Postseismic slip following the 2005 Nias-Simeulue, Sumatran Indonesia earthquake Ya-Ju Hsu1, Mark Simons1, Jean-Philippe Avouac1, Kerry Sieh1, John Galetzka1, Yehuda Bock2, Danny Natawidjaja3 and Linette Prawirodirdjo2 1. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. 2. Institute of Geophysics and Planetary Physics, University of California San Diego, California, U.S.A. 3. Research Center for Geotechnology, Indonesian Institute of Sciences, Bandung, Indonesia. Abstract The March 28, 2005 Nias-Simeulue Mw 8.7 earthquake ruptured the plate boundary megathrust on the northwest coast of Sumatra, southeast of the December 26, 2004 Mw 9.1 Aceh-Andaman earthquake. The availability of 3-component continuous GPS data and microatoll observations of vertical displacements for this event, all located above or near the region of coseismic rupture, permits us to address a disparate suite of questions regarding the mechanical nature of the megathrust. For the first time, we unambiguously determine that significant post-seismic slip occurred updip from the coseismic rupture, consistent with the notion that the shallow-most portions of the subduction interface primarily slips aseismically. Generally, the regions of major coseismic and postseismic slip are mutually exclusive and after 9 months many portions of the megathrust are still slipping at rates several times the long-term average interseismic rate. The observed relationship between the spatial and temporal evolution of cumulative postseismic displacement and the cumulative number of aftershocks is consistent with a model in which the location and number of aftershocks is driven by induced afterslip and not directly by coseismic stress changes. The difference in directions between the interseismic displacement field and our modeled postseismic slip suggests that the ambient absolute level of stress on the megathrust near the Batu Islands is about 5 MPa. .