2002 Volume 111 Issue 2 Pages 233-247
Traditionally, long-term earthquake probability has been estimated using regional seismicity or a characteristic earthquake hypothesis without considering any recent stress perturbations caused by sudden crustal deformation or nearby earthquakes. The conventional calculation method ignores such processes observed time dependent clustering of earthquakes, and the occurrence of aftershocks or anti-shocks (seismicity rate decreases). I have thus introduced a method to seek the time-dependent seismicity rate based on stress interaction, incorporating the rate-and state-dependent friction law. Regarding earthquake productivity response to the stress state, coseismic stress step controls amplification of seismicity rate increase. In addition, assuming that the constitutive parameter and normal stress are constant throughout time, the aftershock or anti-shock duration is inversely proportional to the regional tectonic stressing rate. Thus, influence of the stress step lasts longer where the loading rate is slow, and the long-term probability retains the stress-related change longer. In addition, change in the loading rate proportionally causes the change in the earthquake productivity, in other words earthquake probability, with some delay estimated by a new loading rate. The response time to the sudden loading rate change depends on a new loading rate as predicted by the rate-and state-friction theory.
To make further validations about this theory and method, I have investigated two cases of stress triggering, 2000 seismic swarm activity in and around the Izu Islands, and the October 6, 2000, M7.3 Tottori-ken-seibu earthquake. In the Izu swarm activity, the factor of the stressing rate changes was focused. Observed hundreds-to thousands-fold increases of seismicity rate than usual during the active two-month period are almost equivalent to the increases in the tectonic stressing rate caused by a dike intrusion. In contrast, long aftershock duration and long influence of the stress perturbation associated with a low stressing rate were tested in the Tottori case. Even though we need to incorporate additional factors such as viscoelastic behavior and change in earthquake size distribution, this method is expected to contribute more precise long-term earthquake probabilistic forecasting together with welldetermined data from recent GPS and seismic networks.
