Change of Strain Rate in Thailand after the 26 December 2004 and 28 March 2005 Earthquakes Using GPS Measurements

摘要

During the period 1994-2004, before the Mw 9.3 Sumatra-Andaman earthquake of 26 December 2004, Thailand was moving horizontally constantly eastward with an average rate of approximately 33.2±1.1 millimeters per year in ITRF2000. The magnitude of the horizontal strain rate was less than 30 nanostrain per year, which was considered small but significant. After the occurrence of the mega thrust earthquake, a horizontal movement to the southwest direction is evident at different rates all over the Thai region. Large co-seismic horizontal displacements were observed in the southern part of Thailand, while moderate and small displacements were seen in the central and northern parts of Thailand. The Royal Thai Survey Department (RTSD) carried out multiple Global Positioning System (GPS) field campaigns to monitor the post-seismic displacements. However, their efforts were complicated by the second mega thrust (Mw8.7) earthquake, which occurred at Nias, Sumatra on 28 March 2005. This study focuses on the use of GPS data, collected between 1994 and 2006, gathered from six GPS stations located at Phuket, Chumporn, Chonburi, Uthaitani, Srisaket and Lampang in Thailand and an additional station located in the northern part of Malaysia to derive changes in the strain rate. Here we find that today's deformation in Thailand is dominated by SW-NE trending extension. This feature is in agreement with post-seismic relaxation occurring on the Sumatran trench.