Arrows represent the relative direction of motion of the two plates, which are colliding at a rate of about 60 millimeters (2 inches) per year. The boundary between the Australia Plate and the Sunda Plate is a thick black line. Elevation is shown in shades of brown lighter areas are higher. The depth of the ocean is shown in blue, with darker colors representing deeper water. The aftershocks ranged up to magnitude 5.1. This map shows the location of the magnitude 7.7 earthquake (red star) and the series of weaker aftershocks that occurred over the next several days (red circles). The structure of the region’s sea floor and the topography of the islands give clues about the source of the geological violence: Sumatra is part of an island arc along the boundary where the Australia and Sunda tectonic plates collide. Two recent great earthquakes-a magnitude 8.6 on March 28, 2005, and a magnitude 9.1 on December 26, 2004-occurred within 125 kilometers of the recent quake. Although it was stronger than any earthquake in the continental United States since the Great San Francisco Earthquake of 1906 (magnitude 7.8), it was modest by Indonesian standards. A magnitude 7.7 earthquake struck deep beneath the Selat Mentawai (Mentawai Straight), between Sumatra and the nearby Simeulue and Nias Islands on April 6, 2010.

Sumatra java