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The elusive role of a deep-seated low-angle normal fault activated by the Mw 9.0 Tohoku-Oki megathrust in the triggering of a normal faulting earthquake sequence in northeast Japan
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  • Yohai Magen,
  • Asaf Inbal,
  • Alon Ziv,
  • Gidon Baer,
  • Roland Burgmann,
  • Axel Periollat,
  • Takeshi Sagiya
Yohai Magen
Department of Geosciences, Tel-Aviv University

Corresponding Author:[email protected]

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Asaf Inbal
Department of Geophysics, Tel Aviv University
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Alon Ziv
Department of Geosciences, Tel-Aviv University
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Gidon Baer
Geological Survey of Israel
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Roland Burgmann
University of California, Berkeley
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Axel Periollat
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Takeshi Sagiya
Nagoya University
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Although deep-seated blind normal faults are common in subduction environments, their rheology, kinematics and interaction with the upper crust are poorly constrained. A month-long shallow normal faulting sequence in the Ibaraki-Fukushima prefectural border (IFPB), northeast Japan, which followed the Mw9.0 Tohoku-Oki earthquake (TOE) and culminated in the Mw6.7 Iwaki earthquake, provides a window into megathrust-to-normal fault interaction. Stress change calculations clearly indicate that the IFPB earthquake sequence cannot be explained in terms of direct triggering by the TOE co- and post-seismic slip. In quest for an alternative triggering mechanism, we analyzed post-TOE GNSS data from eastern IFPB. A key step in this analysis is the removal of the large-scale post TOE displacement field, after which a distinct highly-localized strain along the coastline becomes apparent. The accumulation of this strain was mostly aseismic, and migrated with time prior to the Iwaki earthquake in a manner that correlates well with post-TOE local seismicity. We attribute the pre-Iwaki earthquake strain accumulation to aseismic slip along low-angle seaward dipping blind normal fault, activated by the TOE. Stresses transferred by this slip episode accelerated the failure along the IFPB shallow normal faults. This indirect triggering of the Iwaki earthquake sequence by the TOE highlights the complexity of stress transfers in subduction environments.
16 Feb 2024Submitted to ESS Open Archive
16 Feb 2024Published in ESS Open Archive