Crustal Movements Before and After the Wenchuan Earthquake as Detected by GPS Observations

The methods of both best fits by trend surfaces and statistics are used in the aim of looking for the epicentral area from the strain accumulations in the regional network observed from 1999 to 2007 before the Wenchuan Earthquake. Besides the epicentral area of the Kunlun Mountain earthquake of M8.1 in 2001,the results of best fits by trend surfaces of the strain accumulations from 2004 to 2007 in the regional network before the earthquake show that the Wenchuan earthquake occurred at the eastern fringe of a region of large area and large accumulations of the first shear strains and also at the northeastern fringe of a smaller region with significant areal compression accumulations. The statistics of the accumulations of the strain components show that they also show anomalous distribution patterns in this region and its neighborhood with increasing accumulations of both shear strain and areal compression. Positioning solutions of single epochs for continuous GPS observations in or around the earthquake area before the event show that there were eminent precursory crustal movements,particularly vertical movements.