included 12-15 events per week.

Diffuse emission of carbon dioxide from the flanks of Mammoth Mountain showed little change from previous years. Emission rates estimated for the Horseshoe Lake tree-kill area continued to fluctuate between 50 and 150 tons of CO₂ per day, with an average flux of 100 tons per day since 1995. The Horseshoe Lake area produced roughly one-third of the total CO₂ flux from the flanks of Mammoth Mountain.

Values for the helium isotope ratio ³He/ 4He from samples taken in early and mid-2002 from the Mammoth Mountain Fumarole (MMF), located at 3,000 m elevation some 300 m E of the Chair 3 ski lift, averaged 5.5, or essentially the same as the 2001 values. These values are significantly higher than the 1999 value of 3.0. The increase with respect to 1999 is consistent with an increase in the magmatic component in the gas emissions from the fumarole. Whether the elevated values for 2001-2002 are related to the very-long-period (VLP) volcanic earthquakes that occurred at a depth of 3 km beneath the summit of Mammoth Mountain in July and August of 2000 remains to be seen.

Seismic activity in the region surrounding Long Valley caldera continued to be dominated by earthquakes in the SSW-trending aftershock zone of the June and July 1998 and the May 1999 earthquakes in the Sierra Nevada S of the caldera. Activity within this aftershock zone included a cluster of earthquakes near the southern end of the zone centered just E of Grinnell Lake that began on 6 June and persisted through the end of the month. Elsewhere, a M 3.7 earthquake on 15 July just 3.2 km NNW of Bishop produced felt shaking throughout the Bishop area. Earthquakes of M 2.9 and 3.5 on 12 December were located beneath the Volcanic Tableland 19 km NNW of Bishop.

An updated revision of the USGS Response Plan for Volcanic Unrest in the Long Valley Caldera - Mono Craters Region, California was released in March 2002 as USGS Bulletin 2185. This bulletin is available in print and in electronic form at ttp://geopubs.wr.usgs.gov/bulletin/b2185/.

References. Hill, D.P., 2001, Long Valley Observatory quarterly report October-December 2001 and annual summary for 2001: Long Valley Observatory, U.S. Geological Survey, Menlo Park, CA (URL: http://lvo.wr.usgs.gov/Annual/lvc_01.html).

Hill, D.P., 2002, Long Valley Observatory quarterly report July-September and October-December 2002 and annual summary for 2002: Long Valley Observatory, U.S. Geological Survey, Menlo Park, CA (URL: http://lvo.wr.usgs.gov/Quarterly/qrt_rpt3-4-02.html).

Information Contacts: David Hill, Long Valley Observatory, Volcano Hazards Program, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025, USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

May 2004 (BGVN 29:05) Cite this Report

Summary of report noting nearly 5 years of relative quiescence

The following is a summary of Hill (2004) and Sorey, Hill, and McConnell (2000), reports that collectively concluded that with the close of 2003, Long Valley Caldera had sustained nearly five years of relative quiescence. This marked the longest such interval since the onset of unrest in 1978. A summary of 2001-2002 activity was published in March 2003 (BGVN 28:03).

The slow inflation of the resurgent dome at a rate of ~ 1 cm/year that persisted through most of 2002 leveled off in early 2003 with essentially no change through the end of the year. At the end of 2003, the center of the resurgent dome stood only about 0.5 cm higher than in early 1999. It remained roughly 80 cm higher than in the late 1970s.

Seismic activity within the caldera remained low through 2003 as it has for the previous four years, averaging fewer than five earthquakes per day large enough to be located by the realtime computer system (M 0.5 and above). As in the past, most of these earthquakes were confined to the S moat and the S margin of the resurgent dome. The largest intra-caldera earthquake during the year was a M 2.4 event on 19 September 2003 at 0751, associated with a cluster of smaller events in the S moat beneath the E margin of Mammoth Lakes. An earthquake sequence of comparable intensity was centered beneath the SE margin of the resurgent dome on 8 November. This sequence included three M > 2 earthquakes, the largest of which was a M 2.2 earthquake at 2102.

Most of the earthquake activity in the Sierra Nevada block S of the caldera continued to be concentrated in the N-NE lineation of epicenters that represents the aftershock zone of the three M > 5 earthquakes of June and July 1998 and May 1999 (figure 29). A notable exception was the M 4.0 earthquake of 8 March (0735) that was located 1 km S of Laurel Mountain (~5 km S of the caldera boundary and 11 km ESE of Mammoth Lakes). This earthquake was felt in the Mammoth Lakes area and was accompanied by over 50 smaller earthquakes, the largest of which was a M 3.2 event. The Grinnell Lake area near the S end of the seismicity lineation in the Sierra Nevada was one of the more persistently active areas through the year. It produced M 3.2 earthquakes on 15 June and 18 August as well as a host of smaller earthquakes.

Occasional M 3 earthquakes elsewhere in the region included: a M 3.2 earthquake on 23 January 3 km E of Red Slate Mountain (midway along the seismicity lineation in figure 29), a M 3.0 earthquake on 18 March located beneath the Volcanic Tableland 10 km E of Crowley Lake, a M 3.1 earthquake on 31 August located 2 km E of Lake Dorothy in the Sierra Nevada, a M 3.0 earthquake on 26 October located 20 km W of Bishop, and a M 3.5 earthquake on November 10 in Round Valley. Altogether, ten earthquakes of M 3 or greater occurred in the area during 2003, the largest being the M 4.0 event on 8 March near Laurel Mountain. The mid-crustal (10- to 25-km-deep) long period (LP) volcanic earthquakes, which began during the 1989 Mammoth Mountain earthquake swarm, continued beneath the SW margin of Mammoth Mountain but at a much-reduced rate with respect to the activity levels during the first half of 1997. LP activity for 2003 was limited to the first and last quarters of the year with no LP earthquakes detected from April through September.

The carbon dioxide (CO₂) emissions from the tree-kill areas around the flanks of Mammoth Mountain remained similar over the last several years. In particular, data from the CO₂ sensors at Horseshoe Lake were relatively flat and uneventful for 2003 except for the normal winter excursions due to snow accumulation. A soil CO₂ efflux survey of Horseshoe Lake in August gave an emission rate of 135 tons/day, which is slightly higher than the rate for 2002. However, the emission rate trend from 1995 through 2003 based on linear regression was relatively flat at ~100 tons/day, suggesting continued CO₂ emissions. The Horseshoe Lake tree-kill area produces roughly one third of the total CO₂ flux from the flanks of Mammoth Mountain.

Intra-caldera sites contained dead vegetation, elevated soil temperatures, and CO₂ concentrations consistent with ongoing geothermal activity. The areas that produced the greatest CO₂ emissions were in the vicinity of the geothermal plant and have been known for some time. Initially the formation of these areas likely occurred as a result of superficial changes linked to increases in geothermal fluid production in the late 1980s and early 1990s. Some recently identified sites displayed elevated soil temperatures on the resurgent dome above Fumarole Canyon; these may reflect a delayed response to the 1997 earthquake swarm activity in the area. Total CO₂ emissions at these sites are marginally above background levels.

Hydrologic monitoring data show that declining fluid pressures in key monitoring wells over the past several years continued through 2003. Fluid pressures in four of five key monitoring wells during 2003 were at the lowest values since 1995 and for three of these wells the pressures were the lowest since the late 1980s. The data also show a sharp decline in thermal-water discharge from springs in Hot Creek Gorge, an event that began in August 2003 and persisted to the end of 2003. The decline in discharge was ~18% of the long-term mean discharge.

The decline in thermal-water discharge from Hot Creek Gorge springs was consistent with the low fluid pressures recorded in wells CW3 and CH10B, both of which tapped the S-moat hydrothermal system. The reason for this decline was unclear. Geothermal production from the Casa Diablo power plant has not changed significantly over the past year and the caldera has shown no significant unrest.

New instrumentation and an interdisciplinary workshop. During the week of 2 August 2003, a team of scientists and drilling experts from the oil industry successfully installed a 30-m-long geophysical instrument string at a depth ~2.4 km in the Long Valley Exploratory Well (LVEW). The instrument string includes two three-component seismometers (4 Hz natural frequency, one at 2592 m and the other at 2264 m depths), a dilatometer (at 2254 m depth), a 48-m-long vertical-axis optical-fiber strainmeter (centered at 2150 m depth), and pass-through tubes designed to track pore pressure in the open hole beneath the instrument package. As signals from the remaining components of the LVEW deep borehole observatory come on line over the next few months, they will greatly enhance the power of the LVO network as both a monitoring and research tool.

Instrumentation of LVEW as a deep-borehole observatory represents the final stage of a major drilling project that began in the mid-1980s with multi-agency support (Sorey and others, 2000).

A four-day workshop was held 8-12 October 2003. The title was "Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Process in Long Valley Caldera-Mono Craters."

References.Hill, D.P., 2003, Long Valley Observatory quarterly report October-December 2003 and annual summary for 2003: Long Valley Observatory, U.S. Geological Survey, Menlo Park, CA (URL: http://lvo.wr.usgs.gov/Quarterly/qrt_rpt_4-03.htm).

Sorey, M.L., Hill, D.P., and McConnell, V.S., 2000, Scientific drilling in Long Valley Caldera, California—an update, in California Geology, California Geological Survey, v. 53, pp. 4-11, URL: http://www.consrv.ca.gov/cgs/information/publications/california_geology_magazine.htm.

Information Contacts: David Hill, Long Valley Observatory, Volcano Hazards Program, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025, USA (URL: https://volcanoes.usgs.gov/observatories/calvo/); Deborah Bergfeld, Jim Howle, Chris Farrar, and William Evans, U. S. Geological Survey, Menlo Park, and Carnelian Bay, CA.

May 2005 (BGVN 30:05) Cite this Report

Minor seismicity throughout 2004

The relative quiescence in Long Valley caldera that began in early 1999 persisted through 2004 according to the U.S. Geological Survey's weekly reports and the 2004 annual summary of the Long Valley Observatory. Those manuscripts provide the basis for this synopsis. Seismicity in the adjacent Sierra Nevada block S of the caldera gradually died away over the same period, although background levels remained somewhat higher than within the caldera.

The resurgent dome continued to undergo minor fluctuations in deformation as reflected in changes in the lengths of baselines onto the dome. Over the past 6 years, the center of the resurgent dome has sustained the roughly 75-cm uplift that accumulated during the recurring unrest from 1979 through 1999.

Seismicity within both the caldera and the Sierra Nevada block to the S remained low through 2004. The two most notable earthquake sequences within the caldera were a minor swarm at the end of January and the first few days of February in the S moat, and a M 3.0 earthquake on 20 September located at the S margin of the caldera just N of Convict Lake. The latter was the first earthquake greater than M 3.0 within the caldera since the cluster of earthquakes on 4 November 2002, events centered beneath the S moat just S of the Highway 395-203 junction. The swarm in early February 2004 was located in the same general area of the S moat, but the epicenters fell along a SW trend in contrast to the WNW trend shown by most earthquake sequences in that area.

Seismicity within the adjacent Sierra Nevada block continued to be somewhat elevated compared to that in the caldera through 2004. The Sierra Nevada activity included about seven earthquakes over M 3, the largest of which was an M 3.7 earthquake on 12 January 2004 located 2 km E of Red Slate Mountain (19 km S of the caldera and 15 km WSW of Tom's Place). Most of the activity remained concentrated in the NNE-trending aftershock zone associated with the three earthquakes over M 5 during June and July 1998 and May 1999.

The most noteworthy seismic activity in the general vicinity of Long Valley caldera during 2004 was the prolonged earthquake swarm in the Adobe Hills centered roughly 20 km E of Mono Lake and 20 km NNE of Long Valley caldera (figure 30). Its onset was marked by a M 2.3 earthquake at 0002 on 18 September, followed by M 3.2 and 4.1 earthquakes at 0007 and 0008, respectively. Activity intensified through mid-afternoon of 18 September, with M 5.5 and M 5.4 earthquakes at 1602 and 1643, respectively. These produced widely felt shaking in the area from Bridgeport to Bishop. Seismicity declined gradually through the remainder of the year and into early 2005. By the end of December 2004, this Adobe Hill swarm had produced well over 1,000 detectable earthquakes including ~ 48 over M 3 and 6 equal or over M 4.

The mid-crustal long-period (LP) volcanic earthquakes, which began beneath the SW flank of Mammoth Mountain during the 1989 Mammoth Mountain earthquake swarm, continued through 2004 but at a much reduced rate compared with the peak in LP activity from early 1997 through mid-1998.

In early 2005 seismicity was generally minor (up to M 2.5) in and around the caldera. An M 4.2 earthquake occurred S of Long Valley caldera on 13 March 2005 at 1409. The event, which produced light shaking in Mammoth Lakes and Bishop was located in the Sierra Nevada ~ 12 miles SW of Toms Place near Grinnell Lake. It was followed by a series of 18 aftershocks, the largest which were M 2.8 and M 2.3. The last earthquake of similar magnitude in this area occurred in 1999 on 17 May. In addition to the M 4.2 main shock/aftershock sequence, two other significant earthquakes occurred in the Adobe Hills area E of Mono Lake, and a third occurred on 13 March in the Sierra Nevada S of the caldera, near Mount Baldwin. All three had magnitudes under M 2.0. From that time to mid-June 2005, seismicity was generally in the range of M 1-2, with a very few occurring to M 3.

Carbon dioxide (CO₂) concentrations measured in the Horseshoe Lake tree-kill area on the S flank of Mammoth Mountain showed no significant changes for 2004 with respect to the past several years. A survey of scattered areas of vegetation die-off and diffuse CO₂ flux on the resurgent dome completed in 2004 indicated anomalous CO₂ emissions from the kill areas were ~9 metric tons/day (compared with ~ 300 tons/day from Mammoth Mountain). The d13C-CO₂ values of the diffuse emissions were similar to values previously reported for CO₂ from hot springs and thermal wells around Long Valley, indicating a common source. The areas of elevated CO₂ flux tend to be associated with locally elevated soil temperatures. Some of the older areas near the Casa Diablo power plant are likely related to geothermal power production, but development of new areas may reflect a delayed response of the hydrothermal system to the 1997 unrest episode (including an additional 10-cm uplift of the resurgent dome accompanied by intense earthquake swarm activity in the S moat).

Thermal spring discharge in Hot Creek Gorge, which had dropped by about 20% in the last half of 2003, followed by a recovery beginning in January 2004, reached normal discharge values by June 2004. Fluid levels in key monitoring wells continued to decline, with some wells reaching their lowest values since records began in 1985.

Reference. U.S. Geological Survey—Long Valley Observatory, 2005, Long Valley Observatory Quarterly Report, October-December 2004 and Annual Summary for 2004 (URL: http://lvo.wr.usgs.gov/).

Information Contacts: Long Valley Observatory, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025, USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

June 2006 (BGVN 31:06) Cite this Report

Three ski patrol members die in April 2006 at fumarole-derived snow cave

During May 2005 to June 2006, Long Valley caldera was relatively quiet with hazard status remaining at green, the lowest level. During this time, there were two to three small, shallow earthquakes daily; these rarely reached a maximum of M 3 (figure 31). The earthquakes primarily occurred in the Sierra Nevada, an area S of the caldera, S of Mammoth lakes, and W of Tom's Place. The earthquakes were shallow, with focal depths less than 4 km. Deformation was slight. According to geodetic data from June 2006, the largest range of variability was ~ 13 mm, at Hot Creek.

During the second half of 2005, long-baseline tilt measurements revealed little N-S shift. In contrast, there was considerable E-W shift, with a fluctuation from -1.5 µrad around July up to 1.5 µrad in December (figures 32 and 33). The instrument responsible for these measurements, the long-baseline tiltmeter, measures levels in fluid reservoirs separated by ~ 500 m and connected by buried pipes. It records tidal tilts and shows minimal response to diurnal temperature changes along with little secular drift.

The caldera's CO₂ flux rate was closely monitored, in some cases hourly. CO₂ levels have been high since 1996, and the effects have included the killing of thousands of trees (figures 34 and 35). At Horseshoe Lake the tree-kill area underwent CO₂ discharge rate of 50-150 tons per day.

Ski-area accident. On 6 April 2006, three ski-patrol officers died when they fell into a 6.4-m-deep hollow, a snow cave, at the Mammoth Mountain ski resort (figure 36). The accident occurred "on Christmas Bowl run, E of Chair 3" explained Mammoth Mountain chief executive officer Rusty Gregory. Geophysicist Dave Hill, from Long Valley Observatory, noted that "the fumarole is roughly 200 m left (looking uphill) of the midpoint along the Chair 3 lift," and "one gets a whiff of H₂S when riding on the chair lift if the wind is right" (figure 36).

Hill further said, "the accident was more weather related than due to any changes in the fumarole. The exceptionally heavy spring snow fall completely covered over the vent, so that the ~ 85°C gases melted a snow cave above the vent (normally the heat from the vent keeps pace with the snow-fall so that the vent is marked by an open hole in the snow several meters in diameter). With no clear sign of the vent at surface, the ski patrol guys evidently didn't realize that, as they were attempting to re-set the fencing around the vent, they were standing directly over the cavity melted by the hot gas. When the roof gave way . . . [they fell] into an atmosphere that was very likely over 90% CO₂. " A third ski-patroller died trying to rescue them.

It was reported by Brendon Riley of the Tahoe Daily Tribune that seven other ski patrollers in a rescue party were injured due to the inhalation of dangerous gases from the fumarole. All were recovering.

At an undisclosed date after the accident, Mitch Weber, took pictures of the fenced-off hollow (figure 37). Weber posted these and a narrative regarding the incident in the online magazine he produces, Telemarktips.com, as part of a memorial to the victims.

Hill explained that the LVO's follow-up action, "has been to collect another series of gas samples from the fumarole, which show no change from earlier measurements." Hill also noted that "the ski area has taken steps to fence off a much wider area around the fumarole than before the accident."

The temperatures of all the fumaroles in Long Valley caldera are monitored (figure 38). Although this plot ends in 2005, Hill indicated that the data suggested normal conditions at the fumarole (MMF) at the time of the accident.

Long Valley Observatory posts hazard status as a color code in one of four categories: green, yellow, orange, and red (the most serious response). Details of their response plan appear on the USGS-Long Valley website and in a 2002 publication (USGS, 2002).

Reference. U.S. Geological Survey, 2002, USGS Response Plan for Volcanic Hazards in the Long Valley Caldera and Mono Craters Region, California: U.S. Geological Survey.

Information Contacts: David Hill, Long Valley Observatory, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025, USA (URL: https://volcanoes.usgs.gov/observatories/calvo/); Mammoth Local (URL: http://www.mammothlocal.com/); Tahoe Daily Tribune (URL: http://www.tahoedailytribune.com); Mitch Weber, c/o Telemarktips.com, 3 San Bittern Lane, Aliso Viejo, CA 92656, USA.

October 2008 (BGVN 33:10) Cite this Report

Comparative calm continues during 2006 into early 2008

The Long Valley Observatory (LVO) of the United States Geological Survey (USGS) monitors and studies earthquakes, ground deformation, degassing, and other types of geologic unrest in and around the Long Valley caldera. The LVO posts hazard status as a color code in one of four categories: green, yellow, orange, and red (the most serious response). The Long Valley caldera (figures 39 and 40) is located along the E side of the Sierra Nevada in east-central California. The hazard status remained at Green throughout 2007-2008.

The broad resurgent dome in the caldera had essentially stopped inflating in early 1998, then slowly subsided so that by the end of 2006, the center of the resurgent dome was 75-80 cm higher than its height before the unrest in 1980. Seismic activity during 2006 within the caldera remained low with earthquakes less than than M < 2.0. The largest earthquake in the region was an M 4.3 event near Grinnell Lake in the Sierra Nevada 16 km S of the caldera. Brief sequences of small (M < 1.7), rapid-fire earthquakes (spasmodic bursts) beneath Mammoth Mountain occurred on 19 September and 23-24 November 2006.

During 2007, the Long Valley caldera remained comparatively quiet. Earthquake activity within the immediate confines of the caldera included minor swarms beneath Mammoth Mountain on 17-26 January and 13 March and a swarm beneath the SE margin of the resurgent dome (2.5 km WSW of Hot Creek) on 13-15 March . The largest of these swarm earthquakes was a M 2.1 event on 15 March located ~ 2.5 km WSW of Hot Creek. Earthquake activity in the Sierra Nevada S of the caldera was greater than activity within the caldera. The largest earthquake in the region was a M 4.6 event on 12 June near Lake Dorothy (1.5 km SSW of Mount Morrison 9 km SSE of the caldera's margin). Aftershocks to this earthquake persisted through the remainder of June and included ~ 27 earthquakes of M > 2. A cluster of small earthquakes occurred on 21 December 2007; the largest was recorded at M 1.7.

On 22 November 2008, three earthquakes large enough to be located by the automatic earthquake detection system broke the quiescence at Long Valley and the area south of the volcano. One, magnitude M 1.4, was located beneath the resurgent dome inside the caldera. The others, M 1.7 and M 1.3, were located to the S in the Sierra Nevada.

Between 25 November and 1 December 2008, 13 minor earthquakes occurred in the Long Valley area. All were below magnitude 2.0; one was located N of Round Valley and the rest were S of the caldera in the Sierra Nevada.

As of November 2008, the carbon dioxide (CO₂) flux in the vicinity of Mammoth Mountain remained high but showed evidence of a gradual decline since 1995. The relatively high diffuse CO₂ gas flux of 50-150 tons/day in the Horseshoe Lake tree-kill area (16.6 km SSE of Long Valley and 2.67 km SE of Mammoth mountain) has been relatively constant over the past several years. Sporadic episodes of geysering in Hot Creek that began May 2006 continued through December 2007, but at a declining rate.

Information Contacts: David Hill, Long Valley Observatory, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025, USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

March 2012 (BGVN 37:03) Cite this Report

2009 summary, deep seismic swarm at Mammoth Mountain

This report on Long Valley caldera, California, summarizes USGS reports for 2009. The volcano remained non-eruptive. Long Valley Observatory (LVO) is now part of the California Volcano Observatory (CalVO). A tectonic earthquake sequence during 2011 in nearby Hawthorne, Nevada, is also discussed.

Long Valley caldera entered relative quiescence in the spring of 1999 (BGVN 26:07) following unrest that began in 1980 (SEAN 07:05); this relative quiescence continued through 2009.

Seismicity during 2009 was characterized by a low level of seismicity within the caldera, and a typical higher level of seismicity in the surrounding Sierra Nevada range (figure 41). Three recorded earthquakes were larger than M 3.0, yet none of them occurred within the region of Long Valley caldera as delimited by LVO. The largest earthquakes within Long Valley caldera were an M 2.7 on 9 January in the S moat, and a pair of M 2.3 earthquakes on 10 December that were located beneath the resurgent dome.

Deep seismic swarm at Mammoth Mountain.At Mammoth Mountain, increased seismicity began in late May, and a deep seismic swarm occurred on 29 September. The 29 September seismic swarm included over 50 M ≥0.5 high-frequency earthquakes that occurred at depths of 20-25 km, depths inferred to be in the mafic lower crust (figure 42). The high frequencies of these earthquakes indicated brittle-rock failure similar to shallow earthquakes that typically occur at <10 km depth, and were distinctly different than the long-period earthquakes that occur within the silicic upper crust, at depths of 10-25 km. The increased seismicity at Mammoth Mountain during 2009 produced more earthquakes there than occurred within Long Valley caldera (figures 41, 42, and 43).

Slow inflation of the caldera's resurgent dome. Deformation trends during 2007-2009 highlighted slow inflation of the resurgent dome. At the end of 2009, the height of the resurgent dome remained ~75 cm higher than prior to the onset of unrest in 1980. Measurements since 2007 indicated horizontal displacement rates of ~5 mm/year, mostly in a pattern radiating away from the resurgent dome (figure 44).

During 2009, soil CO₂ emission measurements revealed variations typical of most previous years. The increase in seismicity at Mammoth Mountain on 29 September did not produce a corresponding increase in CO₂ emissions.

2011 Hawthorne, Nevada, earthquake sequence. In March 2011, an earthquake sequence (mentioned in LVO weekly activity updates) began in Hawthorne, Nevada (~100 km NNE of the center of Long Valley caldera) that, according to Smith and others (2011), initially sparked brief concerns of unrest at Mud Springs volcano (figure 45). Mud Springs volcano is a probable Pleistocene volcano of the Aurora-Bodie volcanic field, Nevada (Wood and Kienle, 1992). The Hawthorne earthquakes did not show volcanic signatures in near-source seismograms (Smith and others, 2011), and the sequence was quickly identified as tectonic in origin.

According to Smith and others (2011), "An additional concern, as the sequence . . . proceeded, was a clear progression eastward toward the Wassuk Range front fault. The east dipping range bounding fault is capable of M 7+ events, and poses a significant hazard to the community of Hawthorne and local military facilities. The Hawthorne Army Depot is an ordinance storage facility and the nation's storage site for surplus mercury."

Earthquakes of the March 2011 sequence were as strong as M 4.6 (figure 46); the largest earthquakes may have been felt in Bridgeport, CA (~60 km SW of Hawthorne, and ~70 km NNW from the center of Long Valley caldera), according to LVO. The earthquakes occurred along at least two shallow faults, originating at 2-6 km depth (Smith and others, 2011). The earthquake sequence "slowly decreased in intensity through mid-2011" (Smith and others, 2011).

References. Smith, K.D., Johnson, C., Davies, J.A., Agbaje, T., Antonijevic, S.K., and Kent, G., 2011. The 2011 Hawthorne, Nevada, Earthquake Sequence; Shallow Normal Faulting. American Geophysical Union, Fall Meeting 2011, Abstract ##S53B-2284.

Wood, C.A. and Kienle, J., 1992. Volcanoes of North America: United States and Canada, Cambridge University Press, 354 p., pgs. 256-262.

Information Contacts: Dave Hill, California Volcano Observatory (CalVO), formerly theLong Valley Observatory (LVO), U.S. Geological Survey, Menlo Park, CA (URL: http://volcanoes.usgs.gov/observatories/calvo/); Nevada Seismological Laboratory, Laxalt Mineral Engineering Building, Room 322, University of Nevada-Reno, Reno, NV 89557 (URL: http://www.seismo.unr.edu/).