M8.6 Offshore Sumatra: No Tsunami, Minimal Damage

11-Apr-12

A shallow M8.6 earthquake occurred 400 km offshore northern Sumatra, Indonesia on April 11, 2012. Two hours following, a large M8.2 aftershock occurred approximately 200 km to the southwest of the mainshock. Although this earthquake affected wave heights by up to one meter, it did not cause a tsunami of consequence, mainly due to its strike-slip mechanism. The earthquake caused little to no damage or injuries despite being felt at distances of 1000 km or more, from Thailand to Sri Lanka and from Jakarta to Kolkata. Shaking felt onshore was of low to moderate intensity, IV to V on the Modified Mercalli Scale.

Off the Coast of Northern Sumatra - April 11, 2012

Off the Coast of Northern Sumatra Shakemap
Source: USGS

This is the fifth earthquake of magnitude 8.6 or larger to have occurred since 2004, prior to which 40 years passed without a single worldwide occurrence of a M8.6 or larger earthquake.

While it is fortunate that this earthquake caused little to no human consequence, it is of great seismological interest. Since its epicenter was located about 100 km southwest of the Sunda-Sumatra trench, this earthquake was not associated with the plate boundary megathrust fault. A crustal, strike-slip earthquake of this magnitude is very unusual and indicates a previously unknown fault of considerable length at this location. Undoubtedly, research resulting from today's earthquake will determine if its rupture characteristics have implications regarding conventional thought on strike-slip faults worldwide.

EQECAT will continue to monitor this earthquake and provide insight on the scientific understanding that it generates.

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