The 1995 7.2 magnitude Kobe earthquake and the Arima-Takatsuki tectonic line: implications of the seismic risk for central Japan

Abstract

Along the Rokko Mountains and Awaji Island, NE-SW to ENE-WSW oriented active faults exist in an en echelon arrangement. They constitute a portion of the Arima-Takatsuki tectonic line (ATTL) which extends from Kyoto, through Awaji Island, to the Median Tectonic Line. The ATTL is also correlated to lineaments linking the saddles and steep slopes of gravity (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 main 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 accompanying 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 ATTL. The average rate of seismic moment-release is an important parameter used in evaluating the seismic risk of a fault system. The moment-release rate averaged over the late Quaternary has been evaluated through the average slip rates and the dimensions of constituent faults. The average moment-release rate for the central portion of the ATTL has been estimated as 0.0017−0.0063 × 10 26 dyne cm year −1. Evidence of liquefaction and flowage, probably associated with the 1596 M 7.5 Keicho-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 occurred about 400 years ago. From the elapsed time since the Keicho-Fushimi 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 comparable to that of the Kobe earthquake. The obtained result is consistent with the view that the Kobe earthquake was caused by a rupture of the ATTL which released almost all the moment accumulated since the previous event.