Dynamic variations of gravity and the preparation process of the Wenchuan Ms8.0 earthquake
重力场动态变化与汶川MS8.0地震孕育过程

Based on the gravity observations from 1998 to 2007 of China mainland which repeated every 2~3 years, and considering the GPS and leveling measurements and the regional environment of tectonics and geodynamics, we analyze the characteristics of regional dynamic gravity field evolution and the preparation mechanics of the Wenchuan Ms8.0 earthquake in 2008. The result shows that variations of the regional gravity field reflect the dynamic effect of eastward flow of the Tibetan plateau and the preparation information of mid-and-long term, that is 2 to 10 years, of the Wenchuan earthquake. The obvious gravity indicators related to the Wenchuan earthquake preparation are the positive gravity changes in southwest of epicenter which lasted many years, which is in favor of the continuous accumulation of seismic energy, and the relatively large-scale gradient zone of gravity changes, which is in favor of occurrence of seismic rupture. The gravity change process associated with the earthquake preparation is increasing, accelerated increasing and decelerated increasing followed by the occurrence of the earthquake. The total gravity change accumulated over 8 years may come up to 200×10-8m·s-2. The preparation and occurrence of the Kunlunshan Ms8.1 earthquake in 2001 and the adjustment after this earthquake significantly affected the regional dynamic gravity changes and the preparation of the Wenchuan earthquake. The gravity change in the Songpan-Garzê block is negative, which may reflects the local uplift of deep crust-mantle and the crust expansion that resulted from the relatively high temperature inside, which might promote the thrust or nappe movement and the occurrence of the large earthquake.