Seismicity Cycle of Large Earthquake Occurrence Zones

It is known from trench excavation surveys that large earthquakes rupture the whole brittle zone (earthquake occurrence zone) of the crust occur repeatedly [e.g. Ohnaka et al., 2002] on active faults. For inland active faults, after the occurrence of a large earthquake, the aftershocks usually continue for several to tens of years, decreasing with time [Watanabe, 1989]. In the region of the 1891 Nobi Earthquake, it is known that aftershock activity has been continuing for more than a hundred years. Aftershock activity generally fits very well the Omori formula [Omori, 1894] which describes the aftershock occurrence in time [e.g. Yamashita, 1987]. Following the high level of aftershock activity, the earthquake cycle moves into a phase of much lower seismic activity [Toda, 2002], and then the next major earthquake occurs. For the inland active faults, the seismicity cycle is thousands to tens of thousands of years. For plate boundary, it is usually tens to hundreds of years. The duration of a cycle widely varies between various faults and regions. The period of observations for modern seismology is about a hundred years., and is only a small portion of one cycle. Therefore, it is difficult to recognize what portion of the earthquake cycle the current seismic activity corresponds to. For many active faults, however, the current stage of the cycle can be estimated from geological data. By combining both geological data and current seismicity for many different faults, we can view a complete seismicity cycle. We evaluated quantitatively the seismic activity of 98 major active faults in Japan and 17 segments of the San Andreas Fault System from the relation between the lapsed time from the last large earthquake, and the present seismic activity. Although there is large variation in the scales and recurrence times of faults, we developed a method to correct for these factors. We find a clear correlation between the lapsed time from a large earthquake and the present seismic activity. After the occurrence of large earthquakes, the seismic activity decays smoothly inversely proportional to the lapsed time for over a thousand years. Since this decay follows the modified Omori formula, we interpret the observations as indicating that the aftershock decay lasts for almost the entire earthquake cycle.