Scientists demonstrate strengths and shortcomings of method for determining ancient earthquake size

by Dan Walsh
Monday, September 9, 2013

“A giant Cascadia earthquake, with its accompanying tsunami, has the potential to be the biggest natural disaster in this history of the U.S.,” says Simon Engelhart, a seismologist at the University of Rhode Island. On Jan. 26, 1700, a magnitude-9 earthquake associated with the nearby Cascadia Subduction Zone struck the Pacific Northwest. The quake created tsunami waves that left deposits on shorelines as far away as Japan and caused parts of the Pacific shoreline to sink half a meter into the sea. Such a quake will happen again; the question is when. Researchers are attempting to determine how often earthquakes occurred in the past to estimate when the next quake is likely to occur. New experimental research off the coast of Oregon that demonstrates the accuracy of a method for determining the vertical deformation associated with past earthquakes may help scientists answer this question.

“We wanted to check how accurate and precise we are at estimating the size of past earthquakes; we did this by simulating what would happen [to a piece of sediment] if the earthquake were to occur today,” says Andrea Hawkes, a coastal geologist at the University of North Carolina at Wilmington and a co-author of the new study, published in Geology.

To do this, Hawkes, Engelhart and their colleagues carried out an experiment at the South Slough National Estuarine Research Reserve in Oregon. The experiment involved moving an intact piece of marsh peat from the upper estuary to a location where the water was slightly deeper. Moving the peat to deeper waters simulated the coastal subsidence that would be expected to accompany a large earthquake in the region, Hawkes says. The team returned to the site after one year and again after five years to take samples of the peat layer and the fresh sediments that had accumulated on top of it.

Upon examining the layers of tiny marine organisms called foramanifera (forams) in the peat and accumulated mud, the researchers made two important discoveries. First, by applying previous empirical relationships between particular groups of forams and elevation, the team accurately predicted the total vertical displacement related to the earthquake. Their estimate of 0.61 meters of subsidence was within three centimeters of the actual simulated subsidence of 0.64 meters. These results provide much-needed evidence that the method is effective for determining the size and frequency of past earthquakes along the Pacific Northwest coastline, the team wrote.

The second discovery to come from the experiments, however, cast doubt on a potential method of earthquake prediction, Hawkes says. Some evidence suggests that the ground may rise or fall slightly in the months or years leading up to an earthquake. If such changes could be detected for past earthquakes using the foram method, they might allow researchers to find patterns that could help predict earthquakes in the future, the authors wrote.

If the method worked, the sudden subsidence of the land from the earthquake should have shown up as a sudden change in foram type, Hawkes says. However, rather than a sharp transition from high-marsh forams to low-marsh forams at the transition from peat to mud layers marking the time of the earthquake, there was a gradual shift. This was not expected, Hawkes says, and effectively erased any evidence that might have existed for preseismic deformation. The team suggests that burrowing forams dug down into the peat from the mud layers above after the peat was submerged.

Thus, the results show that forams cannot be used to detect subtle changes in water depth related to deformation preceding past earthquakes, the team wrote. For now, Hawkes says, researchers trying to predict quakes using preseismic deformation will have to rely on modern technology like GPS to precisely measure deformation before an earthquake.

Despite this unfortunate finding, Chris Goldfinger, a seismologist at Oregon State University who has studied the Cascadia Subduction Zone in depth, says he is impressed by the team’s research. “This study helps improve the precision of subsidence estimates, raising the bar from the original work done in the 1990s; it’s very well done,” he says. With research such as this, he adds, “eventually, we will have models for past earthquake ruptures.”

Next, Hawkes says, “we need to figure out what the actual limits of the method are. It’s good for reconstructing deformation of less than one meter, but we need to see if we can increase its application to greater depth ranges.”

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