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Earth Planets Space, Vol. 54 (No. 11), pp. 1021-1026, 2002

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

Norihito Umino, Hisako Ujikawa, Shuichiro Hori, and Akira Hasegawa

Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

(Received January 8, 2002; Revised September 20, 2002; Accepted September 26, 2002)

Abstract: 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 25°. 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 ~1.1 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.


Corresponding author E-mail: umino@aob.geophys.tohoku.ac.jp


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