The 2000 western Tottori (Japan) earthquake: Triggering of the largest aftershock and constraints on the slip‐weakening distance

We investigate the stress time relationships between a mainshock and its largest aftershock. We focus on the 2000 western Tottori (Japan) earthquake ( M 6.6–6.8) and on the triggering of its largest aftershock ( M s 5.5) that occurred 48 h later. We compute both the static Coulomb stress change and the complete Coulomb failure function that provides the evolution of the stress field on the fault plane of the largest aftershock. We show that the static Coulomb stress change is always negative. Therefore it cannot explain the triggering of the largest aftershock, located in a stress shadow. On the contrary, the complete Coulomb failure function exhibits a large positive pulse because of the rupture dynamics of the mainshock. This is consistent with the triggering of the largest aftershock by transient stresses. In a second step, we infer a lower threshold for the critical slip‐weakening distance. Using a frictional model based on a nonlinear, slip‐dependent friction law and assuming the initial state of stress on the fault plane of the largest aftershock, we derive for the slip‐weakening distance D c a value greater than 0.2 m to be consistent with the observed delay of 48 h.