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

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.

[1]  L. Brown,et al.  Reanalysis of the cocorp deep seismic reflection profile across the San Andreas fault, Parkfield, California , 1986 .

[2]  Takeyoshi Yoshida.  The Evolution of Arc Magmatism in the NE Honshu Arc, Japan , 2001 .

[3]  T. Parsons.  Seismic-reflection evidence that the Hayward fault extends into the lower crust of the San Francisco Bay area, California , 1998, Bulletin of the Seismological Society of America.

[4]  N. Umino,et al.  Seismic activity and deformation process of the overriding plate in the northeastern Japan subduction zone , 2000 .

[5]  K. Furlong,et al.  Seismic Evidence for a Lower-Crustal Detachment Beneath San Francisco Bay, California , 1994, Science.

[6]  Max Wyss,et al.  Evaluation of Proposed Earthquake Precursors , 1991 .

[7]  Lupei Zhu,et al.  Crustal structure across the San Andreas Fault, southern California from teleseismic converted waves , 2000 .

[8]  A. Levander,et al.  Deformation in the Lower Crust of the San Andreas Fault System in Northern California , 1997 .

[9]  K. Furlong,et al.  Geometry and evolution of the San Andreas Fault Zone in northern California , 1989 .

[10]  T. McEvilly,et al.  Interpretation of seismic reflection profiling data for the structure of the San Andreas fault zone , 1983 .

[11]  Akira Hasegawa,et al.  Foreshock and Aftershock Sequence of the 1998 M 5.0 Sendai, Northeastern Japan, Earthquake and Its Implications for Earthquake Nucleation , 2002 .

[12]  M. Zoback,et al.  Crustal strain near the Big Bend of the San Andreas Fault: Analysis of the Los Padres‐Tehachapi Trilateration Networks, California , 1990 .

[13]  N. Umino,et al.  Double-planed structure of the deep seismic zone in the northeastern Japan arc , 1978 .

[14]  Akira Hasegawa,et al.  Distinct S-wave reflectors (bright spots) detected beneath the Nagamachi-Rifu fault, NE Japan , 2002 .

[15]  Walter H. F. Smith,et al.  Free software helps map and display data , 1991 .

[16]  W. Holbrook,et al.  Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin , 1996 .