Tectonics of earthquake swarms in the Tokara Gap of northern Ryukyu Arc based on marine geological and geophysical surveys

Abstract

Abstract The 2025 Tokara seismic swarm, which began in late June, has been the largest documented sequence of earthquakes in the northern Ryukyu Arc. It has generated more than 2200 earthquakes, including episodes of strong ground-shaking at Akusekijima Island. The mechanisms responsible for generating repeated swarm activity in the Tokara Gap remain poorly understood. The most recent event, characterized by very frequent and intense shaking, rapidly amplified societal impacts. To investigate the tectonic framework of this area, we analyzed marine geological and geophysical datasets, including multibeam bathymetry, multichannel seismic profiles, marine magnetic anomalies, and Bouguer gravity anomalies. Seafloor morphology and seismic profiles revealed two intersecting extensional fault systems: a WNW–ESE set and an ENE–WSW set. Localized magnetic and topographic highs suggested shallow magmatism. Previous geochemical studies have indicated that hydrothermal plumes along the volcanic front, near the western swarm area, comprise a mixture of deep-sourced fluids and volcanic-front components. However, water-column surveys after the December 2021 swarm detected no persistent hydrothermal plumes. While the relative roles of magma and fluids remain uncertain, the evidence suggests that the 2025 swarm reflects processes typical of incipient back-arc rifts, where episodic magmatic intrusion and fluid overpressure interact.