Seismic risk assessment of an active fault system: the example of the Tsurugawan–Isewan tectonic line

The 185 km long Tsurugawan–Isewan tectonic line (TITL) is one of the major active fault systems in central Japan. The TITL, oriented in a NW–SE-trend, passes through the western part of central Japan, extending from the Sea of Japan to the Pacific ocean. It is composed mainly of five large active faults; from the northwest to southeast, the Kaburagi, Yanagase, Sekigahara, Yoro, and Ise Bay faults. As a case study of seismic risk assessment of an active fault system using the parameter of the average moment-release rate, the seismic risk of the TITL is evaluated in the present paper. The average moment-release rate of an active fault system can be calculated from slip-rates and lengths of constituent active faults. In addition, the latest earthquake of the active fault system if unknown can be inferred from data of historical earthquakes and trench excavations. The total moments, which are accumulated over a duration time from the latest earthquake to the present, are estimated by multiplying the average moment-release rate by the duration time. The total accumulated moments can be used to empirically determine the magnitude of an earthquake which would presently occur. The calculated magnitude for each segment or active fault which constitutes the TITL presently is determined as 7.1 at the maximum. Further, the maximum magnitude of a possible earthquake which would occur after a 100 year durability period of concrete-made structures is estimated as 7.2. This implies that the TITL has the potential of producing earthquakes of M7.1 and 7.2 at the present and after 100 years, respectively. The method of seismic risk assessment is important in order to obtain input magnitude data of possible earthquakes for an active fault system, to be used in selecting sites and earthquake-proof designs of large structures in seismically unstable regions.