April 2012 intra-oceanic seismicity off Sumatra boosted by the Banda-Aceh megathrust

The two earthquakes of respective magnitudes 8.6 and 8.2 that occurred off the coast of the Sumatra subduction zone on 11 April 2012 are shown to be part of a continuing boost of the intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes. The 11 April east Indian Ocean earthquakes On 11 April 2012, two of the largest strike-slip earthquakes ever recorded — at magnitudes of 8.7 and 8.2 — occurred in the northeastern Indian Ocean, a few hundred kilometres off the coast of Sumatra. Three groups now report the analysis of seismic data from the days and months before and after these events, as well as the events themselves. Matthias Delescluse and co-authors show that these earthquakes are part of the ongoing boost of intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes. They conclude that the Australian plate, driven by slab-pull forces at the Sunda trench, is gradually detaching from the Indian plate. Han Yue and colleagues show that the 11 April event involved a complex four-fault rupture lasting several minutes, followed two hours later by a magnitude-8.2 aftershock. These great ruptures on a lattice of strike-slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the Indian and Australian plates. Fred Pollitz and colleagues show that, in the six days following 11 April, the global rate of remote earthquakes with magnitudes greater than 5.5 increased nearly fivefold, and events up to magnitude 7 seem to have been triggered. The unprecedented delayed triggering power of this earthquake may arise from its strike-slip source geometry, or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure. Large earthquakes nucleate at tectonic plate boundaries, and their occurrence within a plate’s interior remains rare and poorly documented, especially offshore. The two large earthquakes that struck the northeastern Indian Ocean on 11 April 2012 are an exception: they are the largest strike-slip events reported in historical times 1 , 2 and triggered large aftershocks worldwide 3 . Yet they occurred within an intra-oceanic setting along the fossil fabric of the extinct Wharton basin, rather than on a discrete plate boundary 4 , 5 , 6 , 7 , 8 . Here w