The M 6.1 July 2, 2013 earthquake south of Bireun, Indonesia occurred as a result of strike slip faulting within the crust of the Sunda plate. The earthquake struck several hundred kilometers to the east of the Sunda-Java Trench, which is the seafloor expression of the plate boundary between Sunda and the Indo-Australia tectonic plates. The location and faulting mechanism of this earthquake are consistent with upper plate accommodation of the right-lateral component of oblique convergence between the two plates, which is most clearly expressed along the Sumatran Fault, a major transform structure that bisects the island of Sumatra and whose main strand lies further to the west of the July 2 earthquake. At the latitude of this event, the Indo-Australia plate moves towards the northeast with respect to Sunda at a rate of approximately 60 mm/yr.


While earthquake activity in the region is more frequently associated with thrust faulting along the subduction zone to the west of the July 2 earthquake, upper plate events in Sumatra have not been uncommon; half a dozen M6+ earthquakes have occurred within 250 km of the July 2 event over the past 40 years. Such earthquakes can be very hazardous if close to population centers – as the July 2 event was - because of their shallow depths. A M 6.7 earthquake in November 1990, 120 km to the southeast, resulted in approximately 7 fatalities.