From: Andy Soos, ENN
Published May 9, 2011 07:42 AM

Andean Earthquakes to the East

The region west of the Andes Mountains is the leading edge of the South American continent into the Pacific Ocean. Subduction of the Nazca plate beneath South America has driven the growth of the Andes Mountains. Subduction has routinely generated earthquakes larger than magnitude 8.0 along the western margin of the mountain belt. Lesser known for tectonic activity is the eastern side. The region east of the central Andes Mountains has the potential for larger scale earthquakes than previously expected, according to a new study posted online in the May 8th edition of Nature Geoscience. Previous research had set the maximum expected earthquake size to be magnitude 7.5 (Richter), based on the relatively quiet history of seismicity in that area. This new study by researchers from the University of Hawaii at Manoa and colleagues contradicts that limit and instead suggests that the region could see quakes with magnitudes 8.7 to 8.9.

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This project is a long-term collaborative effort between UHM, Ohio State University, Arizona State University, the Bolivian Instituto Geografico Militar (IGM), the Bolivian Seismological Observatory (Observatorio San Calixto), the Universidad Nacional de Cuyo (Argentina), and University of Memphis. The project's general name is the Central and Southern Andes Project (CAP).

The study found that the velocity of surface movement decreases sharply from west to east across the Subandean ranges. This suggests that a subhorizontal fault underlying the ranges slips freely at depth in the west, but is locked for up to 100”‰kilometers in shallower sections further east. Analysis of fault scarps formed where the subhorizontal fault intersects the surface indicates that the fault has generated repeated large earthquakes.

One of the regions in South America that would be affected by such an earthquake forecast is Bolivia which lies mostly east of the Andes. There are over 2 million people in Bolivia. Historically there have not been recent major earthquakes of this magnitude in this area as opposed to Peru and Chile that lie west of the Andes and have several historic major earthquakes such as the 8.8 one in Chile in 2010. 

There was a 2007 6.1 earthquake and a 6.5 in 2010 in the Brazilian province of Acre to the east of the Andes.

The 1994 Bolivia earthquake occurred on June 9, 1994. The epicenter was located in a sparsely populated region in the Amazon jungle, about 200 miles from La Paz. It was estimated at a focal depth of 647 km and a moment magnitude scale of 8.2. The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale.

The frequency of major earthquakes in this region is not clearly known yet.

For further information: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1143.html

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