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. 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.

Fluid Migration Before and During Slow Earthquakes in the Shallow Nankai Subduction Zone

“…Light‐blue triangles show the locations of the dense ocean floor network system for earthquakes and tsunamis stations for nodes KMB, KMC and KMD (b)–(f) Same as (a), but for Events 2–6. Purple and light‐brown circles in (c) and (f) show shallow very low frequency earthquake locations during 2015 (Nakano et al., 2016, 2018) and December 2020–January 2021 (Takemura et al., 2021).…”

“…More details for each event are described in the text and Table S1 in Supporting Information S1. Pink and light‐brown regions are the periods of slow earthquakes that are estimated from the borehole pore‐pressure measurement (Araki et al., 2017; Ariyoshi et al., 2021) and the shallow very low frequency earthquake (sVLFE) activity (Takemura et al., 2021), respectively. Periods from first CC drop and subsequent slow earthquake activities are noted.…”

“…The catalog of shallow very low frequency earthquake in Takemura et al. (2021) can be obtained from https://doi.org/10.5281/zenodo.5211090. The data set in dv/v and CC for each pair of two stations obtained in this study (Figure 3 and Figures S4–S6 in Supporting Information S1) and the data for creating the figures can be downloaded through a repository (https://doi.org/10.5281/zenodo.5637605).…”

Source Characteristics and Along‐Strike Variations of Shallow Very Low Frequency Earthquake Swarms on the Nankai Trough Shallow Plate Boundary

“…After detection and relocation (gray circles in Figure 1), we estimated the moment rate functions of the shallow VLFEs in Region C, which were constructed using a series of 6‐s Küpper wavelets. The weights of each Küpper pulse were estimated using a Monte‐Carlo‐based simulated annealing method (Takemura et al., 2022). In our previous work (Takemura et al., 2022), we already estimated moment rate functions of shallow VLFEs in Regions A and B.…”

“…Shallow VLFEs in regions A and B are from Takemura et al. (2022). Shallow VLFEs in region C are from this study.…”