Origin of the seismic belt in the San-in district, southwest Japan, inferred from the seismic velocity structure of the lower crust

Abstract

A long linear distribution of epicenters is seen along the Japan Sea coast in the San-in district located in southwestern Japan. This linear distribution of epicenters is called the seismic belt in the San-in district. The localization of intraplate earthquakes in the San-in district, far from plate boundaries, is not well understood. To answer this question, we look at the seismic velocity structure of the lower crust beneath the San-in district using seismic travel-time tomography. Our results show the existence of a low-velocity anomaly in the lower crust beneath the seismic belt. We infer that the deformation was concentrated in the low-velocity zone due to compressive stress caused by the subduction of oceanic plates, that stress concentration occurred just above the low-velocity zone, and that the seismic belt was therefore formed there. We also calculated the cutoff depths of shallow intraplate earthquakes in the San-in district. Based on the results, we consider the possible causes of the low-velocity anomaly in the lower crust beneath the seismic belt. We found that the cutoff depths of the intraplate earthquakes were shallower in the eastern part of the low-velocity zone in the lower crust beneath the seismic belt and deeper in the western part. Thus, the eastern part is likely to be hotter than the western part. We inferred that the eastern part was hot because a hot mantle upwelling approaches the Moho discontinuity below it and the resulting high temperature may be the main cause of the low-velocity anomaly. On the other hand, in the western part, we inferred that the temperature is not high because the mantle upwelling may not exist at shallow depth, and water dehydrated from the Philippine Sea plate reaches the lower crust, and the existence of this water may be the main cause of the low-velocity anomaly.