Long-Period Ground Motion Characteristics in Osaka Basin, Western Japan, from Strong Motion Records of Large Earthquakes

Abstract eng:
Site-specific ground motion characteristics such as horizontal-to-vertical Fourier amplitude spectral ratio (HV) of seismograms have been commonly used to study underground velocity structure beneath each site. In this paper, we studied the relation between the long-period (3-20 sec) ground motion characteristics and the three-dimensional (3D) underground velocity structure in the Osaka sedimentary basin, western Japan, using HV of seismic records from large earthquakes and an existent 3D basin velocity structure model. First we demonstrated that HV of coda-part of the seismic records from eleven events with various azimuth and depth had site-specific stable values, independent of events. Then we compared the observed HV with 1DHV and 3DHV, where 1DHV is the theoretical ellipticity of the fundamental mode of Rayleigh wave obtained from one-dimensional (1D) velocity structure beneath each site and 3DHV is the HV obtained from a 3D ground motion simulation of a M6.5 event. Comparison of the peak periods of HV and 1DHV did not correspond with that of HV and 3DHV at several deep sedimentary stations. The spatial distribution of 1DHV peak and 3DHV suggested that 3DHV is affected by two- or three-dimensional structure of the model. These results emphasized the importance of 3D waveform modeling for constructing a reliable basin velocity structure model.

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