Estimation of Site Amplification near KiK-net Haga Station from Observation of Aftershocks of 2011 off the Pacific Coast of Tohoku Earthquake and Microtremor Explorations

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  • 2011 年東北地方太平洋沖地震の余震観測と微動アレイ観測によるKiK-net 芳賀観測点周辺での地盤震動特性の評価


Site amplifications around the KiK-net strong motion station at Haga (TCGH16) in Tochigi prefecture were investigated from earthquake observation and microtremor array exploration in order to understand the characteristics of the strong ground motion recorded during the 2011 off the Pacific coast of Tohoku Earthquake with a seismic intensity of 7 in the intensity scale by Japan Meteorological Agency. We temporarily installed 8 seismometers within 2km around the strong motion site to obtain aftershock records in a few days in May, 2011. The spectral ratios at each site to that of a reference site were derived from the aftershock records. They are commonly characterized by an existence of a dominant peak at periods of 0.2 to 0.3 seconds. A similar dominant peak can be also identified in the weak-ground motion spectra at the TCGH16 station, suggesting significant effects of shallow soil layers. Then we revealed S-wave velocity profiles of the shallow soils down to a depth of about 30 meters over the engineering bedrock having an S-wave velocity of 400 m/s from the microtremor array explorations at all the aftershock observation stations. The S-wave velocity profiles are not significantly different at the stations within several hundred meters around TCGH16. The similar dominant peaks can be seen in the one-dimensional amplification factors of S-waves calculated for the profiles of the shallow soils at the aftershock stations around TCGH16. It is clearly indicates that the strong ground motion records at the KiK-net Haga station during the main shock can be regarded as representative one in the surrounding area.


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