Monday 16 January 2012

Series of Earthquakes in the South Shetland Islands.

On Sunday 15 January 2012 at approximately 9.40 am local time (1.40 pm GMT) Elephant Island in the South Shetland Islands was shaken by an Earthquake measuring 6.6 on the Richter Scale at a depth of approximately 10 km, according to the United States Geological Survey. This was followed by a magnitude 6.2 quake at a depth of about 14.2 km which took place slightly after 10.20 am local time (2.40 pm GMT), a magnitude 5.6 quake at a depth of 10 km a few minutes before midnight local time (4.00 am on 16 January GMT) and a magnitude 5.1 quake at a depth of roughly 15.7 km, about 50 km to the east, of the coast of Clarence Island. Given the remote location of the islands there are unlikely to have been any casualties, and no tsunami warning has been issued.

Map showing the location of the quakes (blue squares, larger squares are more powerful quakes). Red lines are plate margins. Palmer Station at the bottom of the map is a long term ecological research centre supported by the US National Science Foundation. Map from the United States Geological Survey.

The South Shetland Islands lie on the boundary between the Antarctic Plate and the South Shetland Plate, a small tectonic plate more-or-less surrounded by the Antarctic Plate, and being subducted beneath it from the southeast in the South Shetland Trench, with the majority of the South Shetlands forming an island arc above the subduction zone. Island arcs form above subduction zones when the plate being subducted is partially melted by the heat of the Earth's interior, causing hot liquid magma to rise up through the overlying plate forming volcanoes at the surface. The most famous volcano of the South Shetland Islands is Deception Island, A largely submerged active volcano with a crater that forms an open lagoon into which cruise ships can sail. The surrounding rim reaches 539 m above sea level at its highest point, is covered in glaciers and has a number of hot springs.

Elephant and Clarence Islands lie away from the rest of the South Shetland Islands, on the boundary between the South Shetland Plate and the Scotia Plate, though they probably share a common origin with the other islands. The margin between the South Shetland Plate and the Scotia Plate is a Transform Margin, where the two plates move past one-another, the Scotia Plate moving east with regard to the South Sandwich Island Plate. This is not a smooth process, the plates often sticking, leading to a pressure buildup, then an earthquake (or series of earthquakes) as the rocks give way and the pressure is released.

The Scotia Plate (yellow), showing its margins. Boundary with the South Shetland Plate (pink) is in the southwest. Map from Carlton College, Minnesota.