M 7.6 - Scotia Sea

Tectonic Summary

The August 4, 2003, M 7.6 Scotia Sea earthquake in the southern Atlantic Ocean, to the east of the South Orkney Islands, occurred as the result of shallow, oblique transform faulting between the Scotia Sea and Antarctica plates. Focal mechanism solutions indicate that rupture occurred on either a right-lateral south-southwest-striking fault or a left-lateral east-striking fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the east-striking (left-lateral) fault. At the location of the earthquake, plate motion is also left-lateral; the Antarctica plate moves eastward with respect to the Scotia plate at a velocity about 6 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 August 4, 2003, earthquake are typically about 140x20 km (length x width); modeling of this earthquake implies dimensions of about 100x20 km, mostly to the east of the hypocenter.

Though the region surrounding the Scotia Sea frequently experiences earthquakes, the majority occur around the subduction zone adjacent to the South Sandwich Islands, to the east of the August 4th earthquake. Only four earthquakes greater than M 6 have occurred in the 400-km vicinity of this earthquake over the past 40 years. Due to the remote location of these far from population centers that might be vulnerable to earthquake shaking, none have recorded damage or casualties.

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)