An 8.6 magnitude earthquake occurred 62 miles off the coast of Sumatra on April 11th, 2012. 

Along with being severe by any measure, in one way it was the largest earthquake in observed human history; it originated within the plate rather than at a plate boundary.

The quake originated under the Wharton Basin in the Indian Ocean, where hundreds of miles of rock were under crushing tension, causing the plate to deform at its base. This 'deforming zone' was also absorbing tension as two plates, the Indian and Australian plates, rotated toward each other. 

One month after the earthquake, researchers visited the Wharton Basin to reconstruct the rupture history of the events of April 11th in order to gain a better understanding of the general nature of these rare large within-plate earthquakes. They used back projection analysis, which tracks radiation that emanates as new locations along a rupture path become active. 

Calibrating the back projection analysis with aftershocks, which scientists use to calculate the travel time and distance of seismic P and S waves that originate at the epicenter, the authors find that the events of the Sumatra Earthquake had their roots in the old fabric of the Wharton Basin, the floor of which is littered with old fractures. Tension deep within the deformation zone had reactivated one of the north-south aligned fractures.

That rupture in turn triggered a series of ruptures of old fractures, generating a slew of strike-slip faulting that traveled westward and ended at the Ninety East Ridge, the north-south oriented seamount chain that bounds the basin, 230 miles from the epicenter and 120 seconds later-the timing of the last rupture event. 

Citation:Claudio Satriano, Eszter Kiraly, Pascal Bernard, and Jean-Pierre Vilotte: Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Universite Paris Diderot, UMR CNRS 7154, Paris, France. 'The 2012 Mw 8.6 Sumatra earthquake: Evidence of westward sequential seismic ruptures associated to the reactivation of a N-S ocean fabric', Geophysical Research Letters, doi:10.1029/2012GL052387, 2012