We perform a back projection method to image the rupture propagation and short-period energy release of the 2012 Off Northern Sumatra earthquake (Mw8.6) using Hi-net data recorded in Japan. The results show a complex pattern of four conjugate faults over about 180 sec. There is a striking correspondence between the lengths and orientations of our rupture pattern with the distribution of aftershocks. Each of the first three stages of the rupture corresponds to a clear lineation in the aftershocks, with lengths of 200 to 400 km. Rupture speeds for several of the fault segments were very high, about 5 km/s, and exceed the local S-wave velocity. This is the first example of an oceanic earthquake with supershear rupture speed.