The Diverse Slip Behavior of the Banyak Islands Section of the Sunda Megathrust Offshore Sumatra.

Abstract:

We examine the long-term slip history of the Banyak Islands section of the Sunda megathrust, which stretches for ~100 km between the islands of Simeulue and Nias offshore northern Sumatra. In the last decade this section has ruptured in two large earthquakes: the great M_w 8.6 2005 Nias-Simeulue earthquake and a previously unstudied M_w 7.8 earthquake that occurred five years later in April 2010. Using data from the Sumatra GPS Array (SuGAr), we model the coseismic slip distributions of these earthquakes. The 2005 rupture had a low-slip patch near the Banyak Islands (with higher slip patches on either side along-strike), and we show that the 2010 earthquake rupture coincides with this low-slip patch. The uplift pattern in the coral records suggest that the 2005 rupture might have been similar to its 1861 predecessor [Meltzner et al., 2015]. We investigate whether the low coseismic slip behavior of the Banyak Islands section is persistent for great earthquakes in this region. Coral records also show that the Banyak Islands section had a 15-year-long slow slip event between 1966 and 1981, immediately down-dip of the 2010 rupture patch [Tsang et al., 2015, in press]. The complex slip history in this section might result from differences in frictional properties, or structural features that could include a transform fault in the downgoing slab, a bend in the megathrust, and an extension of the Batee fault zone in the upper plate. We explore candidates for the cause of the diverse slip behavior of this patch. Understanding these structural and morphological controls on megathrust slip behavior has important implications for seismic hazard analyses.