Stress Field and Mantle Flow Patterns in the Ryukyu Subduction Zone From a New Moment Tensor Catalog

Abstract

We present a new regional Ryukyu Area Centroid Moment Tensor (RACMT) catalog that documents detailed transitions in deformation pattern and geodynamic processes across the Ryukyu subduction system. Regional CMT solutions for events with magnitudes greater than 4.0 are determined through full-waveform inversion with systematically explored parameters to ensure robustness. The resulting focal mechanisms are broadly consistent with GCMT, while centroid depths for shallow earthquakes are significantly improved. Stress regimes and deformation patterns are characterized using K-means clustering–based stress inversion and analysis of principal stress axes. Within the subducting Philippine Sea Plate (PSP), slab bending produces normal faulting in the eastern segment, transitioning westward to strike-slip faulting driven by compression parallel to oblique convergence at the western Ryukyu Trench. In the overriding plate, two trench-parallel seismic belts exhibit mutually perpendicular extension, corresponding to opening of the Okinawa Trough and stretching of the Ryukyu Arc. Furthermore, widespread lateral extensional deformation is identified throughout much of the deep PSP. These observations are consistent with enhanced trench retreat and slab rollback toward the western Ryukyu margin. We propose that efficient toroidal mantle flow around the western slab edge makes a major contribution to the westward increase in back-arc opening rate, fore-arc stretching, and widespread lateral extension within the deep PSP.