Distinct S wave reflector in the midcrust beneath Nikko-Shirane volcano in the northeastern Japan arc

Distinct S waves reflectedfrom a midcrustal seismic velocity discontinuity are detectedbeneath Nikko-Shirane volcano in the southernmost part of the northeastern Japan arc. A detailedtravel time analysis of the reflectedS waves by using data acquiredthrough a dense seismic network temporarily set up in this region shows that this unusual S wave reflector is distributed over an area of 15×15 km² at depths ranging from 8 to 15 km. The reflector has a conical shape becoming shallow toward the summit of Nikko-Shirane volcano. Observedamplitude spectral ratios of reflectedS waves to direct S waves show that the reflector body has a strong velocity contrast to the surrounding medium and its thickness is of the order of 100 m at most. The reflector body is approximatedby two thin layers probably filled with partially molten materials. Cutoff depth for shallow seismicity in this area is 3-5 km above the reflector and becomes shallow toward Nikko-Shirane volcano, nearly parallel to the reflector. The depth to brittle-ductile transition zone seems to be prescribedby the existence of the reflector body, which is perhaps a thin magma body.