Abstract:

The 2008 Wenchuan MS 8.0 earthquake occurred near the upper reservoir of the Zipingpu, where the river closure was finished in 2002. After the first water filling of the reservoir in October 2005, some earthquake swarms were recorded, of which the most remarkable ones took place at Shuimo, less than 5km away from the Wenchuan main shock, between 2006 and 2008. What is the role of the reservoir in triggering the Wenchuan earthquake has become a focused topic. In the earlier work, the study of the changes of Coulomb failure stress (ΔCFS) in the Longmenshan fault zone (LMSF) has pervaded discussions. To improve our understanding of this devastating earthquake, we made a detailed analysis of the local seismicity recorded by the Zipingpu local seismic network during the period from 31 July 2004 to 11 May 2008, which was constructed with a primary purpose for monitoring possible reservoirtriggered earthquakes. It is noted that a series of NE trending faults running through the reservoir, lying on carbonate beds, which forms the geological background for water percolation. With the waterlevel fluctuation and rising, the upreservoir was involved in the backwater areas. The spatialtemporal distribution of hypocenters demonstrated a clear migration pattern that indicated the pressure diffusion from the dam to upreservoir, showing a hydraulic diffusivity (D) of 0.7 m2/s. The D is closely related with the stream profile, which is the comprehensive reflection of the porepressure and reservoirfilling process. The Wenchuan segment of the LMSF had been locked with a high stress level before the reservoir filling. The range of seismicity extended for three times along the fault immediately after every rising of the water level. Previous experiments showed the existence of the synergism process of the fault under a metainstability state before fault sliding. It enhanced the stress on the stronger portion of the fault and the synergism degree by reducing strength of the weak portions and by increasing the total length of weak portions. We suggest that the three times of increasing seismicity after high waterlevel, in a high tectonic stress context, just reflected the water entering the partial locked fault, leading to weakening of the fault. The generation of the Wenchuan earthquake was the result of joint action of high regional tectonic stress and reservoir water. Many times of high water level have weakened the locked fault segment, on which the synergism degree reached a critical level and rapid instable slip occurred over a longer section due to the stress of the stronger portion. This study provides new facts and arguments for the further discussion on the role of the Zipingpu reservoir in the generation of the Wenchuan earthquake.

Key words: Wenchuan earthquake, Zipingpu reservoir, fluid diffusion coefficient, metainstability stage