Spatiotemporal variations of shallow very low frequency earthquake
activity southeast off the Kii Peninsula, along the Nankai Trough, Japan
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. This finding indicates that the
subducted ridge causes heterogeneous structures and stress conditions
that promote 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. According
to our results and the geometry of the plate boundary, we conclude that
the subducted ridge also plays an important role in the activity area of
shallow VLFE episodes. Different patterns of shallow VLFE migration
could reflect temporal changes in the pore-fluid distribution or stress
conditions of the plate boundary.