Although it’s long been thought that fault creep in the central San Andreas Fault has been instrumental in preventing larger earthquakes from happening, including the so-called “big one” that many experts have been fearing. That movement was said to allow energy to be released steadily and safely, but new research suggests that the creep along the central section might not be that steady after all.
According to Phys.org, a series of “slow earthquakes,” or seemingly insignificant “stick-and-slip” movements, has been taking place in the San Andreas Fault, releasing energy over several months, though not being strong enough for most people to notice. Researchers believe that these events, small as they may seem, might trigger larger earthquakes in nearby areas, such as the Parkfield earthquake of 2004, a magnitude 6.0 tremor that was originally predicted to take place “within five years of 1988,” based on previous events in the area, according to the NASA Earth Observatory website.
“What looked like steady, continuous creep was actually made of episodes of acceleration and deceleration along the fault,” explained Arizona State University School of Earth and Space Exploration (SESE) graduate research assistant Mostafa Khoshmanesh, lead author on a new study published in the journal Nature Geoscience that explained the “slow earthquake” phenomenon.
The researchers based their findings on a mathematical model they created, which blended synthetic aperture radar data from 2003 to 2010, monthly changes observed along San Andreas’ central section, and existing seismic records, wrote Phys.org.
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Study co-author Manoochehr Shirzaei, an assistant professor at SESE, added that movement on the San Andreas Fault happens every one to two years, and would persist for several months on end. The slow earthquake episodes, he noted, might have been responsible for two magnitude 7.9 earthquakes from more than a century ago — the Fort Tejon and San Francisco earthquakes of 1857 and 1906, respectively — due to the buildup of stress on locked segments found in San Andreas’ north and south central sections. The San Francisco quake, in particular, stood out for being one of the most devastating tremors in U.S. history, having killed more than 3,000 people and destroyed 0ver 80 percent of the city, according to Newsweek.
“Periodic compacting of fault materials causes a brief rise in fluid pressure, which unclamps the fault and eases the movement,” said Khoshmanesh, explaining the mechanism that could be responsible for the slow earthquakes.
All in all, the researchers believe that their paper suggests that there’s a strange mechanism that causes the San Andreas Fault to trigger earthquakes, one that has yet to be factored into computer models used to predict the risk of tremors in California. As the likelihood of earthquakes in the state will likely increase if the mechanism is taken into account, the researchers warned that the chances of a significant earthquake shaking California within the coming decades are quite good.
“Based on current time-independent models, there’s a 75 percent chance for an earthquake of magnitude 7 or larger in both northern and southern California within the next 30 years,” Khoshmanesh warned.