Updip rupture of the 2004 Sumatra earthquake extended by thick indurated sediments

During subduction, weak, unlithified sediments are scraped off the down-going plate and accumulate near the subduction trench axis. The weak nature of the sediments usually impedes the propagation of fault rupture during an earthquake. However, measurements of slip during the 2004 Sumatra–Andaman Mw 9.2 earthquake show that fault rupture propagatedupdip, extending unusually close to the subduction trench, in the southern part of the rupture area. Here we present seismic reflection images of the southern part of the 2004 Sumatra–Andaman earthquake rupture area. We show that sedimentary strata, greater than 4?km in thickness, form coherent blocks that have been thrust onto the continental margin during subduction. The blocks form a 130-km-wide plateau overlying the seismogenic zone and the plate boundary megathrust lies near to the base of the sediments. The sediments consist of the Nicobar and Bengal Fan turbidites and exhibit strong internal cohesion. We suggest that dewatering and lithification of the sediments during burial made them unusually competent and strong, thus enabling rupture during the 2004 earthquake to propagate beneath the plateau, close to the Sunda Trench. Extending fault rupture so close to the trench, and thus further seaward, may have enhanced the tsunami hazard by displacing a greater thickness of water