Spatiotemporal variations of shallow very low frequency earthquake activity southeast off the Kii Peninsula, along the Nankai Trough, Japan

Takemura, S.1, Obara, K.1, Shiomi, K.2, and Baba, S.1
(2022). Journal of Geophysical Research: Solid Earth, 127 (3), e2021JB023073, doi:10.1029/2021JB023073
1ERI, Univ. Tokyo, 2NIED

 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 (Figure 1a), 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 (Figure 1b). Different patterns of shallow VLFE migration could reflect temporal changes in the pore-fluid distribution or stress conditions of the plate boundary.

Our paper can be downloaded from The estimated moment rate functions of shallow VLFEs southeast off the Kii Peninsula can be downloaded from

Figure 1. Spatial distributions of (a) the cumulative moments from 17-years shallow VLFEs and (b) the activity areas of shallow VLFE episodes southeast off the Kii Peninsula. The dashed line represents the Nankai Trough. The paleo-Zenisu ridge (Park et al., 2004) is illustrated by dotted line.