The magnitude-8.8 earthquake that pummeled Chile in February 2010 did not relieve seismic stress the way scientists thought it might have, a new study suggests.
Quake risk thus remains high in the region, geophysicist Stefano Lorito of Italy’s National Institute of Geophysics and Volcanology in Rome and his colleagues report online January 30 in Nature Geoscience. In places, risk might even be higher than it was before last year’s quake.
The geologic stress remains because instead of the ground moving the most where stress had been building the longest, the team reports, the greatest slip occurred where a different quake had already relieved stress just eight decades earlier.
Scientists would like to be able to point at a fault segment that built up stress the longest and say it was primed to go next. But the new work shows that stress buildup does not automatically translate to an earthquake happening right in that area, says geophysicist Ross Stein of the U.S. Geological Survey in Menlo Park, Calif., who was not involved in the research. “It’s a very logical approach,” Stein says. “But I don’t think it holds up.”
Geologists weren’t surprised when the quake happened. Off the western coast of South America, the Nazca plate of Earth’s crust dives beneath the South American plate, pushing up the Andes and building up stress that gets relieved occasionally in powerful earthquakes. The biggest quake ever recorded, a magnitude-9.5 whopper, occurred along the Chilean coast in 1960. Some 300 kilometers north of that, a magnitude-8.0 quake struck in 1928.
Between those two ruptures — 1960 in the south, and 1928 in the north — lay a stretch that apparently hadn’t ruptured since 1835, when Charles Darwin visited aboard the H.M.S. Beagle and witnessed a major earthquake. Researchers had thought that this “Darwin gap” would be filled the next time a big quake struck the region.
But it wasn’t, says Lorito. His team used data on how the surface moved during the 2010 quake — from geodetic markers and tsunami observations, among others — to calculate which parts slipped the most.
The scientists found that the greatest slip occurred north of the quake’s epicenter, right around where the 1928 quake struck. South of the epicenter lay a secondary zone of slip. But right in the middle, where the Darwin gap lies, was little to no movement. “The Darwin gap is still there,” Lorito says.
Other earthquake zones, such as Sumatra in 2007, have experienced big quakes that didn’t relieve pent-up geologic stress where scientists thought it was greatest. “It is not strange to see that the rupture is complex, and that some parts can break at one time and some at another time,” Lorito says.
The new work fits with several other scenarios that scientists have developed to explain ground movement during the Chile quake. The scenarios, however, differ in their details. For example, researchers from the GFZ German Research Centre for Geosciences in Potsdam reported in Nature in September 2010 that some of the quake’s slip happened fairly close to the Darwin gap.
The teams reach different conclusions because they use different sets of quake observations and different analytical methods, says Onno Oncken of the Potsdam team. But overall, various groups agree on the broad patterns of how the ground moved — and where seismic risk remains high.
Along with the Darwin gap, another place to worry about may be a stretch between 37 degrees and 36 degrees south latitude, offshore from the city of Concepción. Lorito’s team concludes that stress was transferred there during the 2010 rupture. It could be capable of unleashing another quake of magnitude 7.5 to 8, the researchers write.
The 2010 Chile quake killed more than 500 people by causing both shaking and a tsunami.
Because of the seismic risk, the Chilean coast is one of the most studied regions in the world. For the past decade, Oncken and others have studded the area with seismometers to understand the details of how a diving plate like the Nazca causes quakes. “We now have the unique opportunity to do a detailed comparison from before and after an event,” says Oncken. “Whatever you look at, it’s fantastic data and new observations emerging at an incredible rate.”
