Monitoring Deformation of Thai Geodetic Network due to the 2004 Sumatra-Andaman and 2005 Nias Earthquakes by GPS

Abstract

The Thai geodetic network has been regularly observed with GPS since 1994. The continuous tectonic motions in Thailand located on the Sundaland block are well determined with uncertainties ∼1mm∕yr. The mega thrust earthquake occurred near Banda-Aceh on Dec. 26, 2004 with Mw 9.3 magnitude and caused significant surface displacements in many surrounding countries. The geodetic network within Thailand was also significantly deformed during the earthquake at the cm to dm level and the geodetic network will continue to deform for many months and possibly even years to come. The magnitude of displacements certainly has a direct impact on the Thai geodetic control network. The Royal Thai Survey Department (RTSD) has been carrying out GPS field campaigns in an attempt to monitor the postseismic displacement. However, the situation became more complicated when the second major earthquake occurred near Nias Island, Sumatra on Mar. 28, 2005 with Mw 8.7 magnitude. This paper will analyze the GPS observations obtained from the RTSD GPS campaigns and existing permanent GPS stations in Thailand up to the end of 2006 using the precise point positioning strategy of the GIPSY-OASIS II software. Results obtained from the latest campaign (Nov. 2006) indicated that the earthquakes have resulted in the horizontal displacements, ranging from 55cm in the south to about 6cm in the north of Thailand. The behavior of postseismic motions is investigated in this study and it was found that the postseismic motions can be fitted well by a logarithmic decay function. Finally, a strategy for updating the Thai geodetic network is proposed.