Abstract: Time series of horizontal and vertical displacements at GPS Observation Stations of IGS and the Crustal Movement Network of China (CMONOC) before and after the Sumatra Earthquake with M8.7 on Dec. 26, 2004 and earthquake with M8.5 on March 28, 2005 in Indonesia are obtained from the data of continuous GPS observation for more than 6 years with datum definition of a group of stable stations of small mutual horizontal or vertical displacements for the time series. Though the number of GPS observation stations is small and they scatter in a vast area, the characteristics of the pre-seismic, co-seismic and post-seismic crustal movements for such huge earthquakes can be seen and the results are meaningful for further studies. The datum definition used in the paper effectively reduced the translation and rotation in the horizontal displacement field and the influence of the non-parallelism of the local ellipsoidal surface on vertical displacements is small during the time period covered in this study. They indicate that the displacement field in vast area obtained by the displacement solutions in the paper is reasonable. The co-seismic horizontal displacements and the post-seismic vertical displacement are as large as a few centimeters and are detected 4500 km away from the epicenter of M8.7 earthquake or even farther.

Key words: GPS continuous observation station, Sumatra great earthquake in Indonesia, Crustal movement, Vertical displacement, Horizontal displacement