Seismicity distribution in the Tonankai and Nankai seismogenic zones and its spatiotemporal relationship with interplate coupling and slow earthquakes

Abstract We conducted seismic tomography to estimate the velocity structure and evaluate spatiotemporal distribution of interplate earthquakes Kii Peninsula, central Honshu, Japan, where Tonankai Nankai megathrusts are located. Microearthquakes were quantitatively detected by using data from a cable-type seafloor observation network, completed in 2015. Our model was consistent with previous 2-D active-source surveys, which reported areal extent key structures: high-velocity zone beneath Cape Shionomisaki, subducted seamount off Muroto, Paleo-Zenith Ridge. The absence any other same or larger spatial scale, than identified structures, confirmed. also revealed that there not simple relationship between areas large coseismic slip strong coupling high overriding plate. Relocated hypocenters widely ranged upper plate within slab, while most active region attributed oceanic crust aftershock 2004 off-Kii earthquake. Compared results land-based accuracy focal depth estimation substantially improved. Furthermore, we activity vicinity boundary determined 14 locations for seismicity areas. They primarily distributed range seismogenic temperature (150–350 °C) along located outside zone. Several exhibited clustered during periods slow-slip events, observed accompanied shallow very-low-frequency earthquakes. Thus, regular microearthquakes became at conjunction slow slip. In summary, as provide more accurate source location can detect events smaller magnitude, monitoring such may reveal variations stick–slip conditions on boundary.