May 9, 2011

Megaquake Possible For Bolivia

Bolivia may need to prepare for a possible 8.9 magnitude quake that would be 125 times more powerful than previously calculated to be possible in this area, AFP reports.

Two million people in the region of the east of the central Andes mountains could be affected in what had been thought, until now, to be a relatively calm area for seismic activity, according to a study published in Nature Geoscience.

"No one suspected that the previous estimates were too low," Benjamin Brooks, a geophysicist at the University of Hawaii Manoa and lead author of the study, told the AFP news agency.

The most powerful expected earthquake for the region east of the central Andes mountains was estimated at magnitude 7.5, but a careful analysis of Global Positioning System (GPS) data from the eastern flank of the mountain chain showed a buildup of stress consistent with a maximum magnitude of 8.7 to 8.9, Brooks said.

Tiny changes in the location of GPS stations enable scientists to measure surface movement to within a fraction of a millimeter per year. This data showed that the area to the west of the Mandeyapecua thrust fault, which runs north-south, has moved far more than the area east of the fault which indicates a potentially large quake situation.

There is no way to know when such a quake might happen -- or if it will happen at all, Brooks explained. A series of stress-relieving smaller temblors could occur and diffuse the movement.

To follow up, Brooks is probing the ancient seismic history of the region to determine the dates and sizes of past earthquakes, and to find out if one that size has ever occurred there.

"We hope that this information will be widely disseminated and considered in Bolivia by the people -- including the general population, engineers, planners, policy-makers -- who may be most affected," he said.

Recent major earthquakes, including the magnitude 9.0 monster off the coast of Japan in March, and an 8.8 quake in Chile in February 2010, have moved researchers to reassess their data.

"We probably should re-evaluate our estimates of the maximum sizes of earthquakes that could strike all fault areas," Ross Stein, a seismologist at the US Geological Survey (USGS), said shortly after the Japan quake.


Image 1: Aerial photo of a portion of the Andes between Argentina and Chile. Credit: Robert Morrow/Wikipedia (CC BY-SA 3.0)   

Image 2: Ben Brooks, 'O. Ozcacha and Todd Ericksen stand next to one of the GPS stations that was used in the study. Credit: Ben Brooks, SOEST/UHM


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