One of the biggest Sierra snowpacks ever — Mammoth Mountain has more than 72 feet of snow, setting a new record —is already producing some scary flooding in the foothills and Valley as some of that snowpack melts and flows downhill.
But flooding isn’t the only hazard that Californians could face this year.
The extra-heavy weight of the snowpack is pressing down on the Sierra’s granite slopes and affecting the state’s geology, including its earthquake faults. And once that snow melts and the pressure lifts, there could be a corresponding increase in earthquakes later this year on both sides of the Sierra.
UC Berkeley professor Roland Bürgmann, who has studied the impact of water loads on seismicity, is quick to note that those earthquakes are likelier to be on the lower end of the spectrum. The chance of Big Snow causing The Big One is remote, he told GV Wire.
“I’m pretty sure that we will not see any particular earthquake that in a meaningful way we can argue,’ oh, this is because we had this really big snow year,’ ” he said. “So the last two years (big snow years) were 2011, 2017, I think. And you know, there’s nothing in those years that was particularly unusual.”
Depending on the moisture content of the snow, a cubic foot can weigh anywhere from 4 pounds to over 21 pounds. Over the Sierra’s 26,000 square miles, the snowpack varies from a few inches to nearly 800, which makes it a little tricky to calculate the actual snow load along the entire mountain range. Suffice it to say, it’s pretty heavy.
On Monday the state’s fourth snow survey revealed that the snowpack is 237% of normal, tying the most-ever record that was set in 1957 when surveys were still being done by hand and the most-ever since the state started its snow sensor network in the mid-1980s.
Sierra Snow Depth Levels
Earthquake ‘Seasons’ Not a Myth
Bürgmann, a professor in Berkeley’s Earth and Planetary Science Department who heads the UC Berkeley Active Tectonics Research Group, was one of the authors of a research study published in 2017 that looked at how a number of factors — hydrology (water), atmosphere, tides, and even the planet’s movement — could put extra stress on earthquake faults.
Researchers studied more than 3,000 earthquakes in Central and Northern California from 2006 to 2014 as well as snow and water loads over the same period.
Prior to that research, Bürgmann said, he would have pooh-poohed as urban myth the belief by some Californians that there are “earthquake seasons.”
Not so much anymore.
“We found that when you carefully look at the stresses, the forces that come from those extra loads, as they come and go, you look at how do they affect individual faults, earthquake fault plains, then you do find that if a fault is already close to ready to have its next earthquake and you push it just a little bit harder by the additional removal of snow or water at the surface, you might just get an earthquake to happen,” he said.
This video produced by the Berkeley Seismology Lab shows the seasonal stress changes on California faults from 2006 through 2014 as a result of water loading.
It’s not the pressure itself but the alleviation of pressure that can result in an earthquake, Bürgmann said.
For instance, the Sierra snowpack can affect the San Andreas Fault hundreds of miles to the west.
The San Andreas is where the Pacific and North American tectonic plates meet and are grinding past each other, with the North American plate heading southwestward and the Pacific Plate northwestward.
Quakes More Likely After Snowmelt
The snow that’s weighing down the Sierra is pushing the North American plate westward into the Pacific Plate, jamming the two more tightly together, Bürgmann said. The pressure squeezing them together makes it less likely they will be able to break apart in an earthquake now, he said.
Quakes are likelier to occur after the snow melts, the snow load on the Sierra lifts, the geologic pressure decreases, and the two plates have more wiggle room.
Along the San Andreas “we would expect more favorable conditions for an earthquake during the late summer-early fall,” he said. “So when we looked specifically along the central San Andreas fault, south of the Bay Area, that is when we saw the biggest number of earthquakes. I think it was in September or October.”
Faults in other parts of the state might see an increase in earthquakes at other times of year, Bürgmann said.
The researchers found that the “peak” stress along the Coast Range was in late summer, while it was in early summer on the east side of the Sierra.
The researchers had noted in the 2017 study that different factors — water, atmosphere, tides, even the earth’s movement — can contribute to stress on an earthquake fault. In California, water is the dominant factor, the researchers said.
This video produced by the Berkeley Seismology Lab shows the groundwater storage as derived from the geodetic GPS measurements between 2006 and 2014. Red colors mean less, blue colors mean more water. The gray lines indicate the location of mapped faults. The green outline is the border of the Central Valley.
Multiple Causes Can Prompt Quakes
Bürgmann cautioned about assigning specific causes to earthquakes, however.
He said he had seen some studies that correlated the 7.1 magnitude earthquake in Ridgecrest in July 2019 with water-related stresses.
“But I think you can’t really support that because the earthquake would happen anyways and seasonal snow and water loads happen every year, and they do so for hundreds of years,” he said. “And so to argue that just because there’s one single earthquake, just because it’s big (and) happened at a time where the snow could have made a difference, that to me is not very compelling.”
But in California there is at least one documented case of when an increase in surface water sparked an earthquake, Bürgmann said. A 5.9 magnitude earthquake struck in August 1975 in the vicinity of Oroville Dam and lake in what is known as an “induced” earthquake.
Berkeley scientists hypothesized that the earthquake occurred six years after the dam’s completion because large amounts of water had been released in the winter of 1974-75 to make way for an anticipated large snowmelt, and the rapid filling of the lake with snowmelt water increased underground pressure and may have reactivated a dormant fault.
Does any of this sound familiar? Dam operators in the Valley will be keeping a close eye on snowmelt this spring and summer, trying to time releases so that there’s enough room at Pine Flat, Friant, and San Luis to handle the inflows.
As of Sunday, San Luis Reservoir was at 98% of capacity, Pine Flat was 71%, and Friant was 61%, according to the California Department of Water Resources.
Swarms of Shallow Quakes at Mammoth
There’s yet another hydrologic factor that could have an impact on quakes, and that’s the movement of water seeping down from the earth’s surface into porous rock and affecting how that underground rock fits together — or creating conditions where the rock can start moving along a fault line.
Research by the U.S. Geological Survey that was published in 2019 tracked earthquake swarms near Mammoth Mountain with the rate of snowmelt as measured in creeks and streams and found that the rate of shallow earthquakes was 37% higher during wet seasons than drier ones.
Researcher Emily Montgomery-Brown and colleagues reviewed earthquake records dating back to 1984 and were able to establish a direct correlation between the amount of water channeling in mountain streams with the earthquake activity.
As the meltwater recharged underground aquifers, it changed the pore pressure of rocks 1 to 3 kilometers below the earth’s surface, the researchers learned.
“That’s a system where that water pressure effect is just really evident,” Bürgmann said. “But we don’t see that generally across California.”