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.

Geological Summary. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

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/).