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We detected many reflected S waves at velocity boundaries within the crust or the upper mantle (SxS waves) in seismograms of shallow earthquakes. We estimated locations of those boundaries based on arrival time differences between the reflected SxS and direct S waves. The estimated S-wave reflectors (bright spots) are distributed in the wide area of NE Japan, that is, not only beneath volcanic areas but also beneath active faults. Depths to the reflectors beneath volcanic areas are shallower than those beneath active faults. Deeper cut-off depth of the reflectors has a positive correlation to that of shallow microearthquakes. It seems that the cut-off depths of both the reflectors and earthquakes are principally governed by the geotherms in the crust. Prominent S-wave reflectors are detected beneath the Senya Fault, which slipped at the time of 1896 M7.2 Rikuu earthquake. Some of them are located on the fault plane of the M7.2 earthquake. Other horizontal reflectors are also detected in the deeper extension of the fault; they might be a part of detachment fault. Many reflectors are also detected in the deeper extension of the fault plane of 1998 M5.0 Sendai earthquake. S-wave reflectors are also detected beneath the focal area of 1962 M6.5 northern Miyagi earthquake. It seems that they connect the focal area of M6.5 event with hypocenters of low frequency microearthquakes occurring near the Moho discontinuity. This close spatial relationship between bright spots, earthquake faults and low frequency microearthquakes suggests that the bright spots are important for better understanding of deep slip process in seismogenic inland fault systems.

Distinct S-wave Reflectors (Bright Spots) Extensively Distributed in the Crust and Upper Mantle beneath the Northeastern Japan Arc