International audienceWe present results from multibeam bathymetric data acquired during 2005 and 2006, in the region of maximum slip of the 26 Dec. 2004 earthquake (Mw 9.2). These data provide high-resolution images of seafloor morphology of the entire NW Sumatra forearc from the Sunda trench to the submarine volcanic arc just north of Sumatra. A slope gradient analysis of the combined dataset accurately highlights those portions of the seafloor shaped by active tectonic, depositional and/or erosional processes. The greatest slope gradients are located in the frontal 30 km of the forearc, at the toe of the accretionary wedge. This suggests that long-term deformation rates are highest here and that probably only minor amounts of slip are accommodated by other thrust faults further landward. Obvious N–S oriented lineaments observed on the incoming oceanic plate are aligned sub-parallel to the fracture zones associated with the Wharton fossil spreading center. Active strike-slip motion is suggested by recent deformation with up to 20–30 m of vertical offset. The intersection of these N–S elongated bathymetric scarps with the accretionary wedge partly controls the geometry of thrust anticlines and the location of erosional features (e.g. slide scars, canyons) at the wedge toe. Our interpretation suggests that these N–S lineaments have a significant impact on the oceanic plate, the toe of the wedge and further landward in the wedge. Finally, the bathymetric data indicate that folding at the front of the accretionary wedge occurs primarily along landward-vergent (seaward-dipping) thrusts, an unusual style in accretionary wedges worldwide. The N–S elongated lineaments locally act as boundaries between zones with predominant seaward versus landward vergence
We locate in the Cascadia subduction zone non‐volcanic tremors from an episodic tremor and slip event in February 2002. The tremors occurred during two 10‐day periods separated by a short lull. Tremors that occurred during the first period are broadly distributed between 10 km and 40 km depth at the southern end of Vancouver Island, and over a >50 km wide region measured normal to the margin. During the second period of tremor activity, most tremors were concentrated in a narrower zone, and many tremors occurred near the subduction megathrust at approximately 35 km depth. This relatively large number of tremors near the subduction megathrust is in contrast to the broad depth distribution of the subsequent March 2003 tremor sequence. The change in the pattern of tremor activity observed during the February 2002 sequence may indicate a change in the slow slip process, or in its migration along the margin.
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