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

Abstract

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