Coulomb stress change on inland faults during megathrust earthquake cycle in southwest Japan

In the subduction zone, megathrust earthquakes may modulate the shallow crustal seismicity in the overriding plate. Historical documents indicate the frequent occurrence of large shallow crustal earthquakes in the overriding continental plate 50 years before and 10 years after the megathrust earthquakes along the Nankai trough in southwest Japan. In this study, we model megathrust earthquake cycles in a simple oblique subduction zone considering the viscoelasticity, and calculate the temporal evolution of the Coulomb failure stress changes ( CFS) on the crustal faults in the overriding plate. Further, we examine the variation of CFS depending on the location and fault type, and the active period of crustal earthquakes in which CFS exceeds the previous maximum. Our viscoelastic model suggests that the dependency of the active period on the distance from the megathrust fault is less when the intrinsic loading rate of the inland fault is low. Moreover, it suggests that the viscoelastic stress evolution on faults with negative coseismic CFS renders the active period longer or shorter than those in a pure elastic medium. The temporal evolution of CFS on most major active faults in southwest Japan can be categorized into two groups with the following different characteristics: one is that CFS is positive coseismically and peaks 10 years after a megathrust earthquake. The other is that CFS is negative coseismically, and does not recover to the preseismic one for more than 50 years after a megathrust earthquake. This can explain the temporal sequence of the historical earthquakes in southwest Japan. Our model which includes viscoelastic relaxation successfully expresses the activation of shallow crustal earthquakes in the overriding continental plate not only before the megathrust earthquake, but also after. If the apparent frictional coefficient is less than 0.1, the coseismic CFS on the source faults of the 1943 -Mj7.3 Tottori earthquake, 1596 M7.0 Keicho Iyo earthquake, and 1596 M7.5 Keicho Fushimi earthquake that occurred within 10 years before the megathrust earthquake along the Nankai trough is negative. Therefore, to explain the occurrence of these historical earthquakes, our model suggests that the apparent frictional coefficient must be less than 0.1.