Earthquake off Sumatra

On April 11 2012 at 10.30 MESZ an earthquake with magnitude 8.6 occurred about 400 km west of the Indonesian island of Sumatra. This was the strongest earthquake to occur so far this year. As expected, there has been a vigorous aftershock sequence. As of 5pm CET on April 11, the strongest aftershock has had a magnitude of 8.2, occurring about 2 hours after the mainshock. The earthquake was felt across a very wide area, including South India, a distance of about 2000 km from the epicenter.

The national Tsunami Warning Centers in Indonesia, India, Thailand, Sri Lanka and the regional Pacific Tsunami Warning Center (PTWC) in Hawaii (who each monitor the Indian Ocean) issued tsunami warnings for the affected coast lines. The PTWC have subsequently withdrawn their warning.

Analysis of the earthquake recordings indicate that the sea floor moved predominantly in a horizontal direction. Due to the lack of significant vertical motion on the seabed, a major tsunami, as occurred in 2004, was not generated. A relatively large vertical sea floor displacement is necessary to produce a tsunami.