M6.6 - SOUTHERN SUMATRA, INDONESIA

Tectonic Summary

The October 1st, 2009 earthquake in southern Sumatra occurred as a result of shallow strike-slip faulting within the Sunda plate. In the southern Sumatra region, the Australia plate moves north-northeastwards with respect to the Sunda plate at a rate of approximately 60 mm/yr. Oblique convergence at the Sunda trench, to the west of the earthquake, is partitioned between thrust-fault motion on the principal plate boundary just landward of the trench, where the Australia plate thrusts beneath the Sunda plate, and strike-slip motion in the overriding Sunda plate. Strike-slip motion within the Sunda plate is concentrated on the northwest striking, right-lateral, Sumatran fault. On the basis of the epicenter, focal-depth, and focal-mechanism determined from seismographic data, the earthquake likely occurred along the Sumatran Fault or on a closely related structure.

The earthquake of October 1, 2009 occurred 270 km southeast of the damaging earthquake of September 30, 2009 offshore Padang. Both shocks are associated with the broad scale tectonics of the Australia/Sunda plate boundary in the southern Sumatra region, but in detail their tectonic situations differ. The October 01 shock occurred at relatively shallow depth within the overriding Sunda plate whereas the September 30 earthquake occurred at a depth of about 80 km, probably within the subducted Australia plate.

The 1900-km-long Sumatran fault has ruptured in at least 14 earthquakes larger than M 6.8 since the late 19th century. The earthquake of October 1, 2009 occurred near a section of the Sumatran fault that ruptured in a M 7.0 earthquake in 1995 and a Ms 7.7 earthquake in 1909.

Earthquake Information for Asia

Earthquake Information for Indonesia