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Spatiotemporal variations of shallow very low frequency earthquake activity southeast off the Kii Peninsula, along the Nankai Trough, Japan
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  • Shunsuke Takemura,
  • Kazushige Obara,
  • Katsuhiko Shiomi,
  • Satoru Baba
Shunsuke Takemura
Earthquake Research Institute, the University of Tokyo, Earthquake Research Institute, the University of Tokyo, Earthquake Research Institute, the University of Tokyo

Corresponding Author:[email protected]

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Kazushige Obara
Earthquake Research Institute, The University of Tokyo, Earthquake Research Institute, The University of Tokyo, Earthquake Research Institute, The University of Tokyo
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Katsuhiko Shiomi
National Research Institute for Earth Science and Disaster Resilience, National Research Institute for Earth Science and Disaster Resilience, National Research Institute for Earth Science and Disaster Resilience
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Satoru Baba
Earthquake Research Institute, the University of Tokyo, Earthquake Research Institute, the University of Tokyo, Earthquake Research Institute, the University of Tokyo
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Abstract

Cross-correlation analysis was applied to long-term onshore broadband records from April 2004 to March 2021 to detect and relocate shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula, along the Nankai Trough, Japan. We then determined the moment rate functions of detected shallow VLFEs using the Monte Carlo-based simulated annealing method. According to this new comprehensive catalog, shallow VLFEs are widespread beneath the accretionary prism toe, but shallow VLFEs with large cumulative moments are localized around the western edge of the paleo-Zenisu ridge, which is subducted beneath southeast off the Kii Peninsula. Our results from long-term shallow VLFE catalog are well consistent with previous studies in this region, suggesting that heterogeneous structures and stress conditions due to the subducted paleo-Zenisu ridge promote the occurrence of shallow slow earthquakes. The relocated shallow VLFE epicenters illustrated three major episodes characterized by a similar activity area and five minor episodes characterized by different areas. The three major episodes exhibited slow frontal migration with different initiation locations, directions, and speeds, as well as several rapid reverse migrations. Episodes of minor activity were distributed in different locations within part of the area of major activity. Different patterns of shallow VLFE migration could reflect temporal changes in the pore-fluid distribution or stress conditions of the plate boundary.
Mar 2022Published in Journal of Geophysical Research: Solid Earth volume 127 issue 3. 10.1029/2021JB023073