Is the 2013 Lushan earthquake (Mw = 6.6) a strong aftershock of the 2008 Wenchuan, China mainshock (Mw = 7.9)?

Abstract

Following the devastating 2008 Wenchuan earthquake (Mw=7.9), the 2013 Lushan strong shock (Mw=6.6) occurred again on the Longmen Shan thrust fault. Ever since the earthquake occurred, it has been a topic of hot debate on whether it is a strong aftershock of the 2008 Wenchuan earthquake or a new independent one. To this purpose, we investigated the seismogenic fault for the Lushan earthquake, and found that it is situated on the same fault system as that for the Wenchuan earthquake, and the Lushan shock ruptured one of the asperities which were left unbroken in the 2008 Wenchuan earthquake. In addition, we analyzed the statistical properties of the Wenchuan-Lushan earthquake sequence (WLES) using three established empirical laws on aftershocks. The modelling results showed that the WLES, in which the Lushan event is regarded as one of aftershocks of the Wenchuan mainshock, well satisfied three accepted statistical laws, namely the Gutenberg-Richter relation, modified Omori-Utsu law and Båth’s law. At the same time, we observed that the weak and ductile materials between the hypocenters of the Wenchuan and Lushan shocks resulted in seismic gap in between, and the strong materials in the source region and special curved geometry of seismogenic fault for the Lushan event may be responsible for the approximately 5-year delay of the occurrence. In particular, the focal mechanism of the Lushan event, like most of the other strong aftershocks of the Wenchuan mainshock, is nearly thrust-type fault, suggesting the Lushan earthquake, together with other strong aftershocks, is controlled by the same stress environment. Most of all, we noticed that the Lushan event was triggered by the 2008 Wenhcuan main shock. Taken together, it is reasonable to infer that the 2013 Lushan event is a strong aftershock, possibly being the largest one, of the Wenchuan earthquake. Meanwhile, we propose in the paper that it is an important criterion for identification of aftershocks by means of judging whether they are triggered by the main shock or not. Identifying and forecasting strong aftershocks shall be crucial in evaluating the seismic hazard for large earthquakes.