Evidence for the existence of a mid-crustal reflector in the Beppu-Shimabara Graben, Kyushu, Japan

The Beppu‐Shimabara graben, in Kyushu, Japan, has active seismic regions and active volcanoes. Studies using triangulation, gravity surveys and mechanism solutions of large earthquakes show that the graben is in a state of extensive stress associated with rifting. A temporary seismic study was carried out from February 20 to March 8 in 1992 in the region, during which time many seismic signals were observed. The microearthquake seismograms recorded inside the graben have a later phase dominant on the horizontal component following the direct S‐wave. Two‐dimensional ray‐tracing was applied to investigate the cause of the later phase. The phase was identified as an S‐reflected S phase (SxS‐phase) from a discontinuity with a depth of 20 km. The reflection points are located inside the Beppu‐Shimabara graben that is in a state of extensive stress associated with rifting.