Velocity Structure Model for Tsukuba Area in Japan, Estimated from a Geological Model and H/V Spectral Ratio of Microtremors

Abstract eng:
We made a velocity structure model for Tsukuba area in Japan. The velocity model was based on a geological model and was developed by using microtremor surveys and its H/V spectral ratio analysis. The microtremor measurement was executed about 200 points over the Tsukuba Area. We used a technique of H/V spectrum ratio in the analysis of the data in order to model a velocity structure. The predominant periods estimated from H/V spectral ratios of microtremors become larger from north to south area. The estimated depth of seismic bedrock is approximately 800m at south part of Tsukuba area, and is 0-20m at north-east area where is near Mt. Tsukuba. From comparison between estimated depth and borehole data, we confirmed that the model is reasonable at all part of Tsukuba area. We also estimated the shallow structure that is from engineering bedrock to surface, based on the mean value of the geological layers by using an inversion technique. The theoretical H/V ratios of Rayleigh waves for the estimated velocity model were calculated at all observation sites, and were compared with those of microtremors. It was confirmed that the S wave velocity structural model for Tsukuba area is suitable for simulation of strong ground motion.

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