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 mechanism was consistent with a right-lateral strike-slip fault trending N40°E. Three days after the occurrence of the mainshock, we started to install a highly dense seismic array in and around the fault area of the quake. Two permanent regional seismic networks of more than 30 stations covered the entire area of 200 km × 200 km. The temporarily installed array of 27 stations spanned the fault area of 15 km × 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 using an Internet connection between seismic networks. The aftershock area extended 70 km trending northeast to southwest. Hypocenters determined 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 variation in the rate of occurrence of aftershocks had a periodic component with periods of a half of day, 1 day, and 3 days, that are superimposed on decaying of the rate following Omori's law.