臨時稠密地震観測で見えてきた有珠山の微小地震活動

概要

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We performed temporary dense observation at Usu volcano in summer 2006, aiming toreveal micro-earthquake activity under the summit crater during dormant period of thevolcanic activity. During about 80 days of temporary observation, more than 330 volcanicearthquakes were observed. Of these earthquakes, well-recorded 142 events are selected forrelocation analysis assuming the 3D-velocity structure model which is a result of theexplosion seismic experiment at Usu volcano in 2001. Following the result of relocation, weestimated focal mechanism of 7 earthquakes whose magnitudes are more than 1.0. Thetemporal observation and analyses gave us three new insights about seismic activity; 1)Continuous activity of volcanic earthquake is seen at the shallow part under the southerncrater floor, 2) Hypocentral regions are distributed along the SW end of the U-shaped faultwhich was built in the 1977-1982 eruption associated with the uplift of Usu-Shinzan cryptdome, 3) Strike direction of the 7 major earthquakes are almost parallel to the U-shaped fault,and dominant mechanism is dip-slip type which suggests subsidence of summit domesrelative to the southern crater floor.