Abstract: Five campaigns for non-continuous observations were carried out in the years of 1999, 2001, 2004, 2007 and 2009 from the regional network of 1000 GPS stations of CMONOC(Crustal Movement Observation Network of China). The great Wenchuan M8.0 earthquake of May 12, 2008 occurred in an area with dense GPS observation stations in the regional network. GPS observations in the regional network carried out in many campaigns for a long time can mitigate the effects of periodic annual changes of vertical displacements on long term trend and are favorable for detecting trend variations in vertical displacements. In this paper, the accuracy of vertical displacements obtained from GPS observations is briefly discussed. Ground subsidence, the major contamination in vertical displacements, particularly, large ground subsidence caused by ground water withdraw in large amount, is analyzed. In order to get vertical displacements before the Wenchuan earthquake, at first the results with large subsidence caused by ground water withdraw in large amount must be deleted. Spatial distributions on China mainland from trend surface mappings show 3 areas of significant ground subsidence. In the subsidence area close to the Wenchuan earthquake region there are dense GPS stations that are free from contaminations of ground water withdraw. The vertical displacements from 1999 to 2007 before the Wenchuan earthquake show significant subsidence in the area south east to the earthquake and the northern segment of the Longmenshan fault had been locked as shown in vertical displacements. This subsidence area showed different changes both in time and space from the changes of other 2 subsidence areas. This subsidence area and the area with anomalous strain accumulations appeared at the same time, but they were not overlapping. Large coseismic vertical displacements were located along the northern segment of the Longmenshan fault which was locked before the event. All these facts show that the subsidence area near the epicenter, detected by GPS observations before the earthquake, and the locked segment of the fault shown by vertical displacements were closely related with the occurrence of the Wenchuan earthquake.

Key words: Wenchuan earthquake, Vertical crustal movement, GPS, Earthquake prediction