Structural heterogeneity and megathrust earthquakes in Southwest Japan

Abstract

Large interplate megathrust earthquakes occurred in the Nankai subduction zone off Southwest Japan with an interval of ~100 to 150 years. Because of a fast convergence rate (4–6 cm/yr) and a young buoyant lithosphere, the subducting Philippine Sea plate and the overriding Eurasian plate are strongly coupled in the forearc region along the upper boundary of the subducting plate. However, the causal mechanism of megathrust earthquakes in the Nankai forearc domain is still not very clear. In this study, we present 3-D high-resolution P and S wave velocity and Poisson's ratio images of the Nankai subduction zone, which are determined using a large number of high-quality arrival-time data of well-located onshore and offshore local earthquakes. The obtained seismic images, as well as residual topography and gravity anomalies, show similar structures in both the upper and lower plates above and below the rupture zones of the 1944 Tonankai earthquake (M 8.1), the 1946 Nankai earthquake (M 8.3), and the 1968 Hyuganada earthquake (M 7.5). These results indicate that the nucleation and rupture processes of these megathrust earthquakes were controlled by both the upper and lower plates, similar to those in the Tohoku forearc. However, the physical properties and seismogenesis of the megathrust zone in the Nankai forearc appear more complicated than those in the Tohoku forearc.