Transient postseismic slip and aftershock triggering: A case study of the 2008 M_W7.9 Wenchuan Earthquake, China

In this study, we investigate how a stress variation generated by a fault that experiences transient postseismic slip (TPS) affects the rate of aftershocks. First, we show that the postseismic slip from Rubin-Ampuero model is a TPS that can occur on the main fault with a velocity-weakening frictional motion, that the resultant slip function is similar to the generalized Jeffreys-Lomnitz creep law, and that the TPS can be explained by a continuous creep process undergoing reloading. Second, we obtain an approximate solution based on the Helmstetter-Shaw seismicity model relating the rate of aftershocks to such TPS. For the Wenchuan sequence, we perform a numerical fitting of the cumulative number of aftershocks using the Modified Omori Law (MOL), the Dieterich model, and the specific TPS model. The fitting curves indicate that the data can be better explained by the TPS model with a B/A ratio of approximately 1.12, where A and B are the parameters in the rate- and state-dependent friction law respectively. Moreover, the p and c that appear in the MOL can be interpreted by the B/A and the critical slip distance, respectively. Because the B/A ratio in the current model is always larger than 1, the model could become a possible candidate to explain aftershock rate commonly decay as a power law with a p-value larger than 1. Finally, the influence of the background seismicity rate r on parameters is studied; the results show that except for the apparent aftershock duration, other parameters are insensitive to r.