Large-scale Fault Plane of the Western Nagano Prefecture Region Estimated from Precise hypocentral Distribution

It has been considered implicitly that an earthquake occurs on a pre-existing fault plane. However, it has not been well known how the pre-existing fault planes are distributed in a seismogenic region. It is important to identify whether the pre-existing fault planes are randomly distributed or aligned along a certain direction, which forms a large scale fault plane. We estimated the spatial distribution of large scale fault planes from a precisely determined hypocentral distribution, assuming that a large scale structure exists in the region where a planar hypocentral distribution is seen. A dense seismic array with extremely high resolution has been installed in the aftershock area of the 1984 Western Nagano Prefecture Earthquake (Mj6.8) since June 1995 (Iio et al, 1999). Since the recording system has a sampling frequency of 10kHz, P and S onsets are precisely determined. The hypocenters of 24,656 earthquakes that had been observed in this region for the period from October, 1995 to December, 2004 were relocated by using the Joint Hypocenter Determination (JHD) method (Kissling et al, 1994). Because P wave arrival times are read to an accuracy of 1ms, the relative errors of 82% of the hypocenters are calculated to be within 100m. From precise hypocentral distributions, which are shown in vertical cross sections of widths of 500m, we found that the hypocentral distribution is planar, although it looks rather random in a large scale figure. We found that the estimated large scale fault planes are mainly aligned along two oppositely dipping directions of roughly north and south. These orientations are different from the fault plane of the main shock which is a steeply dipping strike slip fault with an azimuth of roughly E-W.