Subsurface S-wave velocity structures in the middle part of Ibaraki Prefecture, estimated by seismic analyses using aftershock observation data and microtremors observation data of the 2011 off the Pacific coast of Tohoku Earthquake —Focusing on the Higashi-Ibaraki Terrace—

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  • Tsuno Seiji
    Tokyo Institute of Technology, Interdisciplinary School of Science and Engineering Presently: Railway Technical Research Institute, Disaster Prevention Technology Division
  • Chimoto Kosuke
    Tokyo Institute of Technology, Interdisciplinary School of Science and Engineering
  • Yamanaka Hiroaki
    Tokyo Institute of Technology, Interdisciplinary School of Science and Engineering

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Other Title
  • 東北地方太平洋沖地震の余震観測記録と微動観測記録の解析から推定された茨城県中部に於けるS波速度構造　—東茨城台地に注目して—
  • 東北地方太平洋沖地震の余震観測記録と微動観測記録の解析から推定された茨城県中部に於けるS波速度構造 : 東茨城台地に注目して
  • トウホク チホウ タイヘイヨウオキ ジシン ノ ヨシン カンソク キロク ト ビドウ カンソク キロク ノ カイセキ カラ スイテイ サレタ イバラキケン チュウブ ニ オケル Sハ ソクド コウゾウ : ヒガシイバラキ ダイチ ニ チュウモク シテ
  • Subsurface S-wave velocity structures in the middle part of Ibaraki Prefecture, estimated by seismic analyses using aftershock observation data and microtremors observation data of the 2011 off the Pacific coast of Tohoku Earthquake —Focusing on the Higashi-Ibaraki Terrace—

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Abstract

 The 2011 off the Pacific coast of Tohoku Earthqauke (Mw 9.0) was occurred in the plate boundary of the Pacific Plate subducting beneath Tohoku-Japan. Mito and Hitachi, Kasama which are located in the northernmost-east of the Kanto Basin, had the seismic intensity larger than 6, due to the short distance from the southern edge (off Ibaraki) of the earthquake source fault. Especially in Hokota, Ibaraki Prefectre, the seismic intensity of 6.4 and the maximum acceleration of 1762 gal were observed. However, subsurface S-wave velocity structures in the middle part of Ibaraki Prefecture was not obtained quantitatively, because of less geophysical data of velocity structures up to the seismic bed rock, which is the important factor to evaluate and predict strong ground motions. In this study, to quantitatively estimate subsurface S-wave velocity structures and evaluate strong ground motion characteristics in the middle part of Ibaraki Prefecture, especially the Higashi-Ibaraki Terrace where the permanent seismic stations such as K-NET were not installed, we performed continuous aftershock observations of off the Pacific coast of Tohoku Earthqauke (Mw 9.0). By the inversion of both results from seismic analyses of aftershock data (H/V spectral ratios of coda waves) for long periods and continuous microtremors data (cross-correlations using the seismic interferometry method) for short periods, we estimated subsurface S-wave velocity structures up to the seismic bed rock (Vs 3.2 km/sec) in the Higashi-Ibaraki Terrace. In the distribution of dominant periods of H/V spectral ratios using microtremors data recorded by additional single-station microtremors observations, we could clearly confirm the boundary of the seismic bed rock gently dipping from the center of the Higashi-Ibaraki Terrace to the Pacific Ocean. The site amplification factors obtained by the aftershock data could explain those theoretically calculated using the estimated S-wave velocity structures as to the peak periods.<br>

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