San Andreas Fault Earthquakes Are Triggered by Winter Rain and Snow, Scientists Discover

San Andreas Fault
The San Andreas Fault. Ian Kluft/CC

Earthquakes along the San Andreas Fault in California are being triggered by winter rain and snowfall, scientists have discovered.

The finding is important as it helps us understand what triggers earthquakes—and when they are more likely to strike.

Roland Bürgmann and Christopher Johnson, from the University of California, Berkeley, were investigating what impact seasonal loading has on fault systems—with the state's multiple fault lines acting as a "natural laboratory" to track changes.

Seasonal loading refers to how snow and rain over the winter months acts as a weight, causing the land to depress. However, when it dries up, the weight is removed—and the ground rebounds.

This process, the scientists found, changes the stress placed on California's state tectonics, pushing and pulling on the fault lines—including the biggest and most dangerous, the San Andreas Fault. Their findings are published in the journal Science.

The San Andreas Fault forms part of the tectonic boundary between the Pacific and North American Plate, stretching 800 miles through California. In September last year, there was a swarm of around 200 small earthquakes in the Salton Sea, just south of the fault.

This raised concerns that a much larger earthquake could soon take place—the region of the fault where the swarm occurred had not ruptured for over 300 years. Large earthquakes normally occur along the fault every 150 to 200 years, during which stress along the fault builds, so scientists think a "Big One" is overdue.

In the latest study, Bürgmann and Johnson measured vertical movement along the state's fault lines to track changes resulting from seasonal loading. They used nine years' worth of GPS data on vertical deformation to identify the stress changes on the fault lines that produce small earthquakes.

From this, they calculated the seasonal stress time for each fault location to calculate an average stress cycle. Findings showed that the San Andreas Fault has an increase in small earthquakes in late summer and early fall, while the faults along the eastern edge of the Sierra Nevada see more earthquakes in late spring and early summer.

This does not mean there is an "earthquake season," Bürgmann said, but that seasonal loading plays a role.

"What we find is the stresses that result from the flexing of the crust due to seasonal loads correlate with [around] 10 percent change in seismic activity from the background rates," Johnson tells Newsweek.

California faults
Faults throughout central and northern California are shown with the stress change induced by the hydrological loading cycle. Here you can see the stresses during the dry late summer months, when minimal water is stored, and early spring when maximum water is stored. Christopher Johnson, Berkeley Seismological Laboratory, UC Berkeley

While the annual snow and rainfall increases the chance of earthquakes by a small amount, their discovery provides new information on how and why faults rupture, including the different stresses involved.

The study does not look at large earthquakes directly, but the researchers did look at historic events bigger than magnitude 5.5 going back to 1781. They found there was a slight increase in earthquakes when seasonal loading was high compared to when it was low.

"We look at historic records for larger events, and we do see this seasonality, but we are not at the point that we can provide further evidence to hazard estimates that would say that during these periods of time, we would expect more large earthquakes to occur," Johnson said in a statement.

He says the current findings do not explain the swarm of earthquakes at the San Andreas Fault last September: "We did not look at the swarm last year in this study and cannot say the seasonal patterns we observe are related to that activity."

Earlier this year, Stanford University scientists said California will experience more winter flooding and summer droughts in the future as a result of climate change. Johnson said it is not clear whether more extreme would lead to more earthquakes in the future as they did not explore longer-term trends.

Next, the scientists plan to refine their seasonal loading model to better understand "what makes earthquakes go."

We are continuing to look at seasonal loading and the earthquake cycle in Alaska," Johnson says. "In that environment there is more precipitation throughout the year that results in larger loads. By exploring a different tectonic environment with different loading patterns, we hope to learn more about the timing of the earthquakes with respect to the seasonal stress changes."