Recently, a widespread and densely continuous-recording ocean-bottom seismograph network has been deployed in the Japan Trench subduction zone. Utilizing the offshore network data improves azimuthal station coverage for offshore earthquakes in the Japan Trench subduction zone. It has a potential to obtain centroid moment tensor (CMT) solutions more accurately than conventional analyses using onshore networks and a simple one-dimensional seismic velocity structure model. In this study, we conducted CMT inversion for subduction zone earthquakes that occurred between April 1, 2017, and March 31, 2024, with a moment magnitude range of 5.2–7.0. We used seismograms obtained from both the offshore and onshore networks. We calculated Green’s functions using a three-dimensional seismic velocity structure model. Our CMT solutions with thrust-type mechanisms mostly indicated depths and dip angles consistent with the plate interface. For earthquakes in the outer-rise region, our CMT solutions were characterized as normal-fault mechanisms. The joint use of the offshore and onshore networks reduced the estimation errors of the CMT solutions compared with the only use of the onshore network, although the optimal solutions were consistent. The dip angles for the thrust earthquakes determined by our analysis were more consistent with the dip angle of the plate boundary than those determined by conventional CMT analyses. Additionally, we found that the conventional CMT analysis could introduce a systematic bias in depth and magnitude determinations. This finding highlights the importance of an offshore seismograph network and a reliable seismic velocity structure model for CMT inversions.

We detected shallow very low frequency earthquakes (VLFEs) off the Cape Muroto and Kii Channel in the Nankai subduction zone and estimated their moment rate functions. Combining the new and previously estimated catalogs, we obtained the comprehensive catalog of shallow VLFE moment rate functions along the Nankai Trough. We defined the shallow VLFE swarms and investigated the scaling relationships of their cumulative moments, activity area, and durations in each region. Detected swarms were considered candidates for shallow slow slip events. A similar scaling relationship was observed between the cumulative moments and activity areas, irrespective of regions. It indicates similar stress drops in each region. However, the relationship between the cumulative moments and durations varied. This difference was explained by the along-strike variations in the faulting conditions of shallow slow earthquakes, such as material or hydrological properties.

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.