It's no secret that earthquakes continually rattle California. The state is scarred by hundreds of faults, including the notorious San Andreas, an 800-mile (1,287-km) crack in Earth's crust where the Pacific and North American plates meet. Tremors — around 10,000 a year in southern California alone — occur when two sides of a fault slip suddenly against each other. Now scientists say there is something else at work: water.
In a study published in the June 2017 journal Science, Christopher Johnson and Roland Bürgmann, seismologists from the University of California Berkeley, report that snow and winter rain are partly to blame for many of California's earthquakes, including those along the San Andreas Fault. It's a phenomenon they call "seasonal loading."
Seasonal loading occurs when snow and rain fall to the ground in the winter and push against Earth's crust. When snow melts and water flows downhill in summer, the depressed ground rebounds — or flexes — like a tectonic rubber band, triggering small earthquakes.
"The accumulation of snow and water in the mountains and basins during the rainy winter months causes the ground to be pushed down; then during the summer the ground pops back up as the snow melts and water flows down rivers or otherwise goes into the ground," Johnson says in an email. "This flexing of the crust acts to clamp and unclamp the faults. For the San Andreas, we find the clamping effect during the winter and an unclamping during the late summer."
The seismologists say the weight of winter precipitation depresses the Sierra Nevada Mountains by about a centimeter (.04 inch), while stream and ground water depresses the Coast Ranges by about a half a centimeter.
For decades, scientists have wondered how different natural forces impact Earth's crust and earthquake rates. Ocean tides, for example, are known to trigger weak micro-earthquakes deep below the San Andreas, as do heavy rains in the Himalayas. Also, extremely large quakes in one part of the world can trigger earthquakes in another part.
In California, scientists have always wondered why there is an uptick in seismic activity along the San Andreas in the summer and fall — the driest times of the year. In some areas of the San Andreas, for example, the quakes are so weak — around magnitude 2 — that most people don't feel their energy.
Using data from 661 GPS sensors sprinkled around the state, Johnson and Bürgmann measured the vertical movement of the fault lines. The sensors were extremely sensitive and could detect the slightest sinking and raising of the ground. The scientists studied 3,600 earthquakes between 2006 and 2015. They then did the math, calculating the earthquakes when stress on the faults was at its peak. They also looked at the history books and studied larger quakes, magnitude 5.5 or more, which have rattled the state since 1781.
When they looked at the results, the seismologists saw a pattern: As water collected in snow packs, lakes and reservoirs during the winter, its weight depressed Earth's crust. The ground then rebounded when water levels were at their lowest in the summer, generating smaller quakes. As a result, the scientists concluded that small earthquakes occurred about 10 percent more than normal when seasonal loading was high.
"We observed the mountains move 5 to 10 millimeters (0.25 to 0.4 inches) throughout the year," Johnson says. "The largest subsidence is observed in the early spring and then increases throughout the summer. The mechanical models we use are elastic, like a rubber band that returns to its original shape. So, as the earth bounces back up, it is responding to the changing loads. Since the snow melts over many months, the change is gradual over the months."
Could this information be used to predict the "Big One," a massive earthquake that many say will strike California with untold damage in the future, as shown in the video simulation below?
"We still do not have the ability to predict the time, location or magnitude of any earthquake," Johnson says. "What we are finding is that the annual modulation of the small earthquakes is providing insight to the timing of the failure process prior to the earthquake. The more we learn about small earthquakes and the physics of the rupture process is the way forward to improving hazard estimates for faults throughout the state."