Surface rupture associated with the April 11, 2011 Fukushima-ken Hamadori earthquake and paleoseismic history of the causative fault

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  • Tsutsumi Hiroyuki
    Department of Geophysics, Graduate School of Science, Kyoto University
  • Toda Shinji
    Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University

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Other Title
  • 2011年4月11日に発生した福島県浜通りの地震の地震断層と活動履歴
  • 2011ネン 4ガツ 11ニチ ニ ハッセイ シタ フクシマケンハマ ドオリ ノ ジシン ノ ジシン ダンソウ ト カツドウ リレキ
  • Surface rupture associated with the April 11, Fukushima-ken Hamadori earthquake and paleoseismic history of the causative fault

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Abstract

The devastating 11 March 2011 Mw 9.0 Tohoku-oki earthquake ruptured a 500-km-long and 200-km-wide plate interface between the North American and Pacific plates along the Tohoku coast of northeast Japan. The large source area and huge amount of slip (up to 50 m) of this earthquake caused a drastic change in the crustal stress of northeast Japan and triggered widespread seismic activity. Iwaki City at the southeast corner of Fukushima Prefecture is an example of an area that has experienced significantly elevated seismic activity since the 11 March 2011 earthquake. The most striking characteristic of seismicity in this area is that the majority of the earthquakes have normal faulting mechanisms, contrasting significantly with the geodetically, seismologically, and geologically identified E–W compressional stress field of northeast Japan. This suggests that the dominant E–W tensional stress regime in this area may be transient and related to coseismic and post-seismic eastward movement of the northeast Japan Arc. The largest Mw 6.6 normal faulting event (JMA magnitude 7.0) in this area occurred on 11 April 2011 and ruptured two previously mapped faults: the NW-trending Yunodake fault and the NNW-trending Itozawa fault. A clear ∼15-km-long surface rupture of the Yunodake fault and ∼14-km-long rupture of the western trace of the Itozawa fault appeared during the earthquake, with a normal down-to-the-west sense of slip. The maximum vertical offset on the Yunodake fault is 0.9 m, with the western trace of the Itozawa fault having a maximum vertical offset of 2.1 m. We suggest that the 11 April 2011 earthquake was caused by reactivation of these two sub-parallel normal faults, caused by transient extensional stress related to the 11 March 2011 mega-thrust earthquake. Paleoseismic trenching across the western trace of the Itozawa fault revealed that the penultimate surface-rupturing earthquake occurred sometime between 12,500 and 17,000 cal. yr BP, suggesting that the fault was not activated by the 869 Jogan earthquake, which may have been the last mega-thrust earthquake before the 2011 event.

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