5.1 Earthquake rocks Indonesia
Seismotectonics of the Sumatra Region
The plate boundary southwest of Sumatra is part of a long tectonic collision zone that extends over 8000 km from Papua in the east to the Himalayan front in the west. The Sumatra-Andaman portion of the collision zone forms a subduction zone megathrust plate boundary, the Sunda-Java trench, which accommodates convergence between the Indo-Australia and Sunda plates. This convergence is responsible for the intense seismicity and volcanism in Sumatra. The Sumatra Fault, a major transform structure that bisects Sumatra, accommodates the northwest-increasing lateral component of relative plate motion.
Relative plate motion between the Indo-Australia and Sunda plates is rapid, decreasing from roughly 63 mm/year near the southern tip of Sumatra (Australia relative to Sunda) to 44 mm/year north of Andaman Islands (India relative to Sunda) and rotating counterclockwise to the northwest, so that relative motion near Jakarta is nearly trench-normal but becomes nearly trench-parallel near Myanmar. As a result of the rotation in relative motion along the strike of the arc and the interaction of multiple tectonic plates, several interrelated tectonic elements compose the Sumatra-Andaman plate boundary. Most strain accumulation and release occurs along the Sunda megathrust of the main subduction zone, where lithosphere of the subducting Indo-Australia plate is in contact with the overlying Sunda plate down to a depth of 60 km. Strain release associated with deformation within the subducting slab is evidenced by deeper earthquakes that extend to depths of less than 300 km on Sumatra and 150 km or less along the Andaman Islands.
USGS REPORT HERE