Earth Planets Space, Vol. 54 (No. 11), pp. 1027-1031, 2002
Ayako Nakamura, Youichi Asano, and Akira Hasegawa
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
(Received December 28, 2001; Revised September 26, 2002; Accepted September 27, 2002)
Abstract: An extensive seismic experiment was carried out in June 2001 around the Nagamachi-Rifu fault, northeastern Japan. We deployed seismic arrays on the hanging wall of the reverse fault, and observed seismic waves caused by chemical explosions and vibrators. Several later arrivals from reflectors, whose depths are about 9 to 21 km, are detected in the observed shot gathers. Some of them probably correspond to S-wave reflectors and scatterers previously estimated from the data of natural earthquakes. A clear reflector was found in and below the seismogenic layer at 4 to 4.5-s two-way traveltime (TWTT). It is located at the shallower and deeper extensions of the fault plane of M5.0 earthquake that occurred in 1998. Its extension to the surface seems to connect with the surface trace of the Nagamachi-Rifu fault. These suggest that the Nagamachi-Rifu fault has a low dip-angle in the mid-crust as a detachment fault, if the reflector shows the fault structures.