Permalink : http://hdl.handle.net/2115/44457

JaLCDOI : 10.14943/gbhu.73.57

Observed waveforms recorded by the high-density strong ground motion observation network from two intraslab earthquakes are analyzed in detail. One is the 2001 Iwate-ken Nairiku Nanbu earthquake（MJMA ＝ 6.4, depth ＝ 121.5 km）, which occurred in the Pacific slab subducting beneath the Tohoku island arc. The other is the 2006 Oita-ken Seibu earthquake（MJMA ＝ 6.2, depth＝ 145.2 km）, which occurred in the Philippine Sea slab subducting beneath the Kyushu island arc. In both cases, observed strong ground motions show singular spatial distribution, where high-frequency seismic waves at the stations located in backarc side of the volcanic front have much smaller amplitudes compared with those at the forearc side stations. This singular distribution of strong ground motions can be interpreted by considering attenuation of high-frequency seismic waves at the backarc side stations due to the wave propagation through the low-Q value zone beneath the volcanic front. In the case of the 2006 Oita-ken Seibu earthquake, effect of high-frequency seismic waves trapped in the high-Q slab is also important. It may cause the large amplitude of high-frequency waves observed at the stations in Chugoku and Shikoku, the shallower-slab side extent of forearc side.