Analysis on constituents of amplification and propagation of strong ground motion based on precise underground structure data

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The aim of this research is to clearly understand the effects of local geology and/or underground structure model for estimating the earthquake ground motion by using the strong motion records and by carrying out several geotechnical measurements near the observation site.A very high acceleration of 1.6g was observed during the largest aftershock of 1993 Hokkaido Nansei-Oki earthquake at Otobe-cho, south-west coast of Hokkaido. In order to find the reasons of the very high acceleration, we carried out weak-motion array observation, small scale refraction survey and P-S logging using shallow borehole. Combining these data, we could understand as following :1) Shallow sediments overlaying very hard rock contribute to amplification of ground motion as much as 40 times at resonance frequency respect to rock site.2) Topographical effects are negligible.3) Nonlinearity of sediments at Otobe was very slight even in severe shaking, on the other hand, significant decrease of high frequency which implies nonlinear effects at deep sediment site (Kamino kuni) was found. The complexity of soil during an earthquake should be investigated using strong motion records.Propagation of long period strong ground motion depends on 2 or 3 dimensional underground structure. We examined this effects using strong motion records obtained in and around Kanto plain from the 1990 Izu-Oshima Kinkai earthquake and by numerical simulation for surface wave propagation in laterally heterogeneous media.