M 7.9 - South Indian Ocean

Tectonic Summary

The June 18, 2000, M 7.9 earthquake in the South Indian Ocean occurred as a result of shallow strike-slip faulting within the oceanic crust of the Australia plate, several hundred kilometers southwest of the Sunda-Java Trench and the Australia-Sunda plate boundary. Focal mechanism solutions for the earthquake indicate that rupture occurred on either a left-lateral nearly north-south-striking fault or a right-lateral nearly east-west-striking fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the north-south-striking (left-lateral) fault. At the location of the earthquake, the Australia plate is moving to the northeast relative to the Sunda plate at a velocity of approximately 60 mm/yr.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip events of the size of the June 18, 2000, earthquake are typically about 210x25 km (length x width); modeling of this earthquake implies dimensions of about 190x10 km, predominantly up-dip of and bilateral from the hypocenter.

Other than the M 5.4 aftershock that was triggered by this earthquake, about 20 minutes after the initial event, only one other earthquake has occurred within 400 km of the June 18th event greater than M 5 over the preceding 40 years—a M 5.4 earthquake in September 1989. The remote location of all of these earthquakes makes them minimally disruptive.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)