Indonesia

Indonesia: M7.8 Southwest of Sumatra Earthquake of 02 March 2016

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Tectonic Summary

The March 2, 2016 M 7.8 earthquake, 800 km off the west coast of southern Sumatra, Indonesia, occurred as a result of strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate. This event is located 600 km to the southwest of the major subduction zone that defines the plate boundary between the India/Australia and Sunda plates offshore Sumatra. At this location, the India/Australia plates move north-northeast relative to the Sunda plate at a velocity of about 55 mm/yr. The deformation zone defining the defuse boundary between the India and Australia plates is nearby the March 2 earthquake.

Large strike-slip earthquakes are not unprecedented in the diffuse boundary region separating the India and Australia plates, southwest of the Sumatra subduction zone. In 2012, two events of M 8.6 and M 8.2 on the same day (04/12/2012) ruptured a series of oceanic strike-slip structures 650-850 km to the north of the March 3, 2016 event. On June 18, 2000, a M 7.9 earthquake ruptured an oceanic strike-slip structure about 1000 km southeast of the March 3, 2016 earthquake. The focal mechanisms of the all of these earthquakes are consistent in implying that each event could have occurred as the result of left-lateral slip on an approximately north striking fault or right-lateral slip on an approximately west striking fault. The two different orientations of strike-slip faulting are both possible under the same tectonic stress field; perpendicular strike-slip faults that are both compatible with the same stress field are called "conjugate faults". In 2012, in-depth studies of those major events showed that faults of both orientations were involved in their rupture processes, breaking a network of conjugate faults over an area of ~ 200x200 km in size in the Wharton Basin. Because of the remote locations of these oceanic earthquakes, such events rarely cause shaking-related fatalities (the 2012 M 8.6 event caused two). Similarly, strike-slip earthquakes do not typically generate tsunamis.