Two decades have passed since Associate Professor of Public Health and Social Ecology, Lisa Grant Ludwig, first took on “The Big One.” Back then, as a graduate student at the California Institute of Technology in 1989, Ludwig and most seismologists believed as they do today that the southern San Andreas Fault will see another major earthquake. For 20 years, they predicted that the next earthquake to hit the fault will be larger than any before it based on the idea that the tectonic plates that formed the fault moved in a predictable pattern.
Much of this changed recently when Ludwig, along with Sinan Akciz and Gabriela Noriega of UC Irvine and Olaf Zielke and J. Ramon Arrowsmith of Arizona State University, discovered that the fault’s movement deviated from the pattern and has varied with each earthquake.
Working at the Carrizo Plain about 100 miles away from Los Angeles, the team originally wanted to measure the stream channels created by the fault. This plain in particular had a well-preserved earthquake record in its sediment that could potentially provide a lot of valuable information about earthquake recurrence and behavior. While the plain has remained more or less unchanged in 20 years, scientists now have newer technology at their disposal – specifically GPS and LIDAR (Light Detection and Ranging) technology. Combined, these technologies give geologists and seismologists an extremely accurate picture of faults and their movements in any given region by measuring extremely subtle differences in ground elevation.
“In the beginning, we were simply trying to test the method of using LIDAR data to measure offset stream channels and see how well it would work,” said Olaf Ziekle, an ASU Ph.D. student whose dissertation work initially dealt with computer simulations to predict earthquake behavior. “While there were some indications in the previous data set that suggested the 1857 slip may have been around five to six meters, we did not really anticipate our findings. We were rather surprised to identify such a clear signal.”
The team found that none of the known earthquakes in the last 500 years had made the fault slip more than nine meters, as was previously believed. Instead, they found that there had been more previously undetected smaller tremors along the fault. Additionally, they learned that the largest slip, caused by the magnitude 7.8 Fort Tejon quake of 1857, had been only five meters.
“It really just changes the long-standing understanding that earthquake recurrence along that part was dominated by great [movement] – magnitude 7.5 or larger – and relatively infrequent events,” said Arrowsmith, who had worked with Ludwig since the late 1990s and, like her, has been interested in the San Andreas Fault’s behavior for over 20 years. Despite the new data, they have not ruled out the possibility of another phenomenal earthquake. However, they now recognize that far more moderate earthquakes with magnitudes between 6 and 6.7 occur in a far shorter time frame than previously believed – about every 100 years as opposed to 150 or more.
“Using a rather menacing analogy: instead of being shot at with a bazooka, we may end up being shot at with a machine gun – smaller bullets but a lot more of them and certainly still quite deadly,” Zielke said. “A single ‘big one’ may be less likely than previously thought, but the overall risk may not automatically decrease.”
Californians will still have to be as prepared as ever for the occurrence of “The Big One” as well as its smaller cousins. The results of this discovery will go a long way in helping the Working Group on California Earthquake Probabilities (WGCEP) update the current version of the Unified California Earthquake Rupture Forecast (UCERF v3). Awareness events like The Great California ShakeOut in 2008 and 2009 go far in teaching average Californians about earthquake safety and preparedness. Ludwig has noted that we still do not know whether the next large earthquake will be bigger or smaller than the last “Big One,” or whether it will be near a highly populated area. She personally has over 50 gallons of water stored for when it hits.
“It is sort of like changing the rules of a game, but having the same players,” Ludwig said. “The outcome is more difficult to predict, but the game will go on regardless.”
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