THE 1995 7.2 MAGNITUDE KOBE EARTHQUAKE AND THE ARIMA-TAKATSUKI TECTONIC LINE - IMPLICATIONS OF THE SEISMIC RISK FOR CENTRAL JAPAN

Along the Rokko Mountains and Awaji Island, NE-SW to ENE-WSW oriented active faults exist in an en echelon arrangement. They constitute a po rtion of the Arima-Takatsuki tectonic line (ATTL) which extends from K yoto, through Awaji Island, to the Median Tectonic Line. The ATTL is a lso correlated to lineaments linking the saddles and steep slopes of g ravity (Bouguer) anomalies. The main shock of the 1995 magnitude 7.2 ( M 7.2) Kobe earthquake was located at the mid-point of the ATTL. The m ain shock also created a 9-km long rupture on the ground surface along the NE-SW Nojima fault line in the northern part of Awaji Island. The earthquake aftershocks were distributed over a 40-km long zone along the central segment of the line. Surface ruptures and cracks accompany ing the Kobe earthquake were scattered along a 40-km segment centrally located on the ATTL. Spatial correlation of the surface ruptures and aftershock distribution on the ATTL suggests that the Kobe earthquake was the result of a 40-km long rupture of the central segment of the A TTL. The average rate of seismic moment-release is an important parame ter used in evaluating the seismic risk of a fault system. The moment- release rate averaged over the late Quaternary has been evaluated thro ugh the average slip rates and the dimensions of constituent faults. T he average moment-release rate for the central portion of the ATTL has been estimated as 0.0017-0.0063 x 10(26) dyne cm year(-1). Evidence o f liquefaction and flowage, probably associated with the 1596 M 7.5 Ke icho-Fushimi earthquake, has been identified at several archaeological excavation sites along the ATTL. These observations suggest that the ATTL was also responsible for the Keicho-Fushimi earthquake which occu rred about 400 years ago. From the elapsed time since the Keicho-Fushi mi earthquake and the average moment-release rate, the seismic moment accumulated during the interseismic period was calculated for the ATTL . The obtained value amounts to that of the seismic moment released by an earthquake having a magnitude ranging from 6.8 to 7.2, being compa rable to that of the Kobe earthquake. The obtained result is consisten t with the view that the Kobe earthquake was caused by a rupture of th e ATTL which released almost all the moment accumulated since the prev ious event.