Very Low-Frequency Earthquakes and Earthquake Swarms in the Northern Ryukyu Trench

Slow earthquakes, distinct from regular earthquakes in their gradual release of seismic energy, play a key role in stress accumulation and release at subduction plate interfaces and have been proposed as potential precursors to large earthquakes. In the northern Ryukyu Trench, the Philippine Sea Plate subducts beneath the Eurasian Plate, and the region is characterized by a few large earthquakes over the past several centuries, including the magnitude 8.0 event in 1911 near Kikai Island. Although interplate coupling and crustal heterogeneity in this area remain debated, the occurrence of shallow, very low-frequency earthquakes (VLFEs), often associated with slow-slip events (SSEs), highlights the need to clarify the current state of seismicity.

From September 2018 to June 2019, a broadband ocean-bottom seismometers (OBSs) network, supplemented by land-based stations, was deployed to detect and locate VLFEs in the northern Ryukyu Trench. We used the envelope correlation method (ECM) to determine VLFE epicenters. Rayleigh-wave group velocity was estimated at approximately 2.5 km/s for 0.05–0.1 Hz frequencies. Our results reveal that VLFEs predominantly cluster northeast of Amami Island and east of Okinoerabu Island, with minimal spatial overlap with areas exhibiting regular earthquakes. Recurring VLFE activity occurred in distinct locations along the trench, sometimes coinciding with earthquake swarms.

These findings indicate that VLFEs and regular earthquake seismicity in the northern Ryukyu Trench may be influenced by high-pressure fluids migrating along the subducting plate interface—similar to observations in other subduction zones such as Hikurangi. In particular, earthquake swarms that precede or follow VLFE swarms suggest a delayed triggering mechanism, potentially driven by fluid migration or stress changes related to SSEs. Our observations imply weak interplate coupling in the low-seismicity area (LSA) northeast of Amami Island. Although SSEs remain difficult to detect at offshore distances, studying the spatiotemporal distribution of VLFEs alongside regular seismicity offers valuable insight into the dynamic processes governing stress release, fluid migration, and fault mechanics in this subduction environment.