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

A significant correlation has been reported between the intraplate (inland) and the great interplate earthquakes in Southwest Japan, based on historic al earthquake data since 868. The frequency of intraplate earthquakes is hi gh for several decades before and up to a decade after great interplate ear thquakes. To explain the correlation, we treated two types of interaction b etween the inland faults and the plate boundary: (a) in which the stress ch anges caused by great interplate earthquakes affect the occurrence of intra plate earthquakes; and (b) in which the stress changes caused by intraplate earthquakes trigger other intraplate earthquakes and great interplate eart hquakes. 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 th e faults and the plate boundary. The estimated rates of increase here are s maller than those used in previous studies. Thus, in our models, stress cha nges could more strongly affect the seismicity than previously considered. We simulated the earthquake sequences for our models in four cases: (i) wit h no interaction among the inland faults in Southwest Japan and the plate b oundary along the Nankai trough; (ii) with stress changes on the inland fau lts 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 ca used by large intraplate earthquakes. The simulated time series in case (i) , of course, shows no correlation between intraplate and great interplate e arthquakes. Those in cases (iii) and (iv) also show no significant correlat ion. Only the time series of intraplate earthquakes in case (ii) shows a te mporal variation similar to that of the real intraplate earthquakes before and after the great interplate earthquakes. We examined statistically the s imilarity between the simulated time series and the real earthquake data. A bout 40% of the time series in case (ii) are significantly similar, althoug h in the other three cases less than 5% of the time series are. Understandi ng of the effects of stress changes on the inland faults caused by the grea t interplate earthquakes along the Nankai trough is essential to explain th e temporal variation in seismicity in Southwest Japan before and after the great interplate earthquakes. (C) 1999 Elsevier Science B.V. All rights res erved.