Today’s M8.6 strike-slip earthquake off the Sumatran coast is an incredible event for a number of reasons: First, it’s (as far as I’m aware) the largest recorded strike-slip earthquake. Second, it’s an intraplate earthquake–although it is about a hundred kilometers from the Sumatran subduction zone, where the catastrophic 2004 M9.1 earthquake occured), it’s not a plate boundary rupture. However, the stresses resulting from incipient subduction or the effects of the 2004 event likely loaded the fault responsible for this event. This may also make it the largest reported intraplate earthquake, but I’ll have to check on this. And third, with a centroid depth of 22 km, this is a dominantly mantle event, as the crustal thickness in mature oceanic plates is typically ~7 km. Since the energy released from an earthquake scales not simply with the along-strike length of the rupture but the total fault area, it’s possible that the great magnitude of this event is a result of a very broad vertical, as well as horizontal, rupture patch. Additionally, the magnitude scales with the strength of the rock surrounding the fault, and cold (and hence capable of brittle failure) oceanic mantle is made of peridotite and very strong. Although I have no idea what kind of surface observations are possible for this event, we may find that much of the slip and energy released occurred in the upper mantle.
Well I have to board a plane, so I will cut this off here. Expect to hear much more about this event in the future, surely from Austin Elliott at The Trembling Earth if no one else.