Earthquake in a Maze: Compressional Rupture Branching During the 2012 MW 8.6 Sumatra Earthquake

The 11 April 2012 magnitude 8.6 earthquake offshore of Sumatra was the largest measured earthquake along a strike-slip boundary that modern seismological instruments have ever recorded. Despite its size and proximity to a large population, there was no subsequent tsunami and there were no reported fatalities. Meng et al. (p. 724, published online 19 July) used teleseismic data from seismological networks in Japan and Europe to image the source of high-frequency radiation generated by the earthquake to understand the mechanics of this unique event. The resultant back projections showed that the earthquake slowly ruptured along a complex series of faults. The deeper-than-usual rupture path and large stress drop are both features that may not be unique to this earthquake, suggesting that regions in a similar tectonic environment may have the potential for more complex--or larger--intraplate earthquakes than might have been expected. [PUBLICATION ABSTRACT] Seismological observations of the 2012 moment magnitude 8.6 Sumatra earthquake reveal unprecedented complexity of dynamic rupture. The surprisingly large magnitude results from the combination of deep extent, high stress drop, and rupture of multiple faults. Back-projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system, with relatively slow rupture speed. The east-southeast-west-northwest ruptures add a new dimension to the seismotectonics of the Wharton Basin, which was previously thought to be controlled by north-south strike-slip faulting. The rupture turned twice into the compressive quadrant, against the preferred branching direction predicted by dynamic Coulomb stress calculations. Orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure-insensitive strength of the deep oceanic lithosphere. [PUBLICATION ABSTRACT]