有珠山における爆破地震動の観測 : マグマによる走時・伝播異常

概要

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A quarry blast was carried out at a site about 13km north of Usu Volcano, which erupted on Aug. 7, 1977 after 32 years of dormancy since the birth of Showa-Shinzan. The seismic observation of this artificial shock was made by 13 routine and 17 temporary stations among which 12 stations were aligned in fan-shooting observation behind Usu Volcano. At six stations behind Usu Volcano relatively late arrivals were observed. This suggests the existence of the anomalous region beneath the volcano. Not only molten magma body which may exist beneath the volcano, but also surface sediment at the observation site may have lower velocity and could provide the late arrivals. A precise gravity survey, however, indicates no significant variation in thickness of historical deposits in the area where the six stations were located. The seismic rays travelling to these six stations penetrate into the earthquake-free region surrounded by the hypocenters of Usu volcanic earthquakes. The region was estimated to have a diameter of about 700 m. In such a case, it may be concluded that the velocity in this aseismic region is decreased by about 35% in comparison with that in the surrounding region. If the velocity decrease is caused by high temperature in this region, the temperature of the magma body is estimated to be about 1000℃.
1980-07-01