The Sierra Nevada and Coast ranges are pushed down by fractions of an inch each winter by the weight of rain and snow and then rise after melt and runoff, tugging on California’s earthquake faults and triggering small temblors, according to a new study by seismologists at UC Berkeley and Bowling Green State University.
The researchers traced the up-and-down motion of the earth’s crust using nine years of GPS measurements, which stresses the underlying faults by pushing and pulling on them. The study, published Thursday in Science magazine, was able to calculate how much water is stored in the ground and how this weight loss and gain stresses the faults. When the stresses change, the number of roughly 2.0 magnitude earthquakes grows, the researchers found. California experienced thousands of quakes of that magnitude last year, with most of them going unnoticed by humans.
Although the size of those quakes seems insignificant, studying them helps researchers understand the causes of earthquakes and the timing of their formation, said UC Berkeley researcher Christopher Johnson, the lead author of the paper. “We’re using all the big faults throughout the states as a giant experiment,” he said.
The Sierra Nevada saw a record snowpack this past season after four years of drought. The exact way the seasonal water cycle unleashes earthquakes differs from fault to fault, the researchers found. For instance, the weight of water on the Sierra Nevada causes the San Andreas fault to clamp together. When the water weight diminishes, the fault unclamps and causes an increase in small earthquakes in late summer and early fall.
Many small earthquakes would have occurred without the water cycle’s influence, Johnson said, but the nudges cause the earthquakes to rupture earlier than they would otherwise.
“This is a subtle push. It’s kind of like the straw that broke the camel’s back,” he said.
Paul Segall, a professor of geophysics at Stanford University, said the earthquakes caused by the phenomenon shouldn’t be a cause for worry.
“It’s an intriguing correlation, but I wouldn’t say that anybody should do anything differently,” Segall said. The threat of earthquakes in California is always present and people should be prepared, he said.