Potential of Megathrust Earthquakes Along the Southern Ryukyu Trench Inferred from GNSS Data

Abstract

The southern part of the Ryukyu subduction zone has recorded tsunami events with a recurrence interval of several hundred years. Although their source is controversial, one model suggested that the last 1771 Yaeyama tsunami was caused by a shallow megathrust earthquake with a magnitude of 8. However, the current knowledge on interplate coupling based on recent geodetic data is limited. This study analyzed a time series of Global Navigation Satellite System data from January 2010 to February 2021, including newly installed stations by Kyoto and Kyushu Universities to obtain the distance changes between stations and vertical secular velocities. The distance changes ranged 2.4 mm/yr in contraction and 4.7 mm/yr in extension, and the vertical velocities exhibited no clear uplift or subsidence of -2.4 to 1.1 mm/yr. The back slip inversion results indicated the slip deficit of 16–54 mm/yr in the south of the Yaeyama Islands. The large slip deficit area is complementarily intervened between the shallower source area of low-frequency earthquakes and the deeper slow slip region, suggesting the spatial heterogeneity of frictional properties along the plate interface. If the large slip deficit area accumulates stress in the same rate since the last 1771 earthquake, it could result in a megathrust event of at least greater than a moment magnitude of 7.7. Because the limited onshore data cannot resolve the slip deficit on the shallow plate interface, seafloor geodetic observations are essential to clarify the detailed spatial distribution of slip deficit and discuss its earthquake and tsunami potential.