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Seasonal Loading along the San Andreas


Last updated 6/16/2017 at 10:38am

According to new research, Roland Bürgmann and Christopher Johnson, from the University of California, Berkeley, Earthquakes along the San Andreas Fault in California are being triggered by winter rain and snowfall.

The finding is important as it helps us understand what triggers earthquakes—and when they are more likely to strike. Called 'seasonal loading', the term refers to how snow and rain over the winter months acts as a weight, causing the land to depress. However, when it dries up, the weight is removed—and the ground rebounds.

Scientists found the changes placed stress on California’s state tectonics, pushing and pulling on the fault lines—including the the San Andreas Fault. Their findings are published in the journal Science.

In September last year, there was a swarm of around 200 small earthquakes in the Salton Sea, just south of the fault, raising concerns that a much larger earthquake could soon take place. Large earthquakes normally occur along the fault every 150 to 200 years, during which stress along the fault builds, the region of the fault where the swarm occurred has not ruptured for over 300 years.

In the latest study, Bürgmann and Johnson used nine years’ worth of GPS data on vertical deformation to identify the stress changes on the fault lines that produce small earthquakes. They calculated the seasonal stress time for each fault location to calculate an average stress cycle. Findings showed that the San Andreas Fault has an increase in small earthquakes in late summer and early fall, while the faults along the eastern edge of the Sierra Nevada see more earthquakes in late spring and early summer.

"This does not mean there is an “earthquake season,” Bürgmann said, but that seasonal loading plays a role.

“What we find is the stresses that result from the flexing of the crust due to seasonal loads correlate with [around] 10 percent change in seismic activity from the background rates,” Johnson tells Newsweek.

While the annual snow and rainfall increases the chance of earthquakes by a small amount, their discovery provides new information on how and why faults rupture, including the different stresses involved. The study does not look at large earthquakes directly, but the researchers did look at historic events bigger than magnitude 5.5 going back to 1781. They found there was a slight increase in earthquakes when seasonal loading was high compared to when it was low.

He says the current findings do not explain the swarm of earthquakes at the San Andreas Fault last September: “We did not look at the swarm last year in this study and cannot say the seasonal patterns we observe are related to that activity.”

Earlier this year, Stanford University scientists said California will experience more winter flooding and summer droughts in the future as a result of climate change. Johnson said it is not clear whether more extreme would lead to more earthquakes in the future as they did not explore longer-term trends.

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