Aftershocks Decays of the 2015 Gorkha Nepal earthquake and the 2008 Wenchuan China earthquake: Observation, Modeling and Implication

To obtain the knowledge of earthquake process, we need enough information from both experiments and observations. Among all kinds of observations, the locations and timing of the aftershock sequences from mainshocks could tell us some useful clues. The 2015 Gorkha Nepal earthquake (GNE) and the 2008 Wenchuan China earthquake (WCE) sequences occurred in the southern and eastern margins of Qinghai-Tibetan Plateau, respectively. Because the seismotectonic settings in Nepal and Tibet are broadly similar and their mainshock magnitudes are comparable, by using the regional seismicity data and the aftershock catalogs from the USGS and NEIC for these two events, we explored the spatio-temporal characteristics of aftershocks to gain insight into the possible fault frictional processes related to their triggering mechanisms. First, we estimated the minimum magnitude of complete recording for these two sequences within 0.1day. For this period, the magnitude of completeness Mc is estimated to 4.0 by using a maximum likelihood procedure. Then, we fit the seismicity decay both with the empirical and physical models. Our current results indicate that, compared with the WCE sequence, the number of aftershocks is much smaller, and the aftershock duration is also shorter for the GNE sequence. Additionally, for these two sequences the p-value used to describe the aftershock decay rate given by the Modified Omori Law (MOL) is obviously large than 1 which is in contradiction with the static stress triggering model (Dieterich, 1994). Based on the Dieterich seismicity evolution model and the rate- and state-dependent friction law (RSF), we propose that the afterslip behavior could be a possible candidate to explain such kind of aftershock decays. The current results indicate that, with a constant background stress rate, the p > 1 could be well explained and fitted for events M > 4.0 if a postseismic stress rate change is involved in the model fitting. Moreover, the p-value is connected with the ratio of b/a, where a and b are the frictional parameters in RSF. It is necessary to point out that the afterslip triggering model could be used to interpret the parameters appeared in the MOL in terms of b/a, state variables and afterslip rate and tell us the possible physical processes behind earthquake triggering mechanism.