Megathrust earthquakes can nucleate in the forearc mantle: Evidence from the 2004 Sumatra event

Abstract

International audienceCurrent models predict that the seismogenic zone along subduction thrusts, where the largest earthquakes nucleate and propagate, does not extend to the forearc mantle below the crust of the upper plate. Stable sliding conditions have been shown to prevail there, particularly along several circum-Pacific margins that underwent great megathrust earthquakes (Mw > 8.5) during the twentieth century. Based on geophysical investigation, we show that the great 2004 Sumatra-Andaman earthquake (Mw = 9–9.3) contradicts these models: not only did it propagate downdip along the interface between the forearc mantle and the subducting plate, but it actually nucleated along this reportedly aseismic part of the interplate contact. Petrological models can therefore underestimate the downdip extent of rupture zones to be expected in megathrust earthquakes, and need to be revised to account for this observation, albeit unusual

Experimental deformation and recrystallization of olivine – processes and timescales of damage healing during postseismic relaxation at mantle depths

“…In the tectonic environment of subduction zones, shear zones can reach down to mantle depth as continuation of seismically active fault zones, as for example observed for the Sumatra earthquake in 2004 that caused the infamous tsunami (Singh et al, 2008;Dessa et al, 2009). The macroscale time-dependent rheology of such shear zones is controlled by nano-to micro-scale deformation and recrystallization processes in the affected upper mantle rocks (i.e., peridotites).…”

Bathymetry of the Indonesian Sunda margin-relating morphological features of the upper plate slopes to the location and extent of the seismogenic zone

“…The sediment cover on top of the oceanic plate in the north of Sumatra reaches a thickness of more than 5 km (e.g. Franke et al, 2008;Dessa et al, 2009). The major portion for the sediment income originates from the Himalayas and is transported as far south as the Sunda Strait (Moore et al, 1982).…”

“…The sediment thickness of the forearc basins varies, sediments locally reach up to 6 km thickness north of Simeulue (e.g. Dessa et al, 2009) decreasing to the south with a thickness of ∼4-5 km (Kopp et al, 2001). Off eastern Java sedimentary sequences are ∼5 km thick in the forearc basin.…”

“…Several studies were conducted in the area north of Simeulue island at 3 • N to 4 • N using MCS and wide-angle seismic studies Dessa et al, 2009;Klingelhoefer et al, 2010) as well as gravity and thermal modelling (Klingelhoefer et al, 2010). All of them obtain similar depths for the updip limit of the SZ in that region.…”

Structure of the Central Sumatran Subduction Zone Revealed by Local Earthquake Travel Time Tomography Using Amphibious Data

“…However, for the Sumatran margin the seismogenic zone is suggested to reach below the continental Moho based on gravity surveys and wide-angle refraction and local earthquake tomography (Siberut: Simoes et al 2004; Aceh basin and Simeulue: Dessa et al 2009;Klingelhoefer et al 2010Southern Mentawai Islands: Collings et al 2012). For central Sumatra Chlieh et al (2008) 30 estimate the width of the seismogenic zone based on geodetic data between 20 km and 50 km, with the largest width approximately alongside Siberut, and the smallest widths at the Batu Islands and between Sipora and the Pagai Islands.…”