A glimpse of earthquake cycle in the Sumatra region

We analyse temporal variation of crustal deformation derived from Global Positioning System (GPS) measurement at SAMP, a permanent GPS site in Sumatra, Indonesia. The site is located at about 300 km from the epicentres of the giant 26 December 2004 Sumatra–Andaman (Mw 9.0–9.3) and the great 28 March 2005 Sumatra (Mw 8.6) earthquakes. It experienced an interseismic displacement at a rate of about 2 cm/yr towards east in the preceding four months of the 2004 earthquake. The 2004 and 2005 earthquakes caused coseismic displacements of about 15 cm towards west and about 18 cm towards southwest respectively. In the period between the two earthquakes and after the 2005 earthquake, slow transients arising due to postseismic deformation are clearly recorded at this site. The postseismic deformation is in the opposite sense to that of the interseismic deformation, indicating either relaxation of the lower crust and upper mantle or afterslip on the subduction interface that lies further downdip of the ruptured part of the subducting interface. The site also recorded probably a slow earthquake that occurred on 9 June 2005, which caused a westward movement of about 2 cm at SAMP. However, more data from other sites are required to further confirm and analyse this event. Together, these displacements provide a glimpse of crustal movement during the earthquake cycle in the region