The magnitude 7.4 Simeulue, Indonesia earthquake of February 20, 2008 occurred as the result of thrust faulting on the boundary between the Australia and Sunda plates. At the location of this earthquake, the Australia plate moves north-northeast with respect to the Sunda plate at a velocity of about 55 mm/year. The direction of relative plate motion is oblique to the orientation of the plate boundary offshore of the west coast of Sumatra. The component of plate-motion perpendicular to the boundary is accommodated by thrust faulting on the offshore plateboundary, with the Australia plate subducting beneath the Sunda plate.
Much of the component of plate motion parallel to the plate boundary is accommodated by strike-skip faulting on the Sumatra fault, which is inland on Sumatra proper. This earthquake occurred at the south end of the rupture zone of the great magnitude 9.1 earthquake of December 26, 2004 and at the north end of the rupture zone of the magnitude 8.6 Nias Island earthquake of March 28, 2005, where the 2004 and 2005 rupture zones nearly abut. The earthquake of December 26, 2004, produced the devastating Indian Ocean tsunami of that date. Since 2000, much of the Sunda trench between the northern Andaman Islands to Eggano Island, a distance of more than 2,000 km, has ruptured in a series of large subduction zone earthquakes