283110
(was 0802-11)
Aso Volcanological Laboratory
Kyoto University, Choyo-Mura
Aso-Gun, Kumamoto-Ken 869-14
Japan

Geophysical studies of Aso Volcano have been carried out by mainly Aso Volcanological Laboratory (AVL) of Kyoto University and Aso Weather Station of Japan Meteorological Agency (JMA). AVL was founded in 1928 at 7 km west of active craters of Mt. Nakadake which is one of the central cones. The seismic, geodetic and geomagnetic data are transmitted from stations in and around the active crater and in and around Aso Caldera to AVL.

SEISMOLOGICAL MONITORING

• around active craters

AVL established 11 satellite seismic stations around the active crater of Mt. Nakadake, and these data have been transported by 12 GHz micro-wave radio telemetry system to AVL. Volcanic earthquakes beneath the crater can be divided into two kinds, high frequency events and low frequency ones. The former events are dominated with higher than 10 Hz and occur in depths 0-3 km below sea level. The latter located in 2 km range of shallow depth directly beneath the active crater. Their source mechanisms are studied to be different before and after the eruptions. The changes in the amplitudes and the periods of volcanic micro-tremors are closely related to eruptive activities.

b) around the caldera

The telemetric seismic network of AVL around Aso Caldera consists of 8 stations. Several earthquake swarms have occurred in the northern and western regions of the caldera. Their fault plane solutions indicate that the faults are in the direction from Northeast to Southwest, and that the mechanisms are the combination of both normal-type and strike-slip type with the right lateral fault. Tension-axis maintains a horizontal north-south orientation. These evidences indicate that stress-field is characterised by north-southward extension and this situation is general throughout the central Kyushu.

GEODETIC MONITORING

In order to obtain high quality data of the ground deformation and to make a contribution to the prediction of eruptions, AVL has constructed the underground deformation observational vault in 1987 at about 1 km Southwest of the active crater. The vault was built under the ground in the depth of 30 m and was the shape of horizontal and equilateral right-angle triangle. The length of an equilateral side is 25 m. The observational instruments consist of two 25 m water-tube tiltmeters and two 20 m and one 25 m super-invar bar extensometers. Three 25 m laser type extensometers are also installed. In the vault, three components short period seismometers, three component long period seismometers and one STS2 type seismometer are installed. Precise levelling surveys and gravity variation measurements have been made repeatedly along the route from the foot of the central cones to the active crater.

From the gravity surveys, the negative concentric Bouguer anomalies were observed inside the caldera.

From the end of 1994, the superconducting gravity meter was installed at the vault. The gravity data have been recorded continuously.

GEOMAGNETIC STUDIES

The continuous observation of geomagnetic field are made by means of five proton precession magnetometers and two flux-gate magnetometers at the stations in and around the caldera. Since 1979 JMA started the routine observation with proton magnetometers at two points near the crater. Two lines of the ground resistivity variometers were installed across the crater.

GEOTHERMAL STUDIES

Geothermal surveys inside the active crater of Mt. Nakadake were made several times by means of the airborne infrared method by AVL, JMA and GSI (Geographical Survey Institute).

Areas of higher temperature inside the active crater increase in close connection with the volcanic active stages. AVL constructed two drill holes for continuous temperature measurement. The holes are at a point about 200 m west of the active crater. Their depth are 150 m and 70 m. The variation of temperature at the deepest (150 m depth) is closely related to eruptive activities.