A new study shows that the region east of the Andean Mountains in Bolivia is capable of much larger earthquakes than had previously been known.
This new study brings the estimate of a massive earthquake from a comparatively timid 7.5 – a figure based on the relatively quiet seismic history of the region – to a massive 8.7 to 8.9. Figures such as this will have large impacts on how communities in the region will prepare for earthquakes.
Researchers from the University of Hawaiʻi at Mānoa, including Benjamin Brooks, associate researcher in the Hawai‘i Institute of Geophysics and Planetology in the School of Ocean and Earth Science and Technology at UH Mānoa, used GPS data to map the movement of the Earth’s surface in the Subandean margin, specifically, along the eastern flank of the Andes Mountains.
“We relate GPS surface movements to the subsurface via deformation models,” said Brooks. “In this case, we use a simple elastic model of slip on a buried dislocation (fault) and do millions of Monte Carlo simulations to determine probability distributions for the model parameters (such as slip, width, depth, dip, etc.).”
The data gathered by the researcher’s shows that a shallow section about 100 kilometres in length in the east of the region is currently locked in place. Such a large piece of the Earth’s surface locked in one place for so long will build up massive amounts of stress that, when allowed to escape, will result in the massive shaking predicted.
“No one suspected the previous estimates were too low,” said Brooks, who was surprised by the findings. “It was a discovery that came out of my broader interest which is studying the way in which mountains (in this case the Andes) actively grow and deform.”
“The findings here are critical in helping us to continue to provide the most up-to-date and accurate information regarding geological hazards in Bolivia,” noted Major Arturo Echalar of the Bolivian IGM.
“It is not yet known if one of that size has ever happened in the Bolivian Subandes,” added Brooks, who along with his colleagues, were quick to report that their findings only demonstrate the potential for such a quake. “Nonetheless we hope that this information will be widely disseminated and considered in Bolivia by the people ( including the general population, engineers, planners, emergency mitigators, policy makers, etc.) who may be most affected by a potential event here.”
The researchers continue to look into the region and its activity, hoping to better understand its future. They’ll be performing paleoseismolgic research in an effort to determine whether there have been any past earthquakes in the region, their dates and sizes, as well as continuing to monitor the seismic region in the hope of determining whether the accumulated stress can be released aseismically.
“As we state in the paper, we believe that the Mandeyapecua thrust fault at the mountain front exhibits evidence for past earthquake ruptures,” said Brooks. “So by applying techniques like digging trenches and identifying and dating offset layers we’ll be able to quantify the seismic past of the region.”