April 11, 2012 Earthquake: Off West Coast, N Sumatra, Indonesia

Felt Reports

At least 2 people killed, 8 others died from heart attacks, 12 injured and some buildings damaged in Aceh. Felt (VII) at Banda Aceh and Meulaboh, (VI) at Padang and (V) at Sibolga and Singkil. Also felt (V) at Gunungsitoli, Nias and at Jitra, Malaysia. Felt in much of Sumatra and Java. Felt widely in South and Southeast Asia, including Bangladesh, Bhutan, Brunei, Burma, India, Laos, Malaysia, Maldives, Singapore, Sri Lanka, Thailand and Vietnam. Felt as far as Bombay, India and Broome, Australia. A small tsunami was recorded, with maximum wave heights (one-half peak-to-trough) at the following selected tide stations: 1.08 m at Meulaboh, 37 cm at Sabang and 14 cm on Pulau Enganno, Indonesia; 8 cm at Cocos Island, Australia; 23 cm at Chittagong, Bangladesh; 21 cm at Male, Maldives; 17 cm at Port Louis, Mauritius; 11 cm at Trincomalee, Sri Lanka.

Tectonic Summary

The M8.6 April 11, 2012 earthquake off the west coast of northern Sumatra, Indonesia, occurred as a result of strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate. The quake was located approximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates offshore Sumatra. At this location, the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52 mm/yr.

Large strike-slip earthquakes, while rare, are not unprecedented in this region of the Indo-Australian plate. Since the massive M 9.1 earthquake that ruptured a 1300 km long segment of the Sumatran megathrust plate boundary in December of 2004, three large strike-slip events have occurred within 50 km of the April 11, 2012 even. These earthquakes occurred on April 19 2006 (Mw6.2), October 4 2007 (Mw6.2) and January 10, 2012 (Mw7.2). In all three cases, the style of faulting was similar. These events align approximately with fabric of the sea floor in the diffuse boundary zone between the Indian and Australian plates.

(above from reference #1053)

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. Small tsunamis from these strike-slip earthquakes were registered with maximum amplitudes of 0.5−1.5 cm on DARTs and 1−19 cm on tide gauges for the 2016 event, and the respective values of 0.5−6 and 6−40 cm for the 2012 event. (reference #11923)

2012 April 11 08:38:36.76 UTC, 2.273 N 93.051 E, 20 depth km, 8.58 Mw (reference #10476, v.10)

2012 April 11 08:38:36.72 UTC, 2.327 N 93.063 E, 20 depth km, 8.6 Mw (reference #13456, access date 2023)

West Aceh Regency: 1 bridge collapsed. The collapsed bridge connects the village and subdistrict of Jatmalaka to the Samatiga subdistrict in West Aceh. 1 girls’ dormitory building at the Arrazatun Nabawi-yah Islamic boarding school, 8 rooms, was damaged. 5 deaths. data from 12 April 2012 (reference #14350)