Estimation of deep fault geometry of the Nagamachi-Rifu fault from seismic array observations

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. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.