[S-CG52] Inter-segment Tectonics: Interdisciplinary Research on Responses to Plate Subduction

[SCG52-07] Geodetic and seismic insights into seismogenic features and fault segmentation along the southern Ryukyu Trench

★Invited Papers

*YA-JU HSU¹, Pei-Ru Jian², Chi-Hsien Tang³ (1.Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, 2.Department of Geosciences, National Taiwan University, Taipei, Taiwan, 3.International Research Institute of Disaster Science, Tohoku University, Sendai, Japan)

Keywords:Ryukyu subduction zone, earthquake, segmentation, plate coupling

The Ryukyu subduction zone stretches 2,200 km from northern Taiwan to Kyushu, Japan, and is associated with the formation of the Okinawa Trough, a young back-arc basin. Along the Okinawa Trough, the rifting rate increases southwestward, from 25 mm/yr in the north to 50 mm/yr in the south. This results in a progressive increase in the convergence rate across the Ryukyu subduction zone, from approximately 70 mm/yr offshore Kyushu to around 130 mm/yr near Taiwan. Despite relatively low seismic activity in the Ryukyu subduction zone, large tsunami earthquakes occurred south of Okinawa island in 1791 and southeast of Ishigaki island in 1771. This study investigates seismogenic features and fault segmentation along the southern Ryukyu Trench using GNSS velocity fields, earthquake focal mechanisms, and data from slow slip events (SSEs), very-low and low-frequency earthquakes (VLFE, LFE). We find that the source areas of large tsunami earthquakes coincide with source regions of large strike-slip earthquakes (e.g. 1998 M_w 7.4 south of Miyoko island and 2010 M_w 7 south of Okinawa islands) near the trench, suggesting the plate interface is at least partially locked to accumulate strain for large earthquakes. Earthquake focal mechanisms reveal compressive and tensile axes sub-parallel and sub-normal to the trench axis in the incoming Philippine Sea plate and near the trench. Aftershock distributions of large historical strike-slip earthquakes suggest ruptures along NW-SE trending structures. Integrating data from tsunami modeling, SSEs, VLFEs, LFEs, earthquake focal mechanisms, and GNSS velocities, we propose that the southernmost Ryukyu subduction zone can be divided into two segments: (1) from offshore Taiwan to the Gagua Ridge (longitudes 120.8-123°E), and (2) from the Gagua Ridge to Ishigaki island (123-124.5°E). A potential third segment may exist between Ishigaki island and Miyako island (124.5-126°E), though its eastern extension remains unclear due to limited GNSS and seismological data between the Miyako Saddle and the Kerama Gap (126-128°E). South of Okinawa Island, large strike-slip earthquakes and inferred shallow interplate coupling correlate with the source of the 1791 tsunami and the downdip extension of SSEs. Additional seafloor observations are essential for illuminating the fault slip behavior along the Ryukyu margin.