March 2, 2016 Earthquake: Sumatra, Indonesia

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 2, 2016 event. On June 18, 2000, a M 7.9 earthquake ruptured an oceanic strike-slip structure about 1000 km southeast of the March 2, 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.

Felt (V) at Air Bangis; (IV) at Padang and Padangpanjang; (III) at Bukittinggi. Also felt at Banda Aceh, Bengkulu, Medan and Selat Panjang. Felt (II) in Singapore. Felt at George Town, Kampung Koh, Kota Bharu, Kuala Lumpur and Pasir Gudang, Malaysia. Also felt at Port Blair, India. Port Blair, Andaman and Nicobar Islands, India - CDI: 4; Kampong Koh, Perak, Malaysia - CDI: 3

(above from ref #13456)

A massive and shallow earthquake struck on Wednesday off the west coast of the Indonesian island of Sumatra, a region devastated by the 2004 Indian Ocean quake and tsunami, and there were early reports of deaths. A tsunami alert was called off, television reports said. Neighbouring Australia issued a tsunami watch for parts of its western coast and then cancelled it. "There are some who have died," said Heronimus Guru, the deputy head of operations with the National Search and Rescue Agency. He did not know how many, but any rescue operation will be hampered by the dark, which falls early in the tropical archipelago. There were no immediate reports of damage, but the shallower a quake, the more dangerous it is. The US Geological Survey originally put the magnitude at 8.2, and then 8.1, before lowering it to 7.9. The epicentre was 808km (502 miles) southwest of Padang, USGS said. It was 10 km (six miles) deep. "So far there have been no reports (of damage)," Andi Eka Sakya, an official of the National Meteorological Agency, told TVOne. "In Bengkulu (on the southwestern coast of Sumatra) they didn't feel it at all." (reference #11480)

The Wharton Basin, off southwest Sumatra, ruptured to a large intraplate left-lateral strikeslip Mw 7.8 earthquake on 2016 March 2. The epicentre was located ∼800 km to the south of another similar-mechanism intraplate Mw 8.6 earthquake in the same basin on 2012 April 11. (reference #11923)