抄録
A geodetic observation network with GPS and EDM at Asama volcano detected pre-and co-eruptive crustal deformation. Observed horizontal displacements by GPS before the 2004 eruption indicated radial pattern outward from the active summit crater. An east-west baseline indicated secular extension with 7mm/yr from 2002 to the occurrence of the 2004 eruption. Rapid extension of 10mm was detected at the baseline near the summit, which may suggest magma discharge into the shallower chamber. We estimated the magma supply system from the observed geodetic data assuming buried two pressure sources. One is at a height of 2200m above sea level beneath the summit crater (Source A) and another is 1.5km east of the summit at sea level (Source B). Our data indicated that before the first eruption discharged magma from the deeper to source B was 200×103m3, and from source B to source A was 14×103m3. Total amount of increase in volume of source A was estimated to be more than 50×103m3 by EDM and GPS data by the end of the eruptive activity. These small but significant signals could be observed only in our network near the crater. Our results indicated that dense geodetic observation near the active crater or vent could give important and interesting data to clarify and monitor the active volcano.
