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

Felt Reports

Felt (V) at Sibolga. Also felt at Banda

Aceh, Bukittinggi, Duri, Medan and Padang. Felt at

Jakarta, Java. Felt (V) at Ayer Itam; (IV) at Petaling

Jaya and Sungai Ara; (III) at Butterworth, Gelugor,

Georgetown and Kuala Lumpur, Malaysia. Felt (IV) at

Chetpet and (III) at Bangalore and Madras, India. Felt

(IV) at Colombo and (II) at Kotte, Sri Lanka. Felt

(II) at Kathu and Phuket, Thailand. Also felt in

Bangladesh, Brunei, Maldives, Singapore and Vietnam. A

local tsunami with a maximum wave height (one-half

peak-to-trough) of 22 cm was recorded on Pulau Enggano.

Tectonic Summary

The April 11, 2012, M8.6 and M8.2 earthquakes 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 quakes were located respectively 100 km and 200 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 with respect to the Sunda plate at a velocity of approximately 52 mm/yr.

Large strike-slip earthquakes are not unprecedented in the diffuse boundary region separating the India and Australia plates, southwest of the Sumatra subduction zone. 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 earlier large strike-slip events had occurred within 50 km of the first large April 11, 2012 event. These earthquakes occurred on April 19 2006 (Mw6.2), October 4 2007 (Mw6.2) and January 10, 2012 (Mw7.2). The focal-mechanisms of the three earlier earthquakes and the two great earthquakes of April 11, 2012, are consistent in implying that each earthquake could have occurred as the result of left-lateral slip on a north-northeast striking fault or right-lateral slip on a west-northwest 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". Possibly faults of both orientation have been involved in the recent earthquake activity.

(above from reference #1053)