Geodetic observations of an earthquake cycle at the Sumatrasubduction zone: Role of interseismic strain segmentation

摘要

We use survey mode and continuous GPS data from 1991 to 2007 to examine faultsegmentation in the earthquake cycle at the Sumatra megathrust, site of the 26 December2004 M_w9.1 Sumatra-Andaman, the 28 March 2005 M_w8.7 Nias-Simeulue, and the12 September 2007 M_w 8.4 Mentawai earthquakes. These data, including new observationsfrom 2006 and 2007, allow us to observe the final few years of one earthquake cycle andthe beginning of the next. Our analysis reveals that the megathrust is segmented, acharacteristic that may persist through multiple earthquake cycles. The Nias-Simeulueearthquake ruptured approximately the same region that broke in 1861, a 300 km longsegment abutting the Sumatra-Andaman rupture zone. Farther southeast, the Mentawaisegment of the megathrust (0.5°S-5°S), which produced M > 8 earthquakes in 1797 and1833, is fully locked in the interseismic period but is flanked by two freely slippingregions, the Batu Islands in the NW and Enggano in the SE. The 12 September 2007Mentawai earthquake sequence ruptured only the southern one third of the 1833 rupturezone. We model postseismic deformation from the Sumatra-Andaman and Nias-Simeulueearthquakes and find that afterslip was concentrated updip and downdip, respectively,from the main shocks. Comparing the velocity fields before and after 2001, we find thesubduction zone underneath the Batu Islands and Enggano, which, prior to theearthquakes, was partially to fully coupled, appears now to be slipping freely. Thus, whilethe segmentation of the subduction zone is preserved, interseismic coupling on thesubduction fault may vary with time.