URGENT JOINT OBSERVATION OF AFTERSHOCKS OF THE 1995 HYOGO-KEN-NANBU EARTHQUAKE

A disastrous earthquake with a magnitude of 7.2 hit the southern part of Hyogo Prefecture on January 17, 1995. The mainshock was located on an active fault of the Arima-Takatsuki-Rokko fault system. Its focal m echanism was consistent with a right-lateral strike-slip fault trendin g N40 degrees E. Three days after the occurrence of the mainshock, we started to install a highly dense seismic array in and around the faul t area of the quake. Two permanent regional seismic networks of more t han 30 stations covered the entire area of 200 km x 200 km. The tempor arily installed array of 27 stations spanned the fault area of 15 km x 50 km. All data were telemetered to a temporary observation center at Uji. We located about 3,100 aftershocks in real time for 1 month usin g an internet connection between seismic networks. The aftershock area extended 70 km trending northeast to southwest. Hypocenters determine d by the network were delivered automatically through the Internet. We found that the aftershock distribution was heterogeneous in space and time: seven clusters of hypocenters were identified and temporary var iation in the rate of occurrence of aftershocks had a periodic compone nt with periods of a half of day, 1 day, and 3 days, that are superimp osed on decaying of the rate following Omori's law.