ACTIVE DEFORMATION ACROSS THE SUMATRAN FOREARC OVER THE DECEMBER 2004 M9.3 RUPTURE

A 220-km-long, single-channel seismic reflection profile, which crosses the northern Sumatra margin and the presumed rupture zone of the December 2004 M9.3 tsunamigenic earthquake, images active deformation across the entire fore arc. Structure and bathymetry co-vary at three distinct wavelengths. At the largest wavelength (tens of kms), 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 the forearc basin. Superimposed on these prism-wide variations are anticlinal ridges spaced ~13 km apart; landward and seaward fold vergence characterize the inner and outer slopes, respectively. Between anticlines, growth strata deposited in slope basins are folded at ~1-2 km wavelengths. These small folds deform the seafloor and increase in amplitude with depth, verging toward anticlinal hinges. We suggest that the long-wavelength variations are consistent with variations in strength across the forearc, from weaker material accreting beneath the outer slope to stronger material beneath the upper slope. The ~13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior. Growth strata geometries for the smallest folds indicate ongoing deformation within individual basins. Our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 M9.3 event, peeling up shallower and less competent trench fill, deforming the toe and the upper slope of the forearc, and producing uplift that is likely the cause of the observed tsunamigenesis.