Permalink : http://hdl.handle.net/2115/44476

JaLCDOI : 10.14943/gbhu.73.217

The largest aftershock（Mw 7.3）of the 2003 Tokachi-oki earthquake（Mw 8.3）generates larger peak ground acceleration（PGA）values than the main shock in the western side of the epicenters despite their magnitude difference. We study the features of strong ground motions from the largest aftershock in detail using PGA values. Spatial distribution maps of PGA ratio between the largest aftershock and adjacent earthquake shows obvious azimuth dependency compared with the other earthquake pairs occurring around Japan. Attenuation relationship of the largest aftershock shows a large azimuth-dependent scattering at distances from 200 to 300 km compared with three smaller earthquakes occurring around the largest aftershock. Because this large scattering appears only in the largest aftershock, we conclude that the azimuth dependency is a peculiar feature of the largest aftershock. Fourier spectral ratios between the largest aftershock and adjacent earthquake at several stations show azimuth dependency at higher frequencies than 1 Hz, which is consistent with the azimuth dependency of PGA values. The azimuth dependency in high-frequency range cannot be explained simply by the source and path effects, and therefore, we make a hypothesis that the azimuth dependency is an apparent one generated by two earthquakes occurring simultaneously at different regions. Synthetic PGA distributions generated by two simultaneous events calculated by the empirical attenuation relationship are in agreement with the observed PGA distribution of the largest aftershock.