California Is at Risk of a Devastating Megaquake
USA — June 21, 2018
Researchers have been warning that swathes of Southern California are long overdue for a massive magnitude seven-plus earthquake.
New research by two Arizona State University geophysicists shows that the central section moves in a way that was previously unexpected and makes big quakes more likely. The central section of California's famed San Andreas Fault—from San Juan Bautista southward to Parkfield, a distance of about 80 miles— has displayed a steady creeping movement—which is sometimes called a "slow earthquake"—that releases energy over a period of months. Although these slow earthquakes pass unnoticed by people, the researchers say they can trigger large destructive quakes in their surroundings.
"What looked like steady, continuous creep was actually made of episodes of acceleration and deceleration along the fault," says Mostafa Khoshmanesh, a graduate research assistant in ASU's School of Earth and Space Exploration (SESE). He is the lead author of a Nature Geoscience paper reporting on the research.
"We found that this part of the fault has an average movement of about three centimeters a year, a little more than an inch," said Khoshmanesh, one of the researches. "But at times the movement stops entirely, and at other times it has moved as much as 10 centimeters a year, or about four inches."
"These episodic slow earthquakes lead to increased stress on the locked segments of the fault to the north and south of the central section," warns Shirzaei. He points out that these flanking sections experienced two magnitudes 7.9 earthquakes, in 1857 (Fort Tejon) and 1906 (San Francisco).
"Based on our observations, we believe that seismic hazard in California is something that varies over time and is probably higher than what people have thought up to now," Shirzaei explained. He added that accurate estimates of this varying hazard are essential to include in operational earthquake forecasting systems.
According to Khoshmanesh, "Based on current time-independent models, there's a 75% chance for an earthquake of magnitude 7 or larger in both northern and southern California within next 30 years."
Experts say that if seismic activity triggered the collapse of the structure and the wider San Andreas fault, the effects would likely be felt across a 15 square mile area.
The strength of the earthquake would depend on whether the whole structure collapsed at once, or if each of the fault lines was triggered individually.
Seismologists say further studies are now needed before they can determine just how dangerous this new fault section really is.