The 11 April 2012 Mw 8.6 earthquake offshore Sumatra is the largest of the rare great intraplate earthquakes of the instrumental era. This major strike-slip event occurred in the diffuse zone of deformation that accommodates differential rotation between Indian and Australian plates. We perform a back projection analysis – calibrated with well- located aftershocks – of short-period teleseismic P-waves recorded by the European array to image the rupture process during the mainshock. In complement, a Love wave analysis is conducted for tracking azimuthal change in the apparent global source duration due to the source spatio-temporal extent. The combined analysis reveals a complex rupture pattern, characterized by three main episodes of energy release, the latest being located 370 km west of the epicenter, on the Ninety East Ridge, with a delay of 120 s. We interpret the 11 April 2012 Mw 8.6 offshore Sumatra earthquake as a complex westward-propagating sequence of dynamically triggered strike-slip fault ruptures, associated to the reactivation of the inherited NNE–striking sea floor fabric. The dynamic triggering mechanism could result from the interaction between transient surface wave stress perturbations and fluids.