Deformation within the downgoing oceanic lithosphere seawards 3 of subduction zones is typically characterised by regimes of shallow 4 extension and deeper compression, due to the bending of the oceanic 5 plate as it dips into the subduction zone. However, o shore Suma- 6 tra there are shallow compressional earthquakes within the down- 7 going oceanic plate outboard of the region of high slip in the 2004 8 Aceh-Andaman earthquake, occurring at the same depth as exten- 9 sional faulting further seaward from the trench. A clear separation is 10 seen in the location of intraplate earthquakes, with extensional earth- 11 quakes occurring further seawards than compressional earthquakes at 12 the same depth within the plate. The adjacent section of the fore- 13 arc prism west of Aceh is also anomalous in its morphology, charac- 14 terised by a wide prism with a steep bathymetric front and broad, 15 gradually-sloping top. This shape is in contrast to the narrower and 16 more smoothly-sloping prism to the south, and along other subduction 1 17 zones. The anomalous near-trench intraplate earthquakes and prism 18 morphology are likely to be the result of the geologically-rapid gravi- 19 tational collapse of the forearc, which leads to induced bending within 20 the subducting plate, and the distinctive plateau-like morphology of 21 the forearc. Such collapse of the forearc could be caused by changes 22 through time of the material properties of the forearc rocks, or of the 23 thickness of the sediments entering the subduction zone.