seasonal earthquake stress
Depicted are the stresses during the dry late summer months (left), when minimal water is stored near the surface, and during the early spring (right) when maximum water is stored following the winter months. A new study finds that these changes in stress can help trigger–small–earthquakes in California. Christopher Johnson, Berkeley Seismological Laboratory, UC Berkeley

Natural forces shape every inch of our globe, but in California, the two big players are water falling out of the sky (or the lack thereof) and earthquakes.

For a long time, many researchers figured that the two were unrelated. Earthquakes can mess with groundwater levels and aquifers, but not rain, and for the most part, it was assumed that rain and snow didn’t make enough of an impression on the Earth to really affect earthquakes either.

But a study published today in Science found a connection between seasonal precipitation and earthquakes, especially some of the very small ones.

Much of California has two distinct seasons—wet and dry. During the wet season, reservoirs fill, snowpacks build up on mountaintops, and the ground fills with moisture. In drier months, all that water starts moving, melting, and evaporating. This massive amount of water concentrated in the region collectively weighs so much that it can push down on the Earth by a fraction of an inch.

Earth scientists Christopher Johnson and Roland Bürgmann of UC Berkeley were able to measure those seasonal flexes in the Earth using a network of sensors called EarthScope that tracks tiny movements of the ground under our feet. The network includes GPS measurements (like you have in your phone) that record how the Earth’s surface moves in response to stress. For instance, a bunch of water being dumped on it every season.

“This is related to the weight of snow and water pushing down on the Earth and what we’re seeing is essentially how the Earth flexes under the addition and removal of that,” Bürgmann says.

What they found

By looking at seasonal changes due to precipitation from 2006-2014 and matching that up with records of earthquakes over the same time period, the researchers were able to show a correlation between earthquakes and California’s seasonal rain.

“This suggests there are interactions that are strong enough that changes in the hydrospheres do have an effect on tectonics in terms of small earthquake occurrence,” Bürgmann says.

Most of the earthquakes were small temblors—above a magnitude 2.0 (below that, shaking is usually too small to feel) but still smaller than some of the massive quakes that have occurred in the past.

They found that the effect of the water load on the earthquakes depended a lot on the shape of the fault where the earthquake originated. Faults are cracks in the Earth’s crust that allow for movement, and they can be oriented in a wide variety of ways in the ground.

“When the Sierras pop up during the summer time, that reduces the clamping stresses so there is less of a squeeze across the fault, and we see more of the little earthquakes during that time,” Bürgmann says.

But in other areas of the state, like the Eastern California Shear Zone faults east of the Sierra Nevada, the shape of the faults in the earth is oriented differently. There, adding water in the winter months led to more earthquakes. So there are some areas that might expect to see seasonal variations in earthquakes, but it might not be the same season everywhere. Bürgmann says that the patterns that they’ve seen in these smaller earthquakes should apply to larger earthquakes as well, but more information is needed to figure out how much of an impact the seasonal water cycle might have.

“It’s similar with other earthquake triggers, like induced seismicity in the central U.S. We’re giving [the faults] that extra push that makes them go earlier, or it can make it wait a little longer,” Bürgmann says. “You cannot demonstrate causality, but just by adding a small amount of stress, you never know for an individual earthquake if that made all the difference in the end.”

So does this mean that the massive amounts of snow and rain that California got this season will mean larger earthquakes? Bürgmann’s gotten that question a lot. And the short answer is that they just don’t know yet.

Johnson and Bürgmann’s results looked at the entire nine year span to resolve the seasonal changes, and weren’t able to pick out individual years to see how rainy years compared to drought years, but they hope that future research might be able to look at individual years or seasons.

Money Matters

But for that research to continue, scientists need to have access to the incredibly accurate network of sensors placed across the country.

Bürgmann says that this research wouldn’t have been possible without the construction and investment of that network. “We can really see everything that moves the Earth, from plate tectonics to earthquakes to water loads,” Bürgmann says. But the future of that network is currently up in the air as budget discussions continue.

“The initial funding from the National Science Foundation is running out in 2018,” Bürgmann says. “Everyone assumes that some solution will be found to keep this going, especially because surveyors, transportation, many, many people depend on this data now, but we haven’t figured that out yet. So we’re all a little anxious to see how that will be worked out.”

Future Work

Bürgmann plans to expand his work to look at other ways that water impacts the movement of the Earth. Glaciers, which have long been known to depress the Earth’s crust, are melting fast, and Bürgmann plans to go up to Alaska to see how those much larger shifts in water weight might influence earthquakes in an area of the country which—like California—is constantly shaking.

There is also the hope that this work can be used to help us understand earthquakes in general. By their nature, they occur underground—in places and on a scale that is wildly difficult to observe.

“I don’t think it will help us predict earthquakes,” Bürgmann says. “Ultimately, it could help us better understand earthquake hazards.”

While forecasting specific earthquakes like we do large storm systems is still far too complicated to be feasible, researchers are using work like this to get a generalized, blurry glimpse into the seismic future of an area. And understanding how weather and earthquakes are connected could help residents in earthquake-prone areas know generally what to expect in the future.

“The more we can improve our ability to resolve this connection, the better the opportunity to see how faults work, and the more closely we can track the forces that it is experiencing, the more we’ll understand what makes earthquakes tick,” Bürgmann says.