Seafloor deformation in Taiwan revealed by GNSS-acoustic measurements

Taiwan sits at the junction of the Ryukyu and Manila subduction zones, where a rapid convergence rate of ~90 mm/yr drives intense seismic and tsunami hazards. However, land-based geodetic networks provide insufficient resolution for monitoring offshore deformation. To address this, we have developed and deployed GNSS-Acoustic (GNSS-A) systems to monitor seafloor deformation. A total of six GNSS-A sites were established along the southern Ryukyu subduction zone near Taiwan, with three additional sites located near the northern tip of the Manila Trench. GNSS-A data in the southernmost Ryukyu margin reveal an eastward increase in convergence rate, from 92 mm/yr offshore Hualien to 123 mm/yr near the Gagua Ridge, indicating the potential to generate M_w 7.5–8.4 earthquakes. The 2024 M_w 7.3 Hualien earthquake ruptured a deep 70° east-dipping Longitudinal Valley fault and a 35° west-dipping offshore fault. At seafloor site ORY2, ~ 40 km east of the epicenter, we recorded coseismic displacements of 9.1±12.1 cm eastward and 12.3±11.4 cm southward motions, along with 52.9±13.5 cm uplift. These observations are consistent with coseismic dislocation modeling results. Additionally, multiple slow slip events on fault systems in eastern Taiwan appear to have preceded the 2024 M_w 7.3 Hualien earthquake.

Offshore southern Taiwan, geodetic data reveal N–S-oriented extension in the Tainan Basin and NE–SW extension between the northern Manila Trench and the North Luzon Trough. These strain axes align with the focal mechanisms of the 1994 M 6.5 and 2006 M_w 7.0 earthquakes. Notably, deformation and seismicity patterns shift distinctly across the Eurasian Plate–South China Sea continent–ocean boundary near 20°N. Together, these integrated observations provide new insights into fault segmentation, strain accumulation, and regional seismic and tsunami hazards.