[S-GD01] Geodetic Advances in Crustal Dynamics and Environmental Change for Taiwan and Neighboring Regions

[SGD01-07] A long-term slow slip event beneath the southern Ryukyu forearc revealed by 25 years of GNSS observations

★Invited Papers

*Chi-Hsien Tang^1,2, Ya-Ju Hsu², Yutaro Okada¹, Yusaku Ohta³, Takeshi Sagiya⁴, Pei-Ru Jian⁵, Yoshiaki Tamura⁶, Takaaki Jike⁶ (1.International Research Institute of Disaster Science, Tohoku University, 2.Institute of Earth Sciences, Academia Sinica, 3.Graduate School of Science, Tohoku University, 4.Disaster Mitigation Research Center, Nagoya University, 5.Department of Geosciences, National Taiwan University, 6.Mizusawa VLBI Observatory, National Astronomical Observatory of Japan)

Keywords:Slow slip event, Ryukyu subduction zone, GNSS, Inversion, Seismicity, Tsunami earthquake

Slow slip events (SSEs) are now recognized as a ubiquitous tectonic process in subduction zones that potentially reflect the frictional properties and stress conditions on plate interfaces, making them important indicators for assessing earthquake and tsunami hazards. However, the spatiotemporal distribution of SSEs is still often elusive, especially for regions like the southern Ryukyu subduction zone with limited near-trench constraints. Understanding the potential occurrence of SSEs in the southern Ryukyu subduction zone is important as paleotsunami records indicate repeating tsunami events in this region, including the most recent 1771 Meiwa tsunami likely induced by a M_w 8 shallow plate interface earthquake. While previous studies have reported the occurrence of deep SSEs, very-low-frequency, and low-frequency earthquakes (VLFEs and LFEs), whether shallow SSEs occur beneath the forearc remains largely unexplored. Here, we analyze the GNSS observations along the southern Ryukyu Islands from 1998 to 2022 to identify a trenchward transient at longitudes 124.5°-125.5°E between 2009 and 2011. The cumulative GNSS horizontal displacement during the transient was up to 10 mm. The slip inversions suggest that the observed transient can be explained by a SSE on the plate interface beneath the forearc region at depths of 14-32 km offshore Miyako. The geodetic moment release was roughly equivalent to a M_w 6.8 earthquake (assuming a rigidity of 30 GPa). The transient slip likely consists of two subevents, each lasting for about 6 months. The onset times and moment magnitudes of the first and second subevents were mid-2009 and M_w 6.7, and mid-2010 and M_w 6.5, respectively. In addition, the first episode was preceded by a nearby M_w 6.1 interplate earthquake. The aseismic/seismic moment ratio (ASMR) during the 2009-2011 SSE ranges from 770-832% for the first subevent and increases to 1,083-1,229% when two subevents are considered. The estimated ASMR is notably higher than the typical range of 20-400% for afterslip, indicating that the 2009-2011 SSE cannot be solely attributed to the typical afterslip of a M_w 6.1 earthquake. Much of the inferred slow-slip area overlaps with a free-air gravity high associated with the subducted transverse ridges of the trench-parallel Luzon-Okinawa fracture zone, implying the role of subducted topography in providing favorable conditions for shallow SSEs and slow earthquakes. The spatial complementarity between the subducted ridges and the 1771 earthquake source suggests that the SSE-prone zone may act as a barrier to seismic rupture, similar to observations in other subduction zones. Other studies, on the other hand, suggest that seismic ruptures may penetrate SSE-prone regions if these zones experience sufficiently low effective stress or undergo enhanced dynamic weakening due to thermal pressurization. In any case, our study suggests that the shallow southern Ryukyu plate interface has the potential to accumulate elastic strain and release energy either aseismically or seismically. We also emphasize the importance of developing near-trench seafloor geodesy in the future to fully characterize the fault slip behaviors beneath the southern Ryukyu forearc.