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Active deformation across the Sumatran forearc over the December 2004 M_w9.2 rupture

¹ Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA
² Natural Resources Canada, Geological Survey of Canada–Atlantic, 1 Challenger Drive, Dartmouth, Nova Scotia B2Y 4A2, Canada
³ Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas, Austin, Texas 78759-8500, USA
⁴ Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487, USA
⁵ Graduate School of Oceanography and Department of Ocean Engineering, University of Rhode Island, Narragansett, Rhode Island 02882, USA

A 220-km-long, single-channel seismic reflection profile crosses the northern Sumatra margin and presumed rupture zone of the December 2004 M_w9.2 tsunamigenic earthquake and images active deformation across the forearc. At the largest wavelength (tens of kilometers), the forearc surface is defined by a steep, 55-km-wide outer slope, a 110-km-wide upper slope forming a broad depression between two forearc highs, and a 25-km-wide steep inner slope between the landward high and forearc basin. Superimposed on these prism-wide variations are anticlinal ridges spaced 13 km apart; the inner and outer slopes are characterized by landward and seaward fold vergence, respectively. Between anticlines, growth strata deposited in slope basins are folded at 2–3 km wavelengths. These small folds deform the seafloor and increase in amplitude with depth, verging toward anticlinal hinges. We suggest that long-wavelength variations are consistent with variations in strength across the forearc. The 13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior. Growth strata geometries indicate ongoing deformation within individual basins. Our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 M_w9.2 event, peeling up shallower and less competent trench fill, deforming the toe and the upper slope of the forearc, and producing seabottom uplift responsible for the tsunami.

Key Words: forearc • Sumatra • growth sedimentation • wedge • tsunami