Deep crustal earthquake swarm and complex seismicity migrations in northern Yamaguchi, southwestern Japan

This study focuses on a deep crustal earthquake swarm that has been occurring since February 2025 in northern Yamaguchi, southwestern Japan. The swarm is located at depths of 25–35 km, approximately 20 km deeper than typical earthquake swarms in Japan. Because direct geophysical observations in the lower crust are limited, deep crustal earthquake swarms provide an important observation for investigating seismogenesis in the deep crustal environment.

We performed hypocenter relocation of 3,886 earthquakes with M ≥ 0.0 that occurred between February and June 2025, using initial hypocenters from the Japan Meteorological Agency. The 3,871 relocated hypocenters show two north–south–aligned planar clusters over a spatial scale of ~5 km. The two clusters are separated by a ~1 km-wide low seismicity zone. Within each cluster, hypocenters are heterogeneously distributed, with localized dense and sparse regions.

To grasp the spatiotemporal characteristics in the swarm, we first investigated seismicity migrations along the strike and dip directions of each planar cluster. In both clusters, seismicity exhibits an overall migration from deeper to shallower areas. Along strike, migration is not simple but shows zigzag-like fluctuations, whereas along dip, seismicity typically migrates upward and downward at a rate of ~1 km/day.

Because seismicity migration appears to be active at ~14 days intervals, we qualitatively compared the timing of seismicity migration with tidal normal stress variations on the fitted planes. As a result, we found that periods of large temporal variations in normal stress, particularly between low tides, corresponded to episodes of seismicity migrations.

We further analyzed the multiple seismicity migrations. To evaluate multiple migration episodes, we treated each earthquake as a spatiotemporal origin and analyzed subsequent events within a fixed time window of several days. Almost all of the migration sequences can be explained by an isotropic pore-fluid pressure diffusion model. Estimated diffusivities of diffusive migrations range from 1.0 to 5.0 m²/s, comparable to values reported for volcanic earthquake swarms.

The Moho depth beneath the swarm area is estimated to be ~40 km, indicating that the swarm occurs in the lower crust just above the Moho. The swarm is located just beneath the Abu monogenic volcano group, where petrological studies suggest partial melting of the lower crust. Considering that the most recent eruption occurred ~8,800 years ago, fluids separated from a magma reservoir may persist and migrate upward. The complex deep to shallow seismicity migration, relatively large diffusivities, and tidal modulation suggest that this deep crustal earthquake swarm is driven by highly pressurized, low-viscosity fluids moving through a structurally and hydraulically heterogeneous swarm area in the lower crust, particularly during periods of tidal normal stress variations.