Prolonged Hypocenter Migration in a Lower‐Crustal Earthquake Swarm in Japan: Positive Feedback Between Heat Influx and Fluid Production

Abstract The Yamaguchi earthquake swarm in western Japan occurred at unusually deep depths (25–40 km), well below the seismogenic zone, and exhibited prolonged zigzag hypocenter migrations over at least ∼180 days. Seismicity initiated in the southwestern part of the swarm area, followed by systematic migration eastward, northeastward, and northwestward. The swarm is confined in an area of ∼10 × 10 km, but a total distance of small‐scale zigzag hypocenter migrations reaches ∼160 km in the period from February 1 to 31 July 2025, with an average velocity of ∼0.9 km/day. Spectral analyses indicate that low‐frequency earthquakes are relatively enriched in low‐frequency energy but form a continuum with regular earthquakes. Fifteen deep earthquakes occur in the early stage of the swarm activity at ∼72 km depth below the swarm area. We propose that high‐temperature fluids ascending in the mantle wedge cause the deep earthquake cluster on their pathways, which eventually infiltrate pre‐existing fault zones in the lower crust. Substantial heat flux, carried by high‐temperature fluids and injected into the lower crust, induces dehydration of hydrous minerals along fault zones, supplying aqueous fluids, reducing the frictional strength, and triggering brittle failure even in the lower crust. We interpret that positive feedback between externally injected high‐temperature fluids and internally produced aqueous fluids sustains long‐term zigzag hypocenter migrations. These interpretations would highlight the potential role of high‐temperature fluid migrations and resultant dehydration reactions along fault zones for triggering and sustaining lower‐crustal earthquake swarms in regions normally dominated by ductile deformation.