En échelon and orthogonal fault ruptures of the 11 April 2012 great intraplate earthquakes

The magnitude 8.7 earthquake that occurred off the coast of the Sumatra subduction zone on 11 April 2012 is shown to have had an extraordinarily complex four-fault rupture; these great ruptures represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates. 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. The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary—from an environment of continental collision to an island arc subduction zone1,2. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude Mw 8.7) occurred about 100–200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest–southeast compression within the plate caused by the India–Eurasia continental collision to the northwest, together with northeast–southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great (Mw 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20–30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW–ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE–SSW) that crosses the first fault. This was followed by westward rupture on a second WNW–ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en echelon WNW–ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE–SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30–40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.