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Improving the constraint on the Mw 7.1 2016 off-Fukushima shallow normal-faulting earthquake with the high azimuthal coverage tsunami data from the S-net wide and dense network: Implication for the stress regime in the Tohoku overriding plate
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  • Tatsuya Kubota,
  • Hisahiko Kubo,
  • Keisuke Yoshida,
  • Naotaka Yamamoto Chikasada,
  • Wataru Suzuki,
  • Takeshi Nakamura,
  • Hiroaki Tsushima
Tatsuya Kubota
National Research Institute for Earth Science and Disaster Resilience

Corresponding Author:[email protected]

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Hisahiko Kubo
National Research Institute for Earth Science and Disaster Resilience
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Keisuke Yoshida
Tohoku University
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Naotaka Yamamoto Chikasada
National Research Institute for Earth Science and Disaster Resilience
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Wataru Suzuki
National Research Institute for Earth Science and Disaster Resilience
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Takeshi Nakamura
Central Research Institute of Electric Power Industry
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Hiroaki Tsushima
Meteorological Research Institute
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Abstract

Tsunamis with maximum amplitudes of up to 40 cm, related to the Mw 7.1 normal-faulting earthquake off Fukushima, Japan, on November 21, 2016 (UTC), were clearly recorded by a new offshore wide and dense ocean bottom pressure gauge network, S-net, with high azimuthal coverage located closer to the focal area. We processed the S-net data and found that some stations included the tsunami-irrelevant drift and step signals. We then analyzed the S-net data to infer the tsunami source distribution. A subsidence region with a narrow spatial extent (~40 km) and a large peak (~200 cm) was obtained. The other near-coastal waveforms not used for the inversion analysis were also reproduced very well. Our fault model suggests that the stress drop of this earthquake is ~10 MPa, whereas the shear stress increase along the fault caused by the 2011 Tohoku earthquake was only ~2 MPa. Past studies have suggested that horizontal compressional stress around this region switched to horizontal extensional stress after the Tohoku earthquake due to the stress change.The present result, however, suggests that the horizontal extensional stress was locally predominant at the shallowest surface around this region even before the 2011 Tohoku earthquake. The present study demonstrates that the S-net high-azimuthal-coverage pressure data provides a significant constraint on the fault modeling, which enables us to discuss the stress regime within the overriding plate around the offshore region. Our analysis provides an implication for the crustal stress state, which is important for understanding the generation mechanisms of the intraplate earthquake.
Oct 2021Published in Journal of Geophysical Research: Solid Earth volume 126 issue 10. 10.1029/2021JB022223