Trapped wave observation in fault zone of the Mid-Niigata Prefecture Earthquake in 2004

The Mid-Niigata Prefecture earthquake in 2004 occurred in northeast Japan at 17:56, October 23 (JST). In this study, knowledge of the fault-zone structure provides the information required to understand the physics of earthquake. In order to understand the fault structure, we must observe the many aftershocks that occur near the active fault by array observation.

Two days after the mainshock, we started an aftershock observation across the active faults in order to obtain the waveform that propagates thorough the fault plane. We succeeded in locating several of the fault-zone trapped waves. The apparent velocities of the fault-zone trapped waves are about 20 - 30 % lower than those of the S waves, and the peaks of the amplitude spectra of the fault-zone trapped waves have low-frequencies between 4Hz and 7Hz. Fault zone trapped waves observed within about 300m of north-west zone of surface fault trace. The ratio of fault-zone trapped waves and aftershocks is only about 1 %. On the other hand, the hypocenter distribution of the characteristic low-frequency (CLF) seismic event corresponds to the region of the large slip distribution of the mainshock. It may be stated that the asperity of the mainshock generates the CLF seismic events and few aftershocks.