Spatiotemporal Variability and Tectonic Implications of Very Low-Frequency Earthquakes in the Southwestern Ryukyu Trench

This study focuses on very low-frequency earthquakes (VLFEs) occurring in the southwestern Ryukyu Trench and investigates their long-term activity as well as their relationship with regional seismicity and tectonic features. VLFEs constitute a type of slow earthquakes characterized by surface waves with periods of 20–50 seconds. As they are detectable over long distances, VLFEs are suitable for effective monitoring of aseismic processes in offshore subduction zones with sparse seismic networks. Using waveform data from Japan’s F-net network and Taiwan’s Broadband Array in Taiwan for Seismology (BATS) network, we constructed two VLFE catalogs, namely the FB-catalog (2000–2024), which was based on both networks, and the B-catalog (1998–2024), which was based only on BATS. VLFEs were identified using waveform cross-correlation with template events.Results show that VLFE activity is concentrated along the trench axis at depths of 10–20 km, occurring periodically in swarm-like episodes approximately every 2–3 months. In some cases, these swarms exhibit migration. VLFE epicenters are spatially complementary to thrust-type earthquake zones, suggesting distinct regions of seismic and aseismic slip. VLFE clusters are distributed just south of areas characterized by recurring slow slip events (SSEs) and GNSS-inferred slip deficits. A clear contrast is observed across the subducted Gagua Ridge: Seismicity is more prevalent to the west, whereas VLFEs dominate to the east. VLFE activity has increased significantly since late 2001, temporally coinciding with moderate earthquakes and afterslip, suggesting the activation of slow slip processes or weakened interplate coupling near the trench axis. This finding provides new insight into the spatial distribution and physical mechanisms of aseismic slip along the plate interface—especially near the trench axis where direct geodetic observations are limited. VLFE monitoring can serve as a valuable tool for characterizing interplate coupling in offshore subduction zones.