Sightly before 8.15 am on Thursday 6 September 2011 the Jujuy Province in the far northwest of Argentina was shaken by a severe earthquake, recorded by the United States Geological Survey as having a magnitude of 6.8 on the Richter Scale and a depth of 9.5 km; this is shallow enough that it is likely to have caused severe problems at the surface, although it is in a rural area with a fairly sparse population, with only 819 people living within 20 km of the epicenter, so it is hoped that there have been no fatalities. The nearest settlement to the epicenter of any size is San Pedro, 62 km to the east, with a population of about 75 000.
The location of the quake.
The province has a largely rural economy, however it is also home to an extensive mining industry, which may have placed more people at risk. The earthquake was felt across the province and in the neighboring provinces of Salta, Tucumán, Catamarca and Santiago del Estero, as well as in southern Bolivia. Public buildings in Jujuy and Tucumán were evacuated, and there have been reports of damage to buildings in Jujuy and Salta. The situation close to the epicenter is as yet unclear.
Jujuy is located in the Andes Mountains, one of the most tectonically active mountain ranges in the world, and has a history of earthquakes. 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.
Diagram showing how the the subduction of the Nazca Plate deforms South America, forming the Andes.
In February 2010 an earthquake with a magnitude of 6.3 on the Richter Scale in the neighboring province of Salta killed two people, and on the same day an unrelated earthquake killed 500 people in Chile. Both the victims, an eight year old boy and a fifty-three year old man, were killed in house collapses. The Chilean earthquake on the same day measures 8.8 on the Richter Scale, the fourth largest earthquake ever recorded (the strongest earthquake ever recorded also hit Chile, in 1960) and caused a tsunami that hit California and Japan.
See also The Puyehue Eruption, Chile, 2011, The Puyehue-Caudón Caulle Volcanic Complex and Earthquakes on Sciency Thoughts Youtube.