Explosive activity, major uplift, and fault formation during 31 March-early May 2000

As reported in BGVN 25:03, an eruption at Usu began on 31 March 2000 following several days of increased seismicity. A description of eruption precursors was presented in BGVN 25:06. This report summarizes activity between the 31 March eruption and early May 2000. Vigorous activity continued throughout April 2000 (figures 29 and 30), and activity was continuing in May.

According to the Geographical Survey Institute, the region including the crater areas was elevated 3.3 m/day during the three days from 31 March to 3 April, suggesting the formation of a new cryptodome similar to Meiji-Shinzan, a cryptodome that formed on Usu's N flank in 1910 and Showa-Shinzan, a cryptodome and lava dome that formed during 1944-45 (figure 31).

Around 6 April 2000, the eruption style changed from phreatomagmatic ash emissions to phreatic ejection of vertical cock's-tail jets reaching 500 m above the craters. By 10 April, more than 30 craters had formed in the Kompira-yama and NW Nishi-yama areas (figures 30 and 32). Cinders ejected by the continuing explosions steamed after they landed around the craters. The temporal changes in eruption style can be explained by assuming that the magma approached an overlying aquifer, estimated to lie between 0 and 300 m below sea level, and passed almost directly through it. According to the Usu Volcano Observatory, the amplitude of volcanic tremors increased after 6 April and increased further after 12 April.

Eruptions continued though late April, although plume activity decreased. White clouds were emitted from the Kompira-yama and NW Nishi-yama craters. Small explosions were observed. Cinders ejected from NW Nishi-yama dusted the Kompira-yama craters. The plumes frequently darkened with ash, implying partial collapse of vent walls. The principal active craters in both areas grew to a few hundred meters across and had dry funnel-shaped floors.

Uplift continued at a decreased rate in the NW Nishi-yama area. The total height of the uplift was 60 m by the end of April according to an air-photo survey by the Geographical Survey Institute. Uplift was < 20 m for the Kompira-yama area except for the crater rims. The total volume of uplift in the two crater areas was estimated to be ~40 x 10⁶ m³; one order of magnitude smaller than the volume of the 1944-45 Showa-Shinzan dome. According to the Joint University Research Group, the uplift rate was the highest in the initial stage of eruption and decreased exponentially to ~10 cm/day by early May.

Information Contacts: Setsuya Nakada and Hidefumi Watanabe, Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Geological Survey of Japan, 1-1-3 Higashi, Ibaraki, Tsukuba 305, Japan (URL: https://www.gsj.jp/); Geographical Survey Institute of Japan (URL: http://www.gsi-mc.go.jp/); Usu Volcano Observatory, Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sohbetsu-cho, Usu-gun, Hokkaido, 052-0103, Japan (URL: http://www.sci.hokudai.ac.jp/isv/english/); Tatsuro Chiba, Asia Air Survey Co. Ltd..

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

August 1977 (NSEB 02:08) Cite this Report

Major eruption on 7 August from a new vent

A major eruption began at 0912 on 7 August 1977 from a new vent [on the SE slope of] Ko-Usu dome (figure 1), after [32] hours of premonitory seismicity. About [0.08] km³ of hypersthene dacite tephra has been ejected, in eruption columns that rose to 10 km or more on four occasions (figure 2). Ashfalls forced the evacuation of about 20,000 tourists and 7,000 residents from nearby towns, and caused serious crop damage, particularly N of the volcano (figures 3 and 4). Table 1 is a summary of the eruption.

Table 1. Summary of explosions from Usu, 7-14 August 1977. Vent numbers correspond to those shown in figure 1. Cloud heights (in km) are approximate. Adapted from Katsui and others (1978).

Date	Time	Vent	Cloud Height	Ashfall Azimuth	Remarks
07 Aug 1977	0912-1140	1	12	S 60 E	Up to 30 cm of ash and breccia fell 4 km away. Airplane windows were cracked by tephra at 3,600 m altitude. Lightning accompanied the eruption cloud. "White smoke."
07 Aug 1977	1331	1	2	--	--
07 Aug 1977	1620	1	1.5	--	--
07 Aug 1977	1822	1	1.5	--	--
08 Aug 1977	1335-1410	2	--	--	--
08 Aug 1977	1537-1800	2	10	N 40 W	Ash reached Asahigawa, 180 km NE. Pumice up to 20 cm diameter fell 3 km away.
08 Aug 1977	1900	2	--	--	--
08-09 Aug 1977	2340-0215	3	10?	N 45 W	Incandescent material rose several hundred meters.
09 Aug 1977	0530-0730	3	6?	N 10 E	Ash reached the Sea of Okhotsk, about 75 km N.
09 Aug 1977	0815-0825	3	1.5	--	--
09 Aug 1977	0855-0905	3	1.5	--	--
09 Aug 1977	0908-0925	3	1	--	--
09 Aug 1977	1020-1105	4	4	N 75 E	--
09 Aug 1977	1120-1420	4	9	S 65 E	--
12 Aug 1977	0812-0900	3	2.5	N 35 W	"Black smoke."
13-14 Aug 1977	2237-0155	3	4?	N 70 E	Two centimeters of pumice nearby; eruption accompanied by lightning.

Earthquake frequency exceeded 100 events/hour during the premonitory swarm, then ranged between 60 and [120]/hour until the afternoon of 16 August, when a decline to 40/hour occurred. [Seismicity declined suddenly when explosions occurred (figure 5).] By 24 August, event frequency had declined further but magnitudes had increased to a maximum of 4, resulting in an increase in total energy release. Hypocenters were 1-2 km beneath the [summit].

Uplift and fissuring were observed on 12 August between Ko-Usu and O-Usu domes. Ground temperatures in this area were elevated to 33°C from the normal 24°C. Usu last erupted 23 June 1944-30 September 1945, producing Showa-Shinzan dome [at the E foot of the volcano].

Reference. Katsui, Y., Komuro, H., and Uda, T., 1985, Development of faults and growth of Usu-shinzan cryptodome in 1977-1982 at Usu volcano, north Japan: Journal of the Faculty of Science, Hokkaido University, ser. IV: Geology and Mineralogy, v. 21, no. 3, p. 339-362.

Information Contacts: T. Tiba, National Science Museum, Tokyo; L. Siebert, SI; J. Bloom, U.S. Embassy, Tokyo.

September 1977 (NSEB 02:09) Cite this Report

Cryptodome rises to 70 m by late September

No explosions have been reported since 14 August, but a new cryptodome at the E foot of Ko-Usu, surrounded by a 1.5-km semicircular fissure zone, had risen about 70 m by late September. Local earthquake swarms continued.

Information Contacts: Y. Katsui, Hokkaido Univ.; JMA, Tokyo; L. Siebert, SI; Asahi Evening News.

October 1977 (NSEB 02:10) Cite this Report

Continued cryptodome uplift and weak emission of steam clouds

Surface activity during September was confined to continued cryptodome uplift and weak emission of steam clouds, which rose a maximum of 600 m from summit vents [produced by the August eruption]. Daily recorded seismic events, including many felt shocks, declined irregularly through September, but remained high at the end of the month (figure 6).

Information Contacts: JMA, Tokyo.

November 1977 (NSEB 02:11) Cite this Report

Small steam explosion from three new vents on 16 November

Weak emission of white to grayish-white steam from small craters and fissures near Ko-Usu and O-Usu continued through October, reaching a maximum height of 400 m above the vents. Uplift of Ogari-yama [a large block making up most of the 2 km-diameter somma's NE quadrant, that has been uplifted] at the E foot of Ko-Usu since August, continued during October at the rate of about 45 cm/day, while Ko-Usu itself subsided about 40 cm/day. Another [unnamed peak of the uplifting block], about 240 m ENE of Ko-Usu's summit, rose 60 cm/day in October.

A small steam explosion from three new vents began between 0300 and 0400 on 16 November (figure 7) and lasted until about 0930. The explosion, the first from Usu since 14 August, produced a 300-m cloud and was accompanied by rumbling. The new vents (one is 6 m in diameter, the other two about 2 m in diameter) are clustered atop a 25-m-diameter rise at the edge of the unnamed new cryptodome. Several hundred local earthquakes per day continued to be recorded (figure 8).

Reference. Katsui, Y., Oba, Y. Onuma, K, Suzukuki, T., Kondo, Y., and others, 1978, Preliminary report of the 1977 eruption of Usu volcano: Journal of the Faculty of Science, Hokkaido University, Ser. 4, v. 18, p. 385-408.

Information Contacts: T. Tiba, National Science Museum, Tokyo; JMA, Tokyo.

December 1977 (NSEB 02:12) Cite this Report

Weak steam emission through November

Weak emission of white to grayish-white steam continued through November from small craters and fissures near the summits of Ko-Usu and O-Usu. During November, daily rates of uplift declined slightly at both [peaks of the uplifting block] (Ogari-yama and another centered about 240 m ENE of Ko-Usu's summit) to about 40 cm/day, from October rates of 45 and 60 cm/day respectively. Subsidence of Ko-Usu had also slowed, from 40 cm/day in October to about 30 cm/day in November. According to the Japan Times, a team of Hokkaido University geophysicists has located a shallow ellipsoidal aseismic zone beneath the two cryptodomes. The zone has a maximum dimension of 1 km and a minimum dimension of 0.5 km, but it was not possible to determine its depth from the newspaper article.

Further Reference. Yokoyama, I., 1978, The 1977 eruption of Usu volcano with a special reference to prediction of volcanic activities: Bulletin of the Volcanological Society of Japan, 2nd series, v. 23, no. 1, p. 65-82.

Information Contacts: Y. Katsui, Hokkaido Univ.; JMA, Tokyo; Japan Times.

January 1978 (SEAN 03:01) Cite this Report

Decreasing ground movement, but light tephra 13 January

Weak white and grayish-white steaming from vents and fissures near the summits of O-Usu and Ko-Usu continued during December, with a maximum cloud height of about 300 m. On 13 January, white steam clouds rose about 500 m from [a new vent (named B)] at 0740 and at 1125. A thin layer of ash was seen on top of snow at Date City (about 10 km SE of Usu) and Sobetsu (about 5 km NW of Usu).

Cryptodome uplift . . . declined slightly (to about 35 cm/day in December, from about 40 cm/day the preceding month). Subsidence at Ko-Usu, about 30 cm/day in November, decreased to about 15 cm/day in December. Daily local seismicity also decreased slightly in December.

Information Contacts: JMA, Tokyo; T. Tiba, National Science Museum, Tokyo; Y. Katsui, Hokkaido Univ.

February 1978 (SEAN 03:02) Cite this Report

Earthquake frequency increases; new cryptodome uplift slows

Weak steaming from vents and fissures on the slopes of . . . Ko-Usu and the new cryptodome NE of Ko-Usu continued during January. Steam clouds rose a maximum of 400 m above the vents. The rate of uplift of the new cryptodome briefly increased in late December to 38 cm/day (from 35 cm/day in mid-December), then declined to 31 cm/day in early January and to 26 cm/day by the end of the month. A team from Hokkaido University measured its summit altitude at 568 m in early January [compared to about 490 m altitude before the August 1977 eruption].

Daily earthquake frequency gradually increased at the end of January, and by late February had returned to the September rate. Epicentral distribution, according to Hokkaido University geophysicists, was unchanged, concentrated beneath Ko-Usu and the N portion of the somma, to slightly N of the somma.

Information Contacts: JMA, Tokyo; D. Shackelford, CA.

March 1978 (SEAN 03:03) Cite this Report

Two small eruptions in late February; cryptodome uplift and seismicity increase

Two small eruptions were observed during February, originating from vents 250-300 m ENE of the summit of Ko-Usu dome. The first occurred at 1559 on the 25th from a vent 2-3 m in diameter, producing a 500-m gray ash cloud and a trace of ashfall near the volcano. The dark gray ash cloud from the second eruption, at 0740 on 27 February, rose about 1,200 m from a 15-20-m-diameter crater, causing 2-3 mm of ashfall a few km SE of Usu.

The rate of uplift at the . . . cryptodome increased substantially, from about 23 cm/day in January to about 39 cm/day in February at Ogari-yama, and from 26 cm/day in late January to about 41 cm/day in February at the unnamed [peak] about 400 m E of the summit of Ko-Usu. February subsidence at Ko-Usu continued at the January rate of about 8 cm/day. Daily earthquake counts at Usu began to increase in late January and remained relatively high through most of February (figure 9).

Information Contacts: JMA, Tokyo; T. Tiba, National Science Museum, Tokyo.

May 1978 (SEAN 03:05) Cite this Report

Small steam explosions continue through April; seismicity and uplift rate decline

The steam explosions of 25 and 27 February were followed by similar events, each lasting a few hours, on 2, 3, 4, 5, 6, 9, 11, and 14 March, and on 24 April, causing slight ashfalls around the volcano. All of the explosions originated from a small area NE of Ko-Usu, where geologists found eight small vents on 5 March.

After a sharp increase to 39 cm/day in February, the rate of uplift at Ogari-yama had declined to 26 cm/day by late March and averaged 24 cm/day in April. . . . Daily earthquake counts also increased substantially in February to a monthly average of 411/day, but decreased to 345/day in March and 260/day in April. . . .

Information Contacts: JMA, Tokyo.

June 1978 (SEAN 03:06) Cite this Report

Seven new steam explosions, causing minor ashfalls

After 30 days of quiescence, steam explosions occurred on 26, 29, and 31 May, and 1, 2, 4, and 7 June, causing ashfalls on surrounding villages. The largest explosion produced a 3-mm ash deposit in a village at the foot of the volcano, and blocks were scattered in the summit crater, 500 m from the vent. Scientists from the University of Hokkaido observed the ejection of incandescent blocks during the night of 2 June, the first incandescent ejecta recognized since the initial large [pumice eruptions] of August 1977. This suggests that the rising magma column is now near the surface.

. . . The general downtrend in daily seismicity that began in February continued through early June. The rate of uplift at the new cryptodome (centered about 400 km E of the summit of Ko-Usu) increased sharply in February, but began to decrease in March and continued to decrease through May (data from I. Yokoyama).

Information Contacts: JMA, Tokyo.

July 1978 (SEAN 03:07) Cite this Report

Uplift slows, but steam explosions continue

Steam explosions continued through late July, with ashfalls around the volcano from those observed on 2, 3, 4, 7, 11, 22, 26, 28, and 30 June, and 2 July. The rate of uplift at the new cryptodome decreased to 17.6 cm/day in May and 13.6 cm/day in June. Subsidence of Ko-Usu dome has virtually stopped.

The Japan Times reported a 5-minute eruption on 16 July, with ejecta rising about 1,000 m above the vent. Reuters reported that eruption columns reached 2,000 m on [29] July. [Ash eruptions occurred frequently 15-31 July.]

Information Contacts: JMA, Tokyo; Japan Times; Reuters.

August 1978 (SEAN 03:08) Cite this Report

Steam explosions and glow; trends of declining seismicity and slower uplift continue

Steam explosions continued, occurring on 2, 9, 10, 15-17, 25, and 28-31 July. The 15 July explosions deposited 10 mm of ash on nearby villages, the largest ashfall since November 1977. Vent I (figure 10), site of explosions in April, late May, and June became quiescent, but reflected glow could be seen above this area at night in June and July. Vents J, K, and L (about 300 m SE of vent I) were formed by explosions on 9, 15, and 16 July, respectively. Vigorous steam emission accompanied by continuous tremor enlarged the vents.

Mean daily seismicity fell to 175 events, of which an average of 23/day were felt. Uplift rates at . . . the new cryptodome ENE of Ko-Usu continued to decrease. . . . Subsidence of Ko-Usu dome has virtually ceased.

The Kyodo broadcasting company reported an explosion of Usu at 0325 on 24 August. The ash cloud rose about 800 m and ash fell on Noboribetsu, about 30 km from Usu. Shortly after the explosion, glow was seen above the summit for several minutes. An increase in local seismicity occurred 2 days before the explosion; 345 events were recorded 22 August, but only 102 on the 23rd.

Information Contacts: JMA, Tokyo; I. Yokoyama and Y. Katsui, Hokkaido Univ.; Kyodo broadcasting company.

September 1978 (SEAN 03:09) Cite this Report

Explosions increasingly frequent; incandescence visible

Steam explosions were more frequent in August than in July. Explosions, most lasting no more than 15 minutes, occurred on 1, 3, 7-14, 16-20, 24-26, and 28-30 August, causing ashfalls in nearby villages. More than 1 cm of ash was deposited on villages a few km E of Usu after explosions on 16 and 24 August. During the 24 August activity, [an incandescenct column 300 m high] was visible from the base of the volcano for the first time since Usu's major eruption of August 1977. The August explosions produced a 350 x 200 m crater in the S portion of the summit area, engulfing the smaller vent active in July. Crater growth persisted through early September.

The rates of uplift at the new cryptodome . . . continued to decline, to 7 cm/day during August. Local seismicity also continued to decline, to an average of 91 events/day, about 1/6 of which were felt. Tokiko Tiba reported that explosions from 2215-2235 on 12 September and 0030-0217 on 13 September produced a 3-km eruption cloud, accompanied by lightning, and 1-5 cm of ashfall on Toyako-spa, about 3 km NNW of the active crater.

Information Contacts: JMA, Tokyo; I. Yokoyama, Hokkaido Univ.; T. Tiba, National Science Museum, Tokyo.

October 1978 (SEAN 03:10) Cite this Report

Stronger explosions, including incandescent ejecta; ashfall to N; seismicity and cryptodome uplift rates increase slightly

Explosions were recorded on 1-2, 5-8, 11-13, 20-22, 24-25, and 27-28 September. Most lasted only a few minutes and produced ashfalls of no more than 0.5 mm in thickness (figures 11 and 12). However, the two explosions that occurred during the night of 12-13 September were the largest since steam explosions began in November 1977. Ash clouds, accompanied by much lightning, rose 2.5 km above the crater and deposited up to 3.5 cm of ash over a broad area N of the volcano. Columns of incandescent tephra, about 600 m high, were present over the crater for three 10-minute periods during the 12-13 September activity, dropping blocks in the summit area. All of the September explosions occurred from the newly-named Gin-numa Crater, which had grown to a roughly circular feature, 350 m in diameter and 70 m deep, by the end of the month.

Cryptodome uplift rates and the number of local earthquakes per day both increased slightly in September, after several months of decline.

Information Contacts: JMA, Tokyo; I. Yokoyama, Hokkaido Univ.

November 1978 (SEAN 03:11) Cite this Report

Lahar kills three nearby; explosions decline

A lahar flowed through the town of Toyako-spa (at Usu's NW foot) on 24 October, killing three persons, destroying 16 houses, and damaging 20 others. However, the number of summit area explosions declined to three in October (one each on the 5th, 18th, and 27th) after one or more had been recorded on 16 days in September. The 5 October explosion originated from Gin-numa crater, active since August, but a new 30-m-diameter vent (labeled N in figure 13) opened less than 100 m to the NE on 18 October and was the source of the explosions on the 18th and 27th.

I. Yokoyama reported that both the average number of earthquakes per day and the rate of cryptodome uplift increased slightly in October, continuing a trend that began the preceding month.

Information Contacts: JMA, Tokyo; I. Yokoyama, Hokkaido Univ.

December 1978 (SEAN 03:12) Cite this Report

Activity declines, but deformation and seismicity continue

The number of days per month in which one or more explosions were recorded has dropped sharply, from 21 in August to three in October.

As of early December, there had been no explosions since 27 October. Local seismicity declined to an average of 79 earthquakes per day in November, and I. Yokoyama reported that the rate of cryptodome uplift decreased to 6.5 cm/day. Both seismicity and cryptodome uplift rates had increased slightly in September and October but have been on a general downward trend since February.

Since the initial explosions of August 1977, the N portion of Usu's somma has been slowly moving outward, causing severe local faulting at the N foot of the volcano. The distance between the N rim of the somma and the shore of Lake Toya decreased by about 65 m September 1977-March 1978 (Yokoyama, 1978). Numerous small faults have formed, damaging or destroying buildings in the area (figure 14). Some roads have been cut by faults (figure 15), and underground water pipes, telephone cables, and tubes for hot spring water have been cut at several hundred points.

Further References. Imagawa, T., 1986, Mud and debris flows on Mt. Usu after the 1977-1978 eruptions: Environmental Science, Hokkaido, v. 9, no. 1, p. 113-135.

Katsui, Y., Yokoyama, I., Watanabe, H., and Murozumi, M., 1981, Usu volcano, in Katsui, Y. (ed.), Symposium on Arc Volcanism Field Excursion guide to Usu and Tarumai Volcanoes and Noboribetsu Spa, Part I: Volcanological Society of Japan, Tokyo, p. 1-37.

Niida, K., Katsui, Y., Suzuki, T., and Kondo, Y., 1980, The 1977-1978 eruption of Usu volcano: Journal of the Faculty of Science, Hokkaido University, ser. IV, v. 19, p. 357-394.

Seki, K. (ed.), 1978, Usu eruption and its impact on environment: Hokkaido University, Sapporo, 496 p. (41 papers).

Yokoyama, I., Yamashita, H., Watanabe, H., and Okada, H., 1981, Geophysical characteristics of dacite volcanism - 1977-1978 eruption of Usu volcano: JVGR, v. 9, p. 335-358.

Japan Meterological Agency, 1980, The eruption of Usu volcano (August 1977-December 1978): Technical Reports of the JMA, v. 99, 203 p.

Information Contacts: JMA, Tokyo; I. Yokoyama, Hokkaido Univ.

January 1979 (SEAN 04:01) Cite this Report

Voluminous steaming continues; no explosions in 3 months

No explosions have occurred since 27 October 1978, although voluminous steam emission has been continuous. The average number of seismic events per day was 84 in December, up slightly from 79/day in November.

Information Contacts: JMA, Tokyo.

February 1979 (SEAN 04:02) Cite this Report

Cryptodome uplift rate continues decline; 2 year history summarized graphically

No explosions occurred in January 1979. The average number of local earthquakes/day declined slightly, from 84 in December to 76 in January (figure 16), and the rate of cryptodome uplift also showed a small decrease. The ground deformation continued NW of the volcano, but stopped to the N, NE, and E (figure 17).

Information Contacts: JMA, Tokyo.

March 1979 (SEAN 04:03) Cite this Report

Seismicity and cryptodome uplift rate continue to decline

The average number of earthquakes per day at Usu declined from 76 in January 1979 to 56 in February, and cryptodome uplift rates also decreased, to about 5 cm/day. No explosions have occurred since October.

Information Contacts: JMA, Tokyo.

April 1979 (SEAN 04:04) Cite this Report

Rates of seismicity and cryptodome uplift remain constant

No explosions have occurred since October 1978. In March, seismicity and the rate of cryptodome uplift remained at the same levels as in February.

Information Contacts: JMA, Tokyo.

May 1979 (SEAN 04:05) Cite this Report

Seismicity declines slightly

Seismicity declined from an average of 64 events/day in March to 61/day in April. No explosions have occurred since October 1978.

Information Contacts: JMA, Tokyo.

June 1979 (SEAN 04:06) Cite this Report

Uplift continues, but seismicity declines slightly and no new explosions

Monthly recorded seismicity continued to decline slightly in May, while cryptodome uplift remained constant at about 5 cm/day (figure 18). Ogari-yama and the Usu-Shinzan (Usu "New Mountain"), once thought to be separate cryptodomes, are now considered to be two peaks of a single [rising block]. Ogari-yama has risen 160 m since uplift began in August 1977. No explosions have occurred at Usu since 27 October 1978.

Information Contacts: JMA, Tokyo.

September 1979 (SEAN 04:09) Cite this Report

Slow seismic decline continues; elevation changes summarized

Yosihiro Sawada provided the following data (table 2) on total summit area elevation changes since activity began in August 1977 [based on results from Hokkaido University]. The uplift rate at Ogari-yama was 5 cm/day in May and 3 cm/day in July.

Table 2. Total elevation change at several features on Usu, as of 31 July 1979. Courtesy of Y. Sawada.

Location	Elevation Change
New cryptodome	+ 159.51 m
Ogari-yama	+ 163.26 m
O-Usu	- 23 m
Ko-Usu	- 54.8 m

Seismicity continued to decline slowly, from an average of 54 events/day in June to 47/day in July. The actual number of events per day sometimes varied considerably from the mean; in June, daily earthquakes ranged from 5 to 293.

When inspected on 31 July, Gin-numa Crater (formed by the activity of August-October 1978) contained a pool of water. Strong fumaroles on its rim had maximum measured temperatures of 635°C.

Information Contacts: Y. Sawada, MRI, Tokyo; D. Shackelford, CA.

July 1980 (SEAN 05:07) Cite this Report

Cryptodome uplift and seismicity continue

Seismic activity (table 3) and cryptodome uplift have continued through May 1980. The rate of cryptodome uplift and outward movement of the N somma wall both averaged about 5 cm/day in November 1979 and 4 cm/day in March 1980. In May, white vapor rose from three vents. Three new parallel faults with a combined throw of about 60 cm passed through the S portions of the active vents.

Table 3. Monthly means of the number of recorded earthquakes per day, July 1979-May 1980. Courtesy of JMA.

Month	Mean Daily Earthquakes
Jul 1979	47
Aug 1979	36
Sep 1979	35
Oct 1979	36
Nov 1979	36
Dec 1979	44
Jan 1980	38
Feb 1980	35
Mar 1980	29
Apr 1980	19
May 1980	22

Information Contacts: JMA, Tokyo; D. Shackelford, Fullerton CA.

January 1981 (SEAN 06:01) Cite this Report

1980 cryptodome uplift and seismicity summarized

Cryptodome uplift and local seismicity continued through 1980. Local seismicity continued an irregular decline through 1980 (figure 19 and table 4). Felt shocks averaged 3/day in 1980, but swarms of 30-40 felt events in a single day occurred about once a month. . . . Careful correlation of seismic records with observed surface deformation and faulting revealed that larger earthquakes occurred simultaneously with measureable fault movements.

Table 4. Monthly number of local seismic events at Usu, 1980-81. Courtesy of JMA.

Month	Recorded Events	Felt Events
Jan 1980	[1177]	234
Feb 1980	1004	216
Mar 1980	890	162
Apr 1980	582	92
May 1980	[674]	121
Jun 1980	[221]	32
Jul 1980	601	112
Aug 1980	486	82
Sep 1980	620	108
Oct 1980	413	69
Nov 1980	604	106
Dec 1980	[571]	94
Jan 1981	357	63
Feb 1981	289	49
Mar 1981	235	41
Apr 1981	485	92
May 1981	153	35
Jun 1981	151	33
Jul 1981	423	89
Aug 1981	317	64
Sep 1981	244	41
Oct 1981	315	52
Nov 1981	[290]	54
Dec 1981	440	105

The rate of cryptodome uplift decreased through 1980, from 5 cm/day in January to 3-4 cm/day in December [at the peak of] Usu-Shinzan . Northward lateral movement of the N flank continued at a similar rate. As a result, compression of the ground N of the volcano also continued, affecting several towns and villages.

Information Contacts: JMA, Tokyo; I. Yokoyama, Hokkaido Univ.

March 1982 (SEAN 07:03) Cite this Report

Cryptodome growth slows; local seismicity continues

"The crustal deformation and local seismicity at Usu continued through 1981. The monthly number of recorded seismic events, having gradually declined since the major 1977 eruption, dropped further to about 308/month in 1981 but remained at about this level through the year (figure 20 and table 4). Gradually weakening steam activity from the craters formed in 1978 has been observed. Around these craters, there have been many fumaroles that vigorously emitted white vapor; highest temperature was 643°C in August 1981. According to the data from the Usu Volcano Observatory (Hokkaido University) the rate of uplift of the Usu-Shinzan cryptodome decreased from about 2/cm per day in 1980 to about 0.8 cm/day in 1981. The northward lateral movement of the N flank continued at a similar rate."

Information Contacts: I. Yokoyama, Hokkaido Univ.

January 1983 (SEAN 08:01) Cite this Report

Decreased seismic activity; negligible ground deformation

Seismicity and ground deformation at Usu ended in spring 1982, after 58 months of activity. The monthly number of recorded seismic events had gradually declined since the major eruption in 1977, but remained above background through 1981, when approximately 308 seismic events were recorded per month. Ground deformation had also continued since the eruption. The rate of uplift of the cryptodome decreased from about 2 cm/day in 1980 to about 0.8 cm in 1981. Northward lateral movement of the N flank also continued through 1981.

In 1982, seismic activity decreased to the background level of about 10 events per month by April; 496 events were recorded in January, 231 in February, 79 in March, 10 in April and 11 in May. Ground deformation has been negligible since April 1982.

Further References. Yokoyama, I. (ed.), 1984, Report of joint geophysical and geochemical observations of Usu volcano in 1982 and Tarumai volcano in 1983, 214 p.

Information Contacts: JMA, Tokyo.

December 1983 (SEAN 08:12) Cite this Report

Fumarole temperatures decline slightly

Usu's major explosions of August 1977 were followed by rapid (but decreasing) cryptodome growth and seismicity. Smaller explosions started in November 1977, became more vigorous and frequent during the following summer, and ended in October. The rate of cryptodome growth increased briefly in early 1978 but slowed gradually, as did accompanying seismicity, for the next several years, ceasing rather abruptly in early 1982. Some points on the cryptodome rose as much as 180 m and a baseline between the N foot of the volcano and its N crater rim had shortened by a similar amount by early 1982 (Yokoyama, Katsui, and Abiko, 1983).

The following is a report from S.N. Williams. "A team of U.S. and Japanese scientists visited Usu's crater in mid-August to continue their study of fumarole mineral geochemistry and mineralogy begun in September 1981. Fumarole locations, distribution of fumarole incrustations, and temperatures were mapped. Samples of gas and fumarole sublimates were collected.

"Peak fumarole temperatures have declined somewhat. Fumarole 2, 770°C in 1981, was 730°C in 1983; no. 3 decreased from 690 to 650°C; no. 1 from 550 to 545°C. Gas discharge continued at a tremendous rate with the associated roaring jet sound. Extensive deposition of fumarole incrustations continued. All of these fumaroles were located in a narrow valley between the 1977-82 cryptodome (Usu-Shinzan) and Ko-Usu, a lava dome extruded in either the 1663 or 1769 eruption.

"Overall, a broad concentric zonation was noted in the temperature distribution of the fumaroles. Large areas of steaming ground on Ko-Usu dome and Usu-Shinzan formed an aureole around the high-temperature fumaroles. These approximately 100°C fumaroles continued to emit large quantities of H₂S. Gin-numa Lake appeared to maintain about the same level as in 1981."

Reference. Yokoyama, I., Katsui, Y., and Abiko, T., 1983, The 1979-1982 Activities of Usu Volcano in Report on Volcanic Activities and Volcanological Studies in Japan for the Period from 1979 to 1982; Bulletin of the Volcanological Society of Japan, v. 28, no. 1, appendix, p. 15-19.

Information Contacts: S. Williams, R. Stoiber, and J. Patterson, Dartmouth College; T. Abiko, Muroran Inst. of Tech.

May 1996 (BGVN 21:05) Cite this Report

Seismic activity increases

The number of earthquakes gradually increased to 18 during the first half of May.

The latest eruptive activity consisted of major explosions in August 1977 that were followed by rapid cryptodome growth. More explosions took place in November 1977, became more vigorous and frequent the following summer, and ended in October 1978. Dome growth and seismicity continued for several years and ceased abruptly in 1982 (SEAN 08:12).

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

March 2000 (BGVN 25:03) Cite this Report

Eruptions began on 31 March and proceeded from multiple craters on the NW flank

Following several days of increased seismicity, an eruption of Usu volcano began on 31 March. The eruption, the first at Usu since 1977-82, was continuing in April. This report is based on information compiled from a wide variety of sources cited in the text by Setsuya Nakada at the Volcano Research Center, University of Tokyo. Vigorous activity continued through April, and by the end of the month there were more than 50 craters; details will be provided in the next Bulletin.

Precursory activity. The number of volcanic earthquakes around Usu began increasing after 0800 on 27 March, prompting a series of Volcano Advisory notices from the Japan Meteorological Agency (JMA). At the site of the seismic station, ~2 km S of the summit, there were 16 earthquakes recorded on 27 March, followed by 599 (including 68 felt earthquakes) on the 28th (table 5). Neither volcanic tremor nor visible change in fumarolic gas had been observed by the night of 28 March.

Table 5. Numbers of earthquake events at Usu during 27 March-4 April 2000. The JMA seismometer is located ~2 km S of the summit. Courtesy of JMA.

Date	Volcanic	Low-frequency	Felt	Tremor
27 Mar 2000	16	0	?	0
28 Mar 2000	599	5	68	0
29 Mar 2000	1629	164	600	0
30 Mar 2000	2454	326	537	0
31 Mar 2000	788	139	299	0
01 Apr 2000	159	24	52	0
02 Apr 2000	48	3	3	0?
03 Apr 2000	22	0	2	0?
04 Apr 2000	9	0	1	0?

On the evening of 28 March the National Coordination Committee of Volcanic Eruption Prediction (chaired by Yoshiaki Ida, Univ. of Tokyo) warned about the high possibility of an imminent eruption. JMA also called resident's attention to the hazard of mudflows triggered due to snow melting during an eruption. A hot spring resort at the N foot of the volcano had about 1,600 guests this night. However, more than 400 persons living around the volcano voluntarily evacuated by the night of 28 March.

A Volcano Alert was issued by JMA on the morning of 29 March. That day the number of volcanic earthquakes totaled 1,629 (600 of them felt), including low-frequency earthquakes whose number increased with time. At 0708 on 29 March a M 3.4 had a hypocenter on the N slope. Later that day, at 1722, one of M 4.2 was centered on the NW slope; analysis indicated faulting along a nearly vertical slip plane. The GPS network around the volcano, maintained by the Geographical Survey Institute (GSI), detected inflation. Neither volcanic tremor nor visible changes in fumarolic gas had been observed yet. People living in a city and two towns around the volcano were required to evacuate on the afternoon of 29 March. By that night, more than 9,000 people evacuated, including all tourists in resorts at the N foot of the volcano.

The number of volcanic earthquakes continued to increase on 30 March, to a total of 2,454, including 326 low-frequency earthquakes and 537 felt events. During a helicopter flight, Yoshio Katsui and Hiromu Okada (Hokkaido University) found chains of cracks as long as 100 m along the NW part of the caldera rim, just above the hypocenters of the volcanic earthquakes. A group of geologists from the same university, the Geological Survey of Japan (GSJ), and the Geological Survey of Hokkaido also found small cracks and signs of ground deformation at the N-NW foot of the volcano. The hot-spring resort on the S shore of Lake Toya, Toyako-Onsen, lies within the area that these phenomena were observed. The Usu Volcano Observatory of Hokkaido University, at the N foot of the volcano, was moved ~8 km S to the western part of Date City. Frequency and magnitude of earthquakes changed in a manner similar to that seen in the 1910 eruption; that event started with a phreatic eruption after 5 days of precursory phenomena. This was followed by explosions from 45 craters aligned along a 2.7-km-long, EW-trending zone. The 1910 eruption ended with a 3-month period of cryptodome emplacement.

Eruptions begin on 31 March. At 1310 on 31 March a phreatic eruption ~4 km NW of the summit and ~2 km NE of the epicenters of the volcanic earthquakes sent ash 3,200 m above the crater. Ash and cinders from the newly formed craters fell on nearby houses. After about 2 hours of strong ash emissions the eruption declined. No injuries were reported. According to Yoshio Katsui, who did a helicopter inspection, five craters formed successively that joined into one larger crater (roughly 200 m long and 60 m wide) during this eruptive episode.

According to the Joint University Research Group (JURG) on the morning of 31 March, additional cracks were observed in the W flank of the volcano, and on the summit of the old lava dome within the caldera. Cracks found the previous day on the NW flank and foot of the volcano became wider. Seismicity had decreased from the previous day.

According to a Volcano Advisory at 0312 on 1 April, a M 4.8 earthquake occurred, the largest so far during this eruption. Observations using a JMA camera indicated that another phase of the eruption started at the W foot around 0250, when explosions formed new craters near the previous craters W of Nishi-yama and also 1.5 km NE, on Konpira-yama. The latter were close to houses in the resort on the shore of Lake Toya. Cock's-tail-shaped jets were frequently observed from the active craters. Sometimes eruption clouds rose more than 2 km above the vents. According to Tadahide Ui, petrological work in Hokkaido University identified a few juvenile fragments in the products of the first eruption. Takayuki Kaneko (VRC, U. Tokyo) inspected the craters from the Asahi Shinbun news helicopter on 1 April (figure 21 and table 6).

Table 6. Venting from west of Nishi-yama dome (craters A1-A6) and near Konpira-yama dome (craters B1-B2) at stated times on 1 April 2000, as seen from helicopter by Takayuki Kaneko (VRC, U. Tokyo). Craters formed in the same order as their numbers, but the opening sequence of A4 and A5 was unclear.

Crater	Status on 1 April (1110-1130)	Status on 1 April (1310-1330)
A1	Inactive	Inactive
A2	Active (white-to-dark cloud)	Active (white)
A3	Active (dark)	Active (dark)
A4	Not open	Active (white)
A5	Not open	Active (white)
A6	Not open	Activity began (white)
B1	Not open	Active (white)
B2	Not open	Activity began (white)

According to the JURG and the GSJ, deformation on the NW part of the volcano continued. During 31 March to 1 April, with respect to the volcano's foot, the NW caldera rim uplifted by as much as 30 cm a day and migrated NE by 25 cm over a period of 19 hours. Small crack systems observed in the N foot also supported some slow but continual migration on the NW part of the volcano.

The GSJ and Hokkaido University noted that on 2 April fresh pumice fragments floated in Toya Lake, ~3.7 km ENE of the 31 May eruption vent. Three faults extending E-W for a few hundred meters were seen on 3 April near the explosion craters (NW of Nishi-yama). The maximum throw of the faults was ~10 m. They form a normal fault system with subsidence in the N side according to Kiyoaki Niida of Hokkaido University. A new fault system was found on 4 April a few hundred meters N of the one near the craters NW of Nishi-yama, according to Okada, and the area between the two fault systems was subsiding to form a graben. In early April, the fracture, which formed near the NW part of the summit caldera rim on 30 March, widened to ~4-5 m. The eruptive activity continued and the Asahi newspaper reported that at 1700 on 4 April at least one new crater lying between the two existing groups of craters sent a white plume 600 m high.

A new crater formed on the morning of 5 April on Konpira-yama near the previous two craters and sent a grayish plume to ~400 m height, according to JMA. On the W slope of Konpira-yama, a mudflow moved down slowly toward the spa, raising steam.

The main edifice consists of basaltic to basaltic-andesites (49-53% SiO₂) with a small summit caldera. Ten dacitic lava or cryptodomes (68-73 % SiO₂) lie on the summit and N slope arranged in two lines trending NW-SE. The eruptions that occurred at the summit (in 1663, 1769, 1822, 1853 and 1977-82) commenced with a strong Plinian phase and, apart from 1977-82, were accompanied by pyroclastic flows. All but perhaps the 1769 eruption also involved the growth of lava or cryptodomes in the middle to final stages. Both summit (O-Usu and Ko-Usu) and flank (Showa-Shinzan) lava domes, along with seven cryptodomes, were erupted in historical time. The war-time growth of Showa-Shinzan was painstakingly documented by the local postmaster, who created the first detailed record of lava-dome growth.

During the flank eruptions (in 1910 and 1943-45) the building of lava or cryptodomes was preceded by phreatic explosions in the initial stage. Each eruption lasted from one month to two years, with between thirty and one hundred years of repose between them. According to Akihiko Tomiya (Geological Survey of Japan), precursory seismicity of the historical eruptions (mainly volcanic earthquakes) lasted from 32 hours (1977-82) to 6 months (1943-45).

Information Contacts: Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Geological Survey of Japan, 1-1-3 Higashi, Ibaraki, Tsukuba 305, Japan (URL: https://www.gsj.jp/); Usu Volcano Observatory, Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sohbetsu-cho, Usu-gun, Hokkaido, 052-0103, Japan (URL: http://www.sci.hokudai.ac.jp/isv/english/); Japan Meteorological Agency, Volcanological Division, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

June 2000 (BGVN 25:06) Cite this Report

Eruption decreasing in intensity; precursors to 31 March eruption

Usu's multi-vent NW-flank eruption that began on 31 March 2000 continued until at least 10 July (see previous report and map in BGVN 25:03). By 10 July the eruption had lost considerable vigor. The last noteworthy ash fall took place on 6 April; a small one occurred on 7 April. Several excellent reports were published rapidly by the Geological Survey of Hokkaido (GSH, 2000a, b). This article provides a summary of those Japanese-language reports as well as excerpts from a formal statement discussing Usu's behavior through 10 July. Satellite imagery also provided ashfall data. The active dome and associated vent group was incorrectly spelled "Konpira-yama" in previous Bulletin reports. According to formal rules of translation this name should be "Kompira-yama."

Prior to the eruption, geological mapping and bore holes had delineated portions of Usu's edifice and surrounding subsurface, enabling workers to draw a generalized cross section (figure 22). In addition to these background studies, as Usu entered an eruptive phase on 31 March a comprehensive suite of monitoring instruments were in place.

Four days prior to the eruption, groundwater levels in instrumented wells (the potentiometric surface) around the volcano began to change (figure 23). A day or two later, these perturbations escalated rapidly. Data from five wells (figure 23) show that at least four underwent roughly synchronous offsets that grew to reach the 2- to 10-m range. These dramatic offsets were inferred to have been driven by the influx of magma. Also, water temperatures increased at hot springs. The level of the groundwater surface in the instrumented wells peaked near the time the eruption started. For the wells with post-eruptive data on figure 23, the groundwater surface began a comparatively gradual steady decrease soon after the eruption started. Ancillary details on well locations and behavior appear in the cited reports.

Global Positioning System (GPS) data helped predict the 31 March eruption. GPS station KMK is near Hokkaido's N coast and ~7 km E of Usu's active vent groups. KMK was compared with three other stations near Usu beginning around 30 March (figure 24). The comparison revealed large vertical motions-tens of centimeters per day- including some beginning on 29 March (not shown). Figure 24 shows how the rates of vertical motion declined in early April at all three close-in stations. The reports also noted horizontal motions measuring tens of centimeters per day.

Clear precursory seismicity appeared at Usu (figure 25). The maxima, ~150 earthquakes in a 2-hour interval (i.e., ~75 earthquakes/hour), occurred ~1 day prior to the eruption. The eruptive pulses on 1 April took place during an interval with comparatively low seismicity.

Figure 26 provides a graphical summary of the episode's eight modest but identifiable ash falls. Most of the ash blew E, but an eruption on 1 April blew SE and one on 4 April blew N. GSH (2000a) features a time line for the two vent groups in eruption, graphically portraying 31 March-7 April plume observation. Figure 27 presents a sample of this larger time line: the portion for 1 April 2000. Figures such as this provide a particularly apt summary of complex phenomena.

Committee's announcement. The Usu eruption committee chaired byYoshiaki Ida made a formal announcement on 10 July. They noted that on this date Usu's phreatic eruption continued at the Kompira-yama and western Nishi-yama vent groups, but the supply of magma from depth had almost ceased. Accordingly, they anticipated a gradual decrease in volcanism.

The committee indicated that the current eruption occurred due to upward movement of magma from depths of ~10 km reaching a shallow reservoir around 4-5 km. Portions of the shallow reservoir traveled NW and then to the vents where magma escaped. The committee noted that on 10 July uplift still continued at western Nishi-yama (~5 cm/day) but that its areal extent and rate were decreasing. The committee noted that by 10 July small faults associated with the eruption ceased moving. They appeared as visible fault traces cutting across roads and other infrastructure (see photos in GSH, 2000a, b).

The committee also noted that the early phases of the eruption had ejected portions of juvenile material, whereas by 10 July the eruptions mainly discharged steam. Similarly, with time, cloud height and explosive vigor decreased. On 10 July Nishi-yama still gave off intermittent weak ash; Kompira-yama still emitted loud blasts with glowing volcanic rocks. But by this time such activities had decreased and ballistic bombs continued to fall several hundred meters from the Kompira-yama vent group. Earthquakes continued on the SW flank of Usu, but by 10 July they became increasingly scarce. The committee suggested a pyroclastic surge was unlikely in the near future .

Satellite imagery. A satellite image from 3 April shows Usu's ash blanketing parts of the largely snow-covered landscape (figure 28). The image caption states that the team planned to image Usu every 3-4 days. The images were captured on ASTER (The Advanced Spaceborne Thermal Emission and Reflection Radiometer), a Japanese-built instrument that obtains high-resolution (15-90 m²/pixel) images at 14 wavelengths from visible to thermal infrared. ASTER registers land surface temperature, emissivity, reflectance, and elevation; it flies on the Terra platform where it serves as a zoom lens for the other Terra instruments. ASTER has the ability to change viewing angles, enabling it to make stereoscopic images and detailed terrain height models. NASA terms the Terra satellite the flagship of the EOS mission. The latter is an effort to better understand planet Earth's atmosphere, land, and oceans, as well as their interactions with solar radiation and with one another.

References. Geological Survey of Hokkaido (GSH), 2000a, Observations of Usu's volcanic eruption, 2000, Preliminary Report (in Japanese), 53 p. (in color on the GSH website and available as a 47 M file.

Geological Survey of Hokkaido (GSH), 2000b, Usu eruption in 2000, GSH News, 2000, 5, vol. 16, (ISSN 1345-1138), (text and captions in Japanese), 4 p. (1 additional sheet with 8 color plates)

Information Contacts: Masahiro Yahata, Geological Survey of Hokkaido, Kita 19, Nishi 12, Kita-Ku, Sapporo, 060-0819, Japan; Yoshiaki Ida, University of Tokyo, Earthquake Research Institute, Yayoi 1-1-1 Bunkyo-Ku, Tokyo 113, Japan; NASA Terra Project, NASA Goddard SFC, MC 613, Greenbelt, MD 20771 USA (URL: https://terra.nasa.gov/); Yasushi Yamaguchi, Japan Outreach Center for ASTER, Nagoya University, Earth & Planetary Sci Dept/Faculty Sci, Furou-cho Chikusa-ku, Nagoya 464-01; Usu Volcano Observatory, Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sohbetsu-cho, Usu-gun, Hokkaido, 052-0103, Japan (URL: http://www.sci.hokudai.ac.jp/isv/english/).

March 2001 (BGVN 26:03) Cite this Report

Explosive activity, major uplift, and fault formation during 31 March-early May 2000

As reported in BGVN 25:03, an eruption at Usu began on 31 March 2000 following several days of increased seismicity. A description of eruption precursors was presented in BGVN 25:06. This report summarizes activity between the 31 March eruption and early May 2000. Vigorous activity continued throughout April 2000 (figures 29 and 30), and activity was continuing in May.

According to the Geographical Survey Institute, the region including the crater areas was elevated 3.3 m/day during the three days from 31 March to 3 April, suggesting the formation of a new cryptodome similar to Meiji-Shinzan, a cryptodome that formed on Usu's N flank in 1910 and Showa-Shinzan, a cryptodome and lava dome that formed during 1944-45 (figure 31).

Around 6 April 2000, the eruption style changed from phreatomagmatic ash emissions to phreatic ejection of vertical cock's-tail jets reaching 500 m above the craters. By 10 April, more than 30 craters had formed in the Kompira-yama and NW Nishi-yama areas (figures 30 and 32). Cinders ejected by the continuing explosions steamed after they landed around the craters. The temporal changes in eruption style can be explained by assuming that the magma approached an overlying aquifer, estimated to lie between 0 and 300 m below sea level, and passed almost directly through it. According to the Usu Volcano Observatory, the amplitude of volcanic tremors increased after 6 April and increased further after 12 April.

Eruptions continued though late April, although plume activity decreased. White clouds were emitted from the Kompira-yama and NW Nishi-yama craters. Small explosions were observed. Cinders ejected from NW Nishi-yama dusted the Kompira-yama craters. The plumes frequently darkened with ash, implying partial collapse of vent walls. The principal active craters in both areas grew to a few hundred meters across and had dry funnel-shaped floors.

Uplift continued at a decreased rate in the NW Nishi-yama area. The total height of the uplift was 60 m by the end of April according to an air-photo survey by the Geographical Survey Institute. Uplift was < 20 m for the Kompira-yama area except for the crater rims. The total volume of uplift in the two crater areas was estimated to be ~40 x 10⁶ m³; one order of magnitude smaller than the volume of the 1944-45 Showa-Shinzan dome. According to the Joint University Research Group, the uplift rate was the highest in the initial stage of eruption and decreased exponentially to ~10 cm/day by early May.

Information Contacts: Setsuya Nakada and Hidefumi Watanabe, Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Geological Survey of Japan, 1-1-3 Higashi, Ibaraki, Tsukuba 305, Japan (URL: https://www.gsj.jp/); Geographical Survey Institute of Japan (URL: http://www.gsi-mc.go.jp/); Usu Volcano Observatory, Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sohbetsu-cho, Usu-gun, Hokkaido, 052-0103, Japan (URL: http://www.sci.hokudai.ac.jp/isv/english/); Tatsuro Chiba, Asia Air Survey Co. Ltd..