Evaluating seismic-risk potential of active faults: What should we do now?

Abstract

In the Japanese Islands, including continental slopes on the Japan Sea side, are numerous active intraplate faults, which would cause disastrous earthquakes in the future. Evaluation of seismic risk potential of each active fault is necessary and urgent, in order to choose a feasible number of target faults, against which disaster-prevention measures should be taken with a high priority. Seismic risk potential of a fault could be evaluated on the basis of the size and timing of earthquakes produced from the fault in recent geologic time. However, the seismic risk evaluation using these two criteria either (1) depends strongly on untested models (such as the characteristic earthquake model and a variety of fault segmentation models) and/or empirical laws derived from observations of historical surface faulting, or (2) requires data that are difficult to observe in the present state of technology. Moreover, possible stochastic nature of faulting processes would raise another issue. In order to avoid these difficulties for the time being, I propose another criterion. The rate of seismic moment release per unit length of a fault averaged over recent geologic time is a good indicator of overall seismic risk potential, and is determined only from the average rate of net slip on and the subsurface geometry of the fault. Seismic risk potential based on this criterion, however, is only a first-order approximation, and must be updated with paleoseismological data in the near future.