Distinct S-wave reflectors (bright spots) detected beneath the Nagamachi-Rifu fault, NE japan

Distinct phases reflected from mid-crustal reflectors (SxS and PxP phases) were observed in seismograms of aftershocks of 1998 M5.0 Sendai earthquake at nearby stations. We estimated the locations of the reflectors (bright spots) by using arrival time differences between these phases and direct waves. A clear reflector is located in the depth range of 15 to 21 km just beneath the fault plane of the M5.0 event. It dips toward the NNE direction with a dip angle of about 25degrees. Other reflectors are also located beneath the fault plane of the event. Internal structure of the S-wave reflector was estimated from spectral amplitude ratios of reflected SxS-wave to direct S-wave. Observed spectral ratios show that S-wave velocity in the reflector body is similar to1.1 km/s, and its thickness is about 50 m. This suggests that a thin reflector body (bright spot) partially filled with fluids exists in the lower crust beneath the focal area of the 1998 event.