Most of California's Big Earthquakes Are Preceded by Ghostly 'Foreshocks' Weeks in Advance

san Andreas fault
An aerial view of the San Andreas Fault. (Image credit: Shutterstock)

How do earthquakes begin? It's an ancient question — and while scientists have ruled out the vengeful gods blamed over the past few millennia, agreeing that tremors are more a matter of grinding plate tectonics than of Poseidon's wrath, many facets of this seismic puzzle remain murky. 

One ongoing mystery is the phenomenon of foreshocks, small, sometimes imperceptible tremors that can precede larger quakes in the same area by several days or weeks. Studies have found that anywhere from 10% to 50% of large earthquakes follow these minishocks. This has led many researchers to wonder whether foreshocks are a geophysical fluke or a standard feature of big quakes that modern instruments just aren't sensitive enough to detect with certainty. 

A study published July 30 in the journal Geophysical Research Letters offers compelling new evidence for the second hypothesis. Using the most comprehensive catalogue of earthquake activity in Southern California ever assembled, a team of researchers found that roughly 72% of large (magnitude 4.0 or greater) quakes in the region between 2008 and 2017 followed distinct foreshocks that hit up to a month before the event. 

"We're hoping that these observations will help inform improved physical models of how earthquakes get started," lead study author Daniel Trugman, a seismologist at Los Alamos National Laboratory in New Mexico, told Live Science. "With this improved physical understanding, we'll eventually be able to improve earthquake forecasting as well."

"If you pick any point in Earth's crust, especially near an active fault zone, there's going to be a background rate of seismicity," Trugman said. "To show that there are foreshocks, you have to demonstrate that there are more earthquakes than you'd expect leading up to the larger event."

"The results suggest that foreshock occurrence in nature is more prevalent than previously thought," the researchers concluded in their study.

And what about the 28% of quakes that lacked a surge in foreshock activity? Trugman said it's likely that many of those quakes did see foreshocks as well but the researchers just couldn't define them with "99% certainty." 

"There are a number of cases where there is an increase in seismic activity, but we're not sure it's statistically significant," Trugman said. As seismic-monitoring equipment improves, so too should foreshock detection, he said.

"What we show in this paper is that most if not all mainshocks are preceded by elevated seismic activity that cannot be explained as simple background seismicity," Trugman said. "But that is a very different statement from saying that 'most upticks in seismicity are foreshocks that signal that a mainshock is impending'."

This all shows that the processes that initiate earthquakes are "quite variable," Trugman said, reminding us that seismologists are still a good ways away from being able to forecast earthquakes with any certainty. Perhaps we shouldn't let Poseidon off the hook yet after all.

Originally published on Live Science.

Brandon Specktor
Editor

Brandon is the space/physics editor at Live Science. His writing has appeared in The Washington Post, Reader's Digest, CBS.com, the Richard Dawkins Foundation website and other outlets. He holds a bachelor's degree in creative writing from the University of Arizona, with minors in journalism and media arts. He enjoys writing most about space, geoscience and the mysteries of the universe.