HR: 0800h
AN: V11C-0747    [Abstracts]
TI: A Source Mechanism of Singular Long-Period Earthquakes Observed Before the 2004 Eruptions at Asama Volcano, Central Japan
AU: * Maeda, Y
EM: ymaeda@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Takeo, M
EM: takeo@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Ohminato, T
EM: takao@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AB: At 11:02 (GMT) on September 1, 2004, a moderate-sized summit eruption (VEI~2) occurred for the first time in the last 21 years. Before the eruption, long-period seismic signals with singular waveforms had been frequently observed at two summit stations (KAC2 and KAH2) at least since September 5, 2002. They have impulsive ground velocities with a dominant period up to 10 sec and repetitive outgoing initial motions from the vent. The signals were so feeble that we could hardly detect them even at the next closest station MAE, about 1.4km away from the summit, and thus, it was difficult to determine the location of their sources based on the conventional hypocenter determination method using travel times. The particle motions suggest that the sources are located to be relatively shallow, just beneath the bottom of the summit crater. In this study we tried to determine their source mechanism by waveform inversion. We prepared the Green's functions by the finite-difference method including 3-D topography of 10m-mesh DEM data, and used long-period band (0.05-0.25Hz) of velocity data in order to avoid error from the lack of structural information. In the inversion analyses, we can use waveform data at only 3 stations (KAC2, KAH2, and MAE): so the resolution of inversion result is poor in estimating 6 components of moment-tensor freely as superposition of short-time basis functions. Then, we made various kinds of inversions to find a possible range of solution: 6 components moment-tensor inversion with free source time function, 6 components moment-tensor inversion with fixed source time function, and fixed mechanism( sphere, cylinder, and crack) with free source time function. Source location and directions for cylinder and crack are derived by grid-search, while the other parameters are estimated by singular-value decomposition and sharp cut-off approach. The source location is estimated to be a bit southwest and about 0-100m below the bottom of summit crater. Source mechanisms derived from our various inversions almost commonly show EW-directed dipole to be prominent, suggesting this dipole component is an essential element in the source process of the singular earthquakes. Now we are trying to make a source model to explain the inversion result.
DE: 7280 Volcano seismology (8419)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting