Strong Earthquake Hits Southern Atlantic Ocean

According to the US Geological Survey, a strong 7.4-magnitude earthquake shook the southern Atlantic Ocean early Friday.

No immediate tsunami warning was issued.

The quake struck at 5:32 am local time (0732 GMT) at a depth of 10 kilometers. It was centered east-southeast of the British administered South Georgia and South Sandwich Islands.

Tectonic Summary

The August 19, 2016 M 7.4 earthquake east of South Georgia Island, in the South Atlantic Ocean, occurred as the result of shallow thrust faulting near the plate boundary between the South America and Scotia plates. In this region, the South America plate subducts to the west-southwest beneath the Scotia Plate and the South Sandwich Island Arc, mainly south and east of the epicenter of todays quake along the South Sandwich Trench. To the west of this event, near South Georgia Island, the Scotia-South America plate boundary is represented by the North Scotia Ridge, a left-lateral transform fault. At the location of this earthquake, the South America plate moves towards the west-southwest with respect to the Scotia plate at a rate of just 9 mm/yr. Rates of subduction along the South Sandwich Trench are in excess of 65 mm/yr, but slow in the region of the August 19th earthquake due to back-arc spreading along the East Scotia Ridge, just east of this event. The focal mechanism solution of this earthquake indicates southwest-oriented thrust faulting, consistent with occurring along the plate boundary interface between the South America and Scotia Sea (and South Sandwich micro-) plates.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust faulting events of the size of the August 19, 2016 earthquake are typically about 70×35 km in size (length x width).

While the South Sandwich Arc subduction zone is active seismically, all moderate-to-large historic events have occurred east of the East Scotia Ridge, >~130 km to the east of the August 19 earthquake. 23 such earthquakes of M 6+ have occurred within 250 km of the August 19 event over the preceding century. These include a M 8.1 earthquake in June 1929, 160 km to the east of the August 19 event, and more recently, two M 6.9 events in September 2004 and June 2014, between 190-230 km to the east. Both of these 21st century events demonstrated oblique-thrust faulting, consistent with South America subduction. Due to the remote location of these events far from population centers that might be vulnerable to earthquake shaking, none are known to have recorded casualties or damage.

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