November 25 2013 saw a major submarine earthquake occurring in the South Atlantic. The tremor, which was recorded as magnitude 7 (M7.0) was one of a series of seven to occur within a relatively small area in the space of less than four hours. All were shallow (at depths of 10-15km) and occurred around 300km south east of the Falkland Islands. This sequence of shocks came just over a week after another major tremor, this time of M7.7 (revised magnitude), struck in the South Atlantic several hundred kilometres to the southeast.
November 25 M7.0 Earthquake: Tectonic Setting
The earthquake and associated shocks occurred just to the south of the boundary between two of the earth’s tectonic plates, in this case the South American and the Scotia Sea plate. Although the remoteness of the area means that we don’t understand its regional tectonics well, the United States Geological Survey earthquake summary notes that available information “suggests it is likely associated with left-lateral faulting along this margin.”
The Scotia Sea plate lies between South America and the Antarctic. Its western margins aren’t clearly defined but the northern and southern margins are delineated by transform boundaries and its western extremity is a short subduction zone where the plate descends beneath the South American plate in the area of the South Sandwich Islands. Plate motions are complex and relative: according to the USGS, the Antarctic plate moves eastwards relative to the Scotia plate and the Scotia plate is moving east-north-east relative to the South American plate. These relative motions set up the stresses and strains which lead to earthquakes.
What’s Happening in the South Atlantic?
The past fortnight has seen unusually high levels of earthquake activity at both the northern and southern margins of the Scotia plate. Without detailed study it’s impossible to speculate on any causal factor but the distance between the two earthquake clusters – several hundred kilometres – suggests that their occurrence so close in time is coincidental.
Large Earthquakes: Unusual Proximity
Two such large earthquakes occurring so close together in time in the same region must, however, certainly be considered remarkable. On the subject of the southern margin, the USGS notes that “just two events of M6 or greater have occurred within 250 km of earthquake over the past 40 years” while for the northern margin the data show the largest tremor in the past 40 years to be an M6.6. In Active Faults of the World, Robert Yeats refers to a tremor of around M7.0 on the northern margin (though some way to the west of the November 25 epicenter) in 1970.
We should note, however, that the region of the Scotia Sea plate is remote with little land, virtually all uninhabited. The lack of information on major earthquakes may just mean that they have gone unrecorded in the past, especially given that even large-magnitude earthquakes along transform margins rarely generate tsunamis and so leave little evidence beyond the immediate vicinity.
USGS. M7.0 – South Atlantic Ocean. Accessed November 25, 2013.
USGS. M7.7 Scotia Sea. Accessed November 25, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.