A physical mechanism for temporal variation in seismicity in Southwest Japan related to the great interplate earthquakes along the Nankai trough

Abstract

A significant correlation has been reported between the intraplate (inland) and the great interplate earthquakes in Southwest Japan, based on historical earthquake data since 868. The frequency of intraplate earthquakes is high for several decades before and up to a decade after great interplate earthquakes. To explain the correlation, we treated two types of interaction between the inland faults and the plate boundary: (a) in which the stress changes caused by great interplate earthquakes affect the occurrence of intraplate earthquakes; and (b) in which the stress changes caused by intraplate earthquakes trigger other intraplate earthquakes and great interplate earthquakes. To simulate the sequences of these earthquakes, we calculated the static stress changes resulting from large intraplate and great interplate earthquakes and also estimate the rate of increase in tectonic stress on the faults and the plate boundary. The estimated rates of increase here are smaller than those used in previous studies. Thus, in our models, stress changes could more strongly affect the seismicity than previously considered. We simulated the earthquake sequences for our models in four cases: (i) with no interaction among the inland faults in Southwest Japan and the plate boundary along the Nankai trough; (ii) with stress changes on the inland faults caused only by great interplate earthquakes; (iii) with stress changes on the plate boundary caused only by large intraplate earthquakes; and (iv) with stress changes both on the inland faults and on the plate boundary caused by large intraplate earthquakes. The simulated time series in case (i), of course, shows no correlation between intraplate and great interplate earthquakes. Those in cases (iii) and (iv) also show no significant correlation. Only the time series of intraplate earthquakes in case (ii) shows a temporal variation similar to that of the real intraplate earthquakes before and after the great interplate earthquakes. We examined statistically the similarity between the simulated time series and the real earthquake data. About 40% of the time series in case (ii) are significantly similar, although in the other three cases less than 5% of the time series are. Understanding of the effects of stress changes on the inland faults caused by the great interplate earthquakes along the Nankai trough is essential to explain the temporal variation in seismicity in Southwest Japan before and after the great interplate earthquakes.