285011-285839

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Usu Volcano Observatory
Institute of Seismology and Volcanology
Faculty of Science
Hokkaido University
Sohbetsu-cho, Usu-gun
Hokkaido 052-01
Japan

Sapporo Office:
Kita 10, Nishi 8, Kita-ku
Sapporo 060-0810
Japan

Scientific staff:

Dr. Hiromu Okada - Volcanology (geophysics, seismology)
Mr. Hitoshi Mori - Volcanology (geodesy, geophysics)
Mr. Yu'ichi Nishimura - Volcanology (seismology, geophysics)

Supporting staff:

Mr. T. Maekawa - Permanent technical
Mr. A. Suzuki - Permanent technical

Usu Volcano Observatory (UVO)

The Observatory was founded in April, 1977 under the National Project for Prediction of Volcanic Eruptions as a research oriented facility belonging to the Faculty of Science, Hokkaido University. Various geophysical researches are conducted using a telemetry, CPU, and field measurements.

Five volcanoes, Usu, Tarumai, Hokkaido-Komagatake, Tokachi and Meakandake are presently under the continuous monitoring by telemetry. Repeated field measurements are also made for understanding the eruptive mechanism, precursory processes and the underground magma behaviour.

Hokkaido is one of the most interesting showcase of calderas and active volcanoes of island arc type. For example, during the recent 350 years, we have 16 large eruptions with VEI equal or greater than 3 (the size of the 1977 Usu eruption) in such a small island of Hokkaido. The largest three are VEI=5 (the size of the 1980 Mount St. Helens).

Even in our short term experience of volcano observation since the UVO establishment, 8 eruptive episodes occurred at 4 active volcanoes during 13 years. Precursory activities such as seismic swarms, tremors, heating/fuming anomalies were luckily detected for 6 of them by the routine or the temporary observations.

The observatory is located at the southern shore of caldera lake Toya, and at the northern foot of Volcano Usu. 24 hours continuous monitoring of the five active volcanoes are performed at this small facility in a country side. The data are recorded on analogue monitor chart recorder, or on the temporary computer disc as a digital data. The later are then transferred on an optical disk for further researches.

In a framework of National Project for the Prediction of Volcanic Eruptions, we must inform all the necessary information to JMA (Japan Meteorological Agency) which has an office of its Coordinating Committee and is only one existing window for information release to the public. Observational report and data are periodically submitted to the office (JMA). Volcano observatories under the National Project (we are one of them) work together at the Comprehensive Volcano Observation Project (2 volcanoes in a year) and at volcanoes in emergency.

As a member institution of Hazard Mitigation Committee of Hokkaido Prefectural Government, we work closely with Sapporo Meteorological Observatory (JMA), Department of Geology and Mineralogy (Faculty of Science, Hokkaido University), Hokkaido Geological Survey, and Volcano Sabo groups. Supports are also due Department of Geophysics and Research Center for Prediction of Earthquake (both at Faculty of Science, Hokkaido University).

Main Collaborators:

Prof. Yoshio Katsui - Sapporo Gakuin College - Volcano geology
Prof. Tadahide Ui - Institute of Seismology and Volcanology (ISV)
Dr. Hiromitsu Oshima - Institute of Seismology and Volcanology
Dr. Minoru Kasahara - ISV Earthquake Prediction Chief - Geodesy
Dr. Yasunori Nishida - Institute of Seismology and Volcanology - Geoelectric and geomagnetic studies
JMA Office
Hokkaido Geological Survey

The former UVO director (1977-1987), Prof. Emeritus Izumi Yokoyama, retired in 1987 but remains active. Our good partner, Prof. Emeritus Yoshio Katsui (volcano geologist, former professor of Hokkaido University) retired in March, 1990 and is now at a private college (Sapporo Gakuin College) continuing close partnership. The former staff, Mr. Hiroki Miyamachi moved to Kagoshima University in March, 1990.

UVO can be reached 2:45 by bus from Sapporo (bus terminal at JR station), ca. 1:25 by JR train from Sapporo Chitose airport (to nearby Toya JR station), or 2:00 drive from the airport. Snow season is from late November to early April, summer is the best season (dry and cool), but may be crowded with many tourists. Volcanoes in Hokkaido are not so high and easy to access; Usu, Tarumai and Komagatake are easy to reach the summit, other two still not so laborious. Some parts of volcanoes are restricted for public access.

I. USU

UVO is presently operating 10 seismic stations; two on the summit and other in the surrounding area in the distance range of 2-10 km. The observation network was designed during the 1977-1982 eruptive-dome building activity, in which numerous large volcanic earthquakes had persistently occurred. Those large events (M=ca.2-4.3) were able to be located at a markedly improved accuracy and their source mechanisms, wave characteristics were well studied.

Since the end of activity after middle March, 1982, no more M > 2 events occurred. Present activity is characterized by minor micro earthquakes, which are scarcely detected by only one or two stations.

Improved high-gain seismic stations using a vault or a deep well are necessary to detect minor precursors which might precede the coming eruption, because of highly explosive nature of Usu's dacitic magma and of necessity of disaster mitigation for the nearby large tourist city, Toyako-Onsen (Spa).

Measurements of temperature, EDM, levelling, waterlevel, and GPS are repeated periodically. One tiltmeter data is telemetered, but that is in a poor S/N condition. Based on the National Project, university group studied the underground structure and location of intruded magma at Usu volcano in 1991 and 1993.

Muroran District Meteorological Observatory (JMA) also operated one seismic station, and is under the close contact with UVO.

The following are some of the observational results:

1. Doughnut-shaped distribution of volcanic earthquakes clearly indicates "earthquake-free magma zone" beneath the summit crater, where new dacitic crypto-dome "Usu-Shinzan" was formed during 1977-1982.

2. Close relation between energies of deformation (dome growth rate), earthquake and eruptions.

3. Earthquake families (an earthquake group characterized by similar wave form events) and long-life asperity slippage due to slowly intruding magma.

4. Clear premonitory phenomena: the important role of large low-frequency earthquakes and large deformations in the pre-eruption process.

5. Contrast between active and inactive periods of dacitic magma; seismicity and deformation clearly show the start and the end of activity. Steady low-level of background activity and gradual subsidence of new dome between eruptive period; subsidence continued during 1982-1990 at the same rate of ca. 6 cm/year.

6. From 31 March to early May 2000, a new cryptodome was formed amid strong seismicity and phreatic explosions. Notable strain-related changes occurred in the level of water wells of the area.

7. Physical conditions of Showa-Shinzan lava dome which was formed during 1943-1945; gradual subsidence (ca. 4 cm/year), decrease of P-wave velocity (4 km/s to 2 km/s), and decrease of fumarolic temperature (1000 to 230°C).

II. TARUMAI

UVO is operating 4 seismic stations. Those data are once centralized to the key station SHI ca. 6 km from the summit, together with the data provided by JMA station, then transmitted to UVO by radio via three relay stations. Short span tilt levelling, measurements of temperature, precise gravity, EDM, and GPS are conducted by UVO periodically.

Since 1667, the volcano had been very active; 4 large eruptions with VEI >= 3, including two with VEI = 5. The destruction of the 1909 dome in the summit crater will be expected type of possible future eruption.

Tomakomai Weather Station (JMA) also operated one seismic station, and is under close contact with UVO.

The following are some of the observational results:

1. Seismicity was quite high during the recent eruptive period (1978-1981). Precursory swarm to the 1981 eruption continued about 4 months and low-frequency M2 events increased before the eruption.

2. Clear long-term decrease of seismicity took place since 1981 eruption, and only few events are recorded monthly after 1985. Geothermal activity, on the contrary, intensified at and around the summit dome during 1983-1984, but no more acceleration since then.

3. Tilt observation at the summit crater indicates small amount of but steady uplift of the dome; ca. 15 sec during 1986-1990. What this result means is not easily answered, because of no other indication that this is caused by at true pre-eruption inflation.

4. A peculiar seismic swarm was observed during 1988. Events are classified into earthquake families and well correlated with earth's tide.

5. Infrared geothermal measurements were tested from the point 6 km from the objective. The results show a clear temperature distribution of the summit dome; thermal power is estimated as 1-1.5 MW.

III. HOKKAIDO-KOMAGATAKE

UVO is operating 5 seismic stations. Those data together with JMA data are linked to UVO via key station SAW, and one relay station. Air waves are also monitored. One station is operated with solar battery. No real time geodetic data. Measurements of EDM, levelling, precise gravity, GPS, and temperature are conducted periodically by UVO.

The 1929 eruption was the one of the largest eruption in the 20th century in Japan. Pumice flow is expected as similar to the past eruptions but precursor might be very minor (if exist) as in the case of its 1929 eruption.

Mori Weather Station (JMA) also operated one seismic station, and is under close contact with UVO.

The following are the main observational results:

1. Seismicity is lowest (several ten events per year) among the volcanoes which were continuously monitored.

2. Precise levelling survey on the southwestern flank revealed subsidence of the edifice. This observation is the reversed one expected from the simple "inflation-deflation model". Expansion of summit crater detected by EDM measurements also disagree with the levelling results.

3. Although the volcano is calm, a few unrests still experienced; the 1983 minor swarm apparently associated with the M7.7 tectonic earthquake, and a few tremor-like bursts in 1989-1990.

IV. TOKACHIDAKE (Mt. TOKACHI)

Most of modernized observation system in Hokkaido is operated by UVO at Tokachi, although station coverage is very poor, due to the natural environments. Six seismic stations are presently linked to UVO by telephone telemetry via FKA, the in-situ key station. A Tokachidake-Konsoku-Koudou (TKC), a 30 m long observational vault, was constructed about 2 km northwest from the main crater.

At TKC, short-period and long-period seismometers, watertube tiltmeter, silica-tube extensometer are installed. A low-frequency microphone is also operated at BGK station for detecting an explosive eruption. Those telemeter data gave us extremely valuable informations about precursors and the eruption mechanisms. Measurements of EDM, GPS, temperature, precise gravity are repeated periodically.

The 1926 eruption produced a snow melt mud flow and that resulted to 144 death toll. The tragic memory together with the recent Nevado del Ruiz disaster let the people and officials prepare well in advance to the 1988-1989 eruptions. Mud flow detection systems, evacuation bridges and roads, evacuation platform, artificial mudflow channel, various Sabo dams, and radio operated information systems are the examples of the preparations.

Tokachidake Volcano Observatory of JMA also operated 3 seismic stations and conducts observations of air wave and visual/infrared images. It is under close contact with UVO.

The following are some of the observational results:

1. Detection of long-term (years) and intermediate-term (weeks) precursors to the 1988-1989 eruption; seismic swarm tremors, migration of hypocenters, mud ejection, natural ignition of sulfur, geothermal and fuming anomalies.

2. Discrimination of explosive eruption during night in the cloud based on the combined observation of seismic waves and air waves.

3. Immediate precursors; small low-frequency events consist of small Rayleigh wave burst and increased its number tens of minutes before the explosive eruption. About one third of explosions are this type.

4. Systematic precursory change of extensometer record which is consistent with shallow pressure source in a vent.

5. Study of pyroclastic producing eruptions with seismic, air wave, and geodetic data.

6. Various kinds of volcanic tremors and their possible origins. Harmonic tremor and low-frequency monochromatic tremor only occurred during the activity building stage or before the small "magmatic" eruption.

V. MEAKANDAKE

Four temporary seismic stations are presently operated by UVO. Air waves are also monitored at the key station. Measurements of EDM, GPS, and precise gravity are also conducted. Under the 4th National Project, the observation is planned to transfer to the permanent observation, instead of temporary one. Kushiro District Meteorological Observatory (JMA) is operating one seismic station, and is under close contact with UVO.

The following are some of the observational results.

1. Based on the JMA observation since 1973, activity after 1987 is moderately high; with intermittent seismic swarms, and minor eruptive episode in 1988.

2. Seismic swarm consisting of a slowly decaying coda with monochromatic spectral characteristics occurred intensively before the recent minor eruption in 1988.

3. Existence of deep low-frequency events, that are never observed at the other 4 volcanoes in Hokkaido.