Stress-driven faulting with weak overpressure in the Tokara Islands, Ryukyu Arc: implications for fault reactivation under backarc extension

Abstract We investigate the shallow stress regime and pore fluid pressure conditions in the Tokara Islands region of the northern Ryukyu Arc, where active seismicity occurs in a complex tectonic setting characterized by backarc extension. Using focal mechanism data from the F-net catalog between 2000 and 2025 (3.0 < M w < 6.1, depth < 20 km), we conduct stress tensor inversion and estimate the pore fluid pressure ratio ( λ *) based on frictional faulting theory. Our results reveal a dominant normal-faulting stress regime with a NNW–SSE extension axis, consistent with the regional backarc tectonics. The estimated pore fluid pressure ratios are generally low ( λ * < 0.4 for 76% of events), indicating that seismic slip in this region can occur without significant fluid overpressure. We observe that larger-magnitude earthquakes tend to be associated with lower λ * values, suggesting that tectonic stress rather than elevated pore pressure primarily governs seismicity in this setting. These findings imply that normal-faulting related to backarc extension in the Tokara Islands is mainly controlled by regional stress accumulation and that fault zones may facilitate continuous fluid discharge rather than episodic overpressure buildup. The stress and fluid conditions identified here are consistent with a tectonic regime in which seismicity reflects mechanical fault behavior rather than fluid-induced weakening. This study provides new insights into the stress–fluid–fault interactions in an actively deforming backarc environment and contributes to understanding the seismic hazard in the northern Ryukyu Arc. Graphical Abstract