GROUND MOTION CHARACTERISTICS IN THE GRABEN-LIKE IRREGULAR UNDERGROUND STRUCTURE FOR FINITE MOVING SOURCE WITH DIFFERENT SLIP SIZE

摘要

Theoretical ground motion characteristics taking into account the variable slip source and the deep irregular underground structure are analytically investigated using hyperelement method. The 3D wave propagation analyses have been performed using the 2D graben-like irregular underground structure model of Kobe City. The ground motion characteristics are discussed for different asperity sizes. Ground motion amplification characteristics at sediment site to rock site are discussed for one-asperity models with different size and also for two asperities models. Three components, fault normal (FN), fault parallel (FP) and vertical (UD), of velocity waveforms and distributions of PGVs are investigated for different wavefields. The following findings are obtained. The amplitudes of the calculated ground motions in the FN direction are larger than the FP direction as recognized in the observed near-source ground motions. The FN/FP ratio becomes larger as slip size becomes smaller. The PGVs in the FN direction become large in the same range corresponding to the heavily damaged area. The vertical PGVs become large at the sediment site in the vicinity of the rock-sediment boundary. These results are the same as those for plane incident waves, deconvoluted from the observation records at Kobe University. The PGVs are larger in the small asperity case compared to the large asperity case when the total seismic moments are the same. The higher frequency contents are generated at the rock site and the higher resonant frequencies are stimulated at the sediment site, as the slip size becomes smaller. The calculated waveforms show strong forward directivity in the propagation direction due to a finite moving source. The shape of the PGV distribution for one asperity can be changed due to the interference of the wavefields for two asperities when they are closed.