Preliminary Evaluation of Site Amplification From Strong Motion Records of the 2011 Tohoku, Japan Earthquake

Abstract eng:
The 2011 Tohoku, Japan earthquake of Mw9.0 produced many strong motion records. The records provide a unique opportunity to examine site amplifications in strong shaking level. The available site data for the records, however, are limited. To strengthen the site data, microtremor measurements are conducted at 467 strong motion sites in the high seismic intensity area of the earthquake where the peak accelerations on rock sites are averagely as high as 0.45 g. As the H/V spectral ratios of microtremors show different shapes for each NEHRP site class, the criteria are determined for the NEHRP site classification from the H/V spectral ratio. By using the criteria, the strong motion sites in the high intensity area are classified into site classes B, C, D and E sites. As the standard deviation of the peak accelerations on rock sites in the area is relatively small, the shaking level inputted to the bedrock in the area may not have large difference. Therefore, the site amplification at strong shaking level is preliminarily evaluated from the ratio of the average spectrum at site class C, D or E with respect to that at class B. The amplifications of site classes C and D to B are almost two, and rather similar to each other. The amplification of site class E to B, however, is larger at longer periods and smaller at short periods, showing stronger site effects. The amplifications are compared with those derived from weak motion records. The difference in both amplifications indicates effects of nonlinear soil response at strong shaking level.

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