Estimation of three dimensional ground structure in Fukui Plain based on inversion analysis of observed ground motion

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During the 1948 Fukui earthquake, most of the heavily damaged areas concentrated in the Fukui plain covered with thick quaternary deposits. This suggests that the sedimentary structure can be one of the most important factors to elucidate the damage distribution. Firstly, I propose an inversion procedure of underground structure based on the earthquake observation records, assuming a multi-reflection of vertically propagating SH-wave in a horizontally layering system. The procedure is formulated as an optimization problem to find the thickness and Q-values of each layer which minimize the different between calculated and measured Fourier spectra. Applying this inversion procedure to the actual strong earthquake records observed at the stations deploying in the center of Fukui plain, I calculated 2D S-wave velocity structure beneath the plain up to the tertiary. It was confirmed that the identified underground structure give a fairly good agreement between monitored and calculated responses, and that estimated structure is harmonized with PS-logging data and P-wave reflection survey. Secondary, 3-dimensional Quaternary structure around Fukui plain was explored based on the microtremor observation. The microtremor observations on the ground surface were carried out every small grid that covered the Fukui pain. Predominant periods and H/V ratios of each site were specified from collected Fourier and H/V spectra. By assuming S wave velocity structures, the thickness of Quaternary system of the sites were estimated based on the predominant periods. By applying the procedure to the sited having little ground condition data, one may be able predict the strong earthquake response with a good accuracy.