The United States Geological Survey recorded a Magnitude 4.7 Earthquake at a depth of 122.6 km in San Juan Province, Argentina, slightly before 11.15 am local time (slightly before 2.15 pm GMT) on Tuesday 6 March 2018. There are no reports of any damage or injuries relating to this event, and nor would any be expected from an Earthquake of this magnitude at this depth but people have reported feeling it in the city of San Juan.
The approximate location of the 6 March 2018 San Juan Earthquake. USGS.
San Juan Province is located on the eastern margin of the Andes Mountains, one of the most tectonically active mountain ranges in the world, and the plains to the east of these mountains. The Andes are being formed as the Nazca Plate to the west is subducted beneath the South American Plate. This causes quakes in a number of ways. Firstly there is friction between the two plates as the Nazca Plate passes under South America. Then there is crumpling and upthrust of the South American as it is pushed from the west by the Nazca Plate and from the east by the expansion of the Atlantic. Finally there is volcanic activity in the Andes, as lighter minerals in the Nazca Plate are melted by the heat of the Earth's interior, then rise up through the overlying South American Plate to form volcanoes.
The subduction of the Nazca Plate beneath the South American Plate, and how it causes Earthquakes and volcanoes. Pacific Earthquake Engineering Research Center.
This means that Earthquakes, even large events, are quite common in the area, but since the province is on the eastern margin of the Andes, such quakes usually happen at considerable depth (the margin between the Nazca and South American plates is angled downwards, so that maximum seismic activity happens close to the surface on the coast of Chile and deep within the Earth beneath eastern Argentina), so that much of the energy from the quake is dissipated before it reaches the surface.
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
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