SAN FRANCISCO, June 15 (Xinhua) -- A new study indicates that winter rains and snow in California depress the Sierra Nevada and Coast Ranges, which then rebound during the summer, changing the stress on the state's earthquake faults and causing seasonal upticks in small quakes.
The study by seismologists with the University of California, Berkeley, reveals that the weight of winter snow and stream water pushes down the Sierra Nevada mountains by about a centimeter, or three-eighths of an inch, while ground and stream water depress the Coast Ranges by about half that.
This loading and the summer rebound, namely the rise of the land after all the snow has melted and much of the water has flowed downhill, makes the earth's crust flex, pushing and pulling on the state's faults, including its largest, the San Andreas, according to a paper appearing this week in the journal Science.
Measuring these vertical motions by using the regional global positioning system (GPS) and thus calculating stresses in the earth due the water loads, the researchers found that on average, the faults in the Golden State on the U.S. West Coast experienced more small earthquakes when these seasonal stress changes were at their greatest.
The central San Andreas Fault, for example, sees an increase in small quakes in late summer and early fall as the water load diminishes in the mountains. And the faults along the eastern edge of the Sierra Nevada see an uptick in late spring and early summer due to this seasonal unloading.
"It's not that all earthquakes happen in September," noted Roland Burgmann, a UC Berkeley professor of earth and planetary science and the senior author of the paper. "It all depends on details of the loading, the location of the fault and the geometry of the fault."
While the impact of this annual up and down movement of the mountains surrounding California's Central Valley is small, increasing the chance of earthquakes by a few percentage points at most, the research provides information about how faults rupture and what kinds of stresses are important in triggering quakes. "This study supports the notion that the state's faults are critically stressed so that these small perturbations can affect the earthquake cycle and sometimes promote failure," first author Christopher Johnson, a UC Berkeley graduate student, was quoted as saying in a news release. "It is advancing the clock on these different faults."
Johnson and Burgmann, members of the Berkeley Seismological Laboratory, looked at 3,600 earthquakes over a nine-year period, 2006-2015, and correlated their occurrence with the calculated peak stress on the fault where they occurred. The stress was calculated from the amount the mountains deformed, as measured by a GPS system, using models of rock mechanics that predict stress changes on faults.
The amount of stress generated by seasonal water loading in California is similar to the stresses induced by the seismic waves from distant megaquakes, Johnson said. "We don't see an increase in large-magnitude earthquakes from these low-amplitude stresses caused by seasonal water storage. What these results are showing, however, is that we do see a correlation with small earthquakes from low-amplitude stresses."