3-D SIMULATIONS OF SURFACE-WAVE PROPAGATION IN THE KANTO SEDIMENTARY BASIN, JAPAN .2. APPLICATION OF THE SURFACE-WAVE BEM

The 3-D surface wave Boundary Element Method is applied to the same se t of strong motion records as those of Part 1 (Kato et al., 1993), nam ely, the long-period strong motion with periods around 8 sec recorded at the JMA (Japan Meteorological Agency) stations in Yokohama and Toky o during the 1980 Izu-hanto-toho-oki earthquake. This method simulates exact Love and Rayleigh waves in a laterally heterogeneous medium inc luding 3-D basin structures. The needed computer memory and CPU time a re made manageable by assuming the inside and outside of the basin to consist of horizontal flat layers. In addition, vertical interfaces ar e assumed between the basin and the outside bedrock for simplicity in this study. After checking the range of validity of the flat-layer app roximation for the Kanto sedimentary basin, the generation and propaga tion of surface waves in the basin are numerically investigated. In pa rticular, our investigation places special emphasis on the cause of th e observed long duration, which could not be explained in Part 1 using the surface wave Gaussian Beam method. When the bedrock outside the w estern margin of the Kanto basin is incorporated into the model, the o bserved long duration at the Tokyo station is successfully reproduced. This can be explained by the strong surface waves generated by the su rface wave to surface wave conversion at the margin and appearing in T okyo after the direct surface waves from the epicenter. This result ag rees with Kinoshita ef al. (1992), who studied records from an array o f seismographs in Tokyo. Moreover, when a sedimentary basin of Sagami Bay is included as was done in Part 1, the simulations are greatly imp roved, particularly at the Yokohama station, which corresponds to the node in the radiation pattern of the source. This is due to the fact t hat the radiation pattern is strongly affected by the near-source hete rogeneity of the Sagami basin. All these results, together with direct comparisons to 2-D modelings, clearly show the strong 3-D effects of the basin structures on the generation and propagation of surface wave s for this earthquake.