Migration of Multimodal Deep Crustal Earthquake Swarm Beneath the Abu Volcano Group, Japan

Abstract Earthquakes in the lower crust, where high pressures and temperatures favor ductile deformation, are rare but occasionally occur beneath active volcanic centers, providing insights into deep magmatic processes. We analyze a deep crustal earthquake swarm (February–August 2025) of over 5,000 events beneath the Abu Volcano Group, Japan. Using spectral characteristics, we distinguish deep low‐frequency earthquakes (DLFEs) from volcano‐tectonic events and identify several anti‐repeating pairs. All events are relocated with cross‐correlation and a double‐difference algorithm, revealing a complex multi‐cluster fault network comprising a steeply dipping conduit and sub‐horizontal fault strands. DLFEs migrate upward at ∼0.3 km/day along the conduit, suggesting fluid or magma ascent that triggers overlying brittle failure. Anti‐repeating earthquakes indicate small‐scale stress heterogeneity within adjacent fault patches. These observations elucidate the interplay between deep fluid migration, stress variability, and faulting mechanisms, providing a framework to interpret precursory deep volcanic seismicity and the structural controls governing deep crustal earthquake swarms.