Spatiotemporal Variations of Shallow Very Low Frequency Earthquake Activity Southeast Off the Kii Peninsula, Along the Nankai Trough, Japan.

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 the 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. Plain Language Summary: Our knowledge of the physical characteristics of shallow slow earthquakes, which may provide frictional properties on plate boundaries and preparation processes of large interplate earthquakes, is still limited. Thus, we comprehensively analyzed shallow very low frequency earthquakes (VLFEs) southeast off the Kii Peninsula, in the Nankai subduction zone, where large tsunamigenic earthquakes have repeatedly occurred with intervals of approximately 100–200 yr. We found that shallow VLFEs occurred around the western edge of the subducted oceanic ridge. We considered that this ridge causes complex stress and subsurface structures around the plate boundary and consequently promotes slow earthquake activity. Key Points: We estimated the locations and moment rate functions of shallow very low frequency earthquakes (VLFEs) that occurred near the Nankai TroughLarge cumulative moments of shallow VLFEs were concentrated in the region with the subducted oceanic ridgeThe migration of shallow VLFEs exhibits various patterns in terms of initiation, migration direction, and area [ABSTRACT FROM AUTHOR]