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2019 M6.7 Yamagata-Oki earthquake in the stress shadow of 2011 Tohoku-Oki earthquake: Was it caused by the reduction in fault strength?
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  • Keisuke Yoshida,
  • Naoki Uchida,
  • Satoshi Hiarahara,
  • Takashi Nakayama,
  • Toru Matsuzawa,
  • Tomomi Okada,
  • Yoshiaki Matsumoto,
  • Akira Hasegawa
Keisuke Yoshida
Tohoku University, Tohoku University

Corresponding Author:[email protected]

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Naoki Uchida
Tohoku University, Tohoku University
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Satoshi Hiarahara
Tohoku University, Tohoku University
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Takashi Nakayama
Tohoku University, Tohoku University
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Toru Matsuzawa
Tohoku University, Tohoku University
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Tomomi Okada
Tohoku University, Tohoku University
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Yoshiaki Matsumoto
Tohoku University, Tohoku University
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Akira Hasegawa
Tohoku University, Tohoku University
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Abstract

Earthquake occurrence in the stress shadow provides a unique opportunity for extracting the information about the physical processes behind earthquakes because it highlights processes other than the ambient stress change in earthquake generation. In this study, we examined the fault structure and the spatiotemporal distribution of the aftershocks of the 2019 M6.7 Yamagata-Oki earthquake, which occurred in the stress shadow of the 2011 M9.0 Tohoku-Oki earthquake, to better understand the earthquake generation mechanism. Moreover, we investigated the temporal evolution of the surface strain rate distribution in the source region by using GNSS data. The earthquake detection and hypocenter relocation succeeded in delineating three planar structures of earthquakes. The results suggest that individual aftershocks were caused by a slip on the macroscopic planar structures. Aftershock hypocenters rapidly migrated upward from the deeper part of the major plane (fault) similar to the recent earthquake swarm sequences triggered by the 2011 Tohoku-Oki earthquake in the stress shadow in the upper plate. East–west contraction strain rate in the source region of the Yamagata-Oki earthquake with E–W compressional reverse fault mechanism changed to the E–W extension as a result of Tohoku-Oki earthquake, and it continued until the occurrence of the Yamagata-Oki earthquake. The upward hypocenter migrations, together with the earthquake occurrence in the stress shadow and in the E–W extension strain rate field, suggest that the reduction in the fault strength due to the uprising fluids contributed to the occurrence of this earthquake sequence. Localized aseismic deformations, such as aseismic creeps, beneath the fault zone may also have contributed to the earthquake occurrence. The results support the hypothesis that aseismic processes in the deeper part of the fault play crucial roles in the occurrence of shallow intraplate earthquakes.
Oct 2020Published in Tectonophysics volume 793 on pages 228609. 10.1016/j.tecto.2020.228609