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 (S¿S-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. ¿ American Geophysical Union 1993