Earthquakes tend to follow wet winters in California, UC Berkeley scientists find

SAN FRANCISCO – California's Mediterranean climate, with its wet winters and dry summers, brings enviable weather to the Golden State but also earthquakes, according to new research out of the University of California, Berkeley.

In a study over nine years, university scientists found that the weight of winter snow and rain on California's mountains puts enough pressure on both the Sierra Nevada and coastal ranges that when the state dries out come summer, the Earth's crust lunges back up, triggering increased fault ­movement.

"It's not like we're seeing an earthquake season, but the timing of this water unloading is when we're [historically] getting more earthquakes," said Chris Johnson, a graduate student at the Berkeley Seismological Laboratory and one of the study's two authors.

Johnson and his research partner, Roland Burgmann, a UC Berkeley professor of earth science, reported in a paper published Thursday in the journal Science that seismic activity can be as much as 10 percent higher when water weight shifts.

In the eastern Sierra, the researchers reported observing increased fault movement with snowmelt in late spring and summer. Closer to the coast, in such spots as the San Andreas fault, they tracked greater quake activity in summer and early fall with drying streams and soils.

Although the uptick in quakes is modest during such drying periods, the research marks headway on the vexing question of how natural forces contribute to earthquakes. Scientists have had limited success explaining how events like rain and faraway geologic activity might prompt local temblors. But the UC Berkeley researchers make a strong case for how the water cycle connects to seismic shifts, their peers say.

"We have a pretty good understanding of earthquakes on a basic level and why they happen," said Nicholas van der Elst, a research geophysicist for the U.S. Geological Survey in Pasadena, who had not seen the new paper but studies earthquake triggers. "However, we don't know things like how quickly stress accumulates and what is the breaking point of a fault."

Johnson and Burgmann used advanced GPS data from 2006 to 2015 to track the amount that California's mountains are depressed because of wintertime snow and rain. In the Sierra, the range is pushed down about ⅜ of an inch while along the coast, the hills drop about half that, the researchers said.

As the dry seasons arrived and the mountains recoiled, the researchers calculated the level of stress exerted on the underlying faults by using computer models of rock mechanics, and found that stress directly correlated with a rise in the number of earthquakes.

Most of the quakes occurring during these stressful drying periods were relatively small, usually no greater than a magnitude 3.0.

Johnson said he's begun to look at whether the winter water weight could also be tied to larger earthquakes. The researchers have gathered historical data on quakes with a magnitude above 5.5, dating back to 1781, and have found that more occurred when stress from water unloading was high, though their findings are preliminary.