The relationship between S-wave reflectors and deep low-frequency earthquakes in the northern Kinki district, southwestern Japan

Abstract

Abstract We conducted high-resolution reflection analysis of data from 168 seismic stations with an average spacing of about 5 km, in northern Kinki district, southwestern Japan. Reflection analysis has previously been conducted in this region, assuming a homogeneous horizontal structure, resulting in an inclined planar zone of high relative reflection strengths (S-wave reflector). However, if the reflector is actually inclined, the location of the S-wave reflector differs from that of an assumed homogeneous horizontal structure. Hence, this study conducted high-resolution reflection analysis to determine the accurate location of the S-wave reflector. We confirm the previously reported S-wave reflector (reflector W). Furthermore, we detected the accurate location of the S-wave reflector and obtained more detailed results that revealed a second S-wave reflection structure (reflector E) to the east of reflector W, in an area that has not be imaged by previous studies. The northern edges of reflector E and reflector W are located near different hypocentral areas of deep low-frequency earthquakes (DLFs). Reflector W exists along the Kyoto Nishiyama fault zone, and its position appears to change along the fault zone as it deepens. Similarly, reflector E exists along the Hanaore and Biwako Seigan fault zones and its position appears to change along these fault zones. The reflector W and reflector E are imaged as separate S-wave reflectors in deeper regions, but they coalesce in shallower regions. According to previous studies, crustal fluid by dehydration from the Philippine Sea plate exists near these epicenters and we infer that this crustal fluid causes DLFs and forms S-wave reflectors.

Keywords

• Reflection analysis
• Lower crust
• Fluid
• Niigata–Kobe Tectonic Zone
• Deep low-frequency earthquakes
• Manten system