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

Satoshi Matsumoto, Akira Hasegawa

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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.

Original languageEnglish
Pages (from-to)3067-3083
Number of pages17
JournalJournal of Geophysical Research: Solid Earth
Issue number2
Publication statusPublished - Feb 10 1996
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science


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