Downgoing plate topography stopped rupture in the A.D. 2005 Sumatra earthquake

Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmentation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the Mw = 8.7, A.D. 2005 Simeulue-Nias earthquake, and the northern termination of a major 1797 earthquake that was partly filled by an Mw = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation. INTRODUCTION Subduction zones are known to rupture in distinct sections or segments. Links between topography on the downgoing plate and this segmentation are debated (Wang and Bilek, 2011; Kopp, 2013). Topography exceeding 1 km correlates with segment boundaries west of South America (Sparkes et al., 2010), but the mechanism, whether by changing mechanical coupling or fault zone physical properties, remains controversial. Some studies argue that subducting topography exceeding the height of the décollement zone strengthens the plate boundary, increasing coupling and acting as a seismic asperity (Cloos, 1992; Scholz and Small, 1997; Bilek et al., 2003); others argue that subducting topography fractures and increases fluid content in the overriding plate, decreasing coupling and limiting strain accumulation to suppress seismogenic rupture (Kelleher and McCann, 1976; Wang and Bilek, 2011; Singh et al., 2011). More detailed observations suggest that some subducted seamounts concentrate stress ahead of them but suppress seismicity above and behind due to fracturing (Mochizuki et al., 2008) and that accompanying erosion of the overriding plate results from hydrofracture (von Huene et al., 2004). A key issue is that there are few examples where rupture during a major earthquake and topography of the plate boundary within the seismogenic zone are both well constrained. Here we examine a dense network of seismic reflection and gravity data from the central Sunda subduction zone to constrain the depth to the top of the downgoing plate. This region spans the termination of the 28 March 2005, Mw 8.7 earthquake with wellcharacterized slip distribution. We show that the southern end of the 2005 rupture corresponds to a 3 km high on the downgoing plate originating at the Wharton fossil ridge. TECTONIC SETTING OF THE SUMATRA MARGIN The central Sumatra margin had major earthquakes in 1797, 1833, and 1861 (Newcomb and McCann, 1987) as well as the modern events since 2004 (Fig. 1). Coral uplift constrains the extent of the Mw = 8.7–8.9 GEOLOGY, January 2016; v. 44; no. 1; p. 71–74 | Data Repository item 2016017 | doi:10.1130/G37258. | Published online XX Month 2015 © 2015 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Figure 1. Swath bathymetry superimposed on global topography with magenta contours of A.D. 2005 Sumatra (Indonesia) earthquake coseismic slip from Hsu et al. (2006). Dashed lines are at 1 m intervals and solid lines are at 2 m intervals; red star is epicenter. Red symbols show earthquake epicenters, circles (Lange et al., 2010) and squares (Tilmann et al., 2010) show local studies, and small x symbols indicate the International Seismological Centre catalogue 1960–2013 (http://www.isc.ac.uk/). The 97E fracture zone (97E FZ) and Investigator fracture zone (IFZ) (Kopp et al., 2008) are yellow dotted lines and hachures, respectively. Yellow dashed lines are segments of the Wharton fossil ridge (WFR) modified after Liu et al. (1983), based on our bathymetry. Yellow lines show multichannel seismic reflection profiles (solid line is cruise SO198–2, dashed line is cruise SO200, heavy lines are the profiles in Fig. 2). The downgoing plate moves in the direction of the arrow at 40–45 mm/yr relative to the forearc sliver (McNeill and Henstock, 2014). Inset is regional tectonic setting, showing slip areas for the A.D. 1797, 1833, 1935 (Natawidjaja et al., 2006), 2004, and 2005 earthquakes. 96 ̊E 97 ̊E 98 ̊E 99 ̊E 2 ̊S 1 ̊S 0 ̊ 1 ̊N 2 ̊N