What’s happening with the San Andreas fault?

Parts of California lying along the San Andreas fault are sinking, while others are rising at the same rate, according to a new study published in journal Nature Geoscience on Monday. The underlying causes of that movement, say researchers, will eventually culminate in a major earthquake.

"Once there is a major event, all of that energy gets released," said Sam Howell, the study's lead author and PhD candidate in geophysics at the University of Hawaii at Manoa (UHM), in an interview with the Los Angeles Times.

Santa Barbara and San Luis Obispo as well as parts of San Bernardino county, the study found, were among the 125-mile "lobes" where the land was rising. In Orange County, the Los Angeles Basin, Bakersfield, and San Diego County, the opposite effect was occurring, with those areas falling a few millimeters per year.

Mr. Howell added that it is "pretty much impossible to say" when an earthquake would occur next. But the study, which used GPS data from an observatory that tracks seismic information on the Pacific and North American tectonic plates, will help scientists better understand how the fault line's behavior might affect the region around it.   

"While the San Andreas GPS data has been publicly available for more than a decade," said Howell in a press release accompanying the study, the question of whether terrain was moving vertically had been ignored in previous research because of "difficulties in interpreting the noisy data." 

The vertical motion of Earth's surface is subject to local, short-term variables that often prove tough to control for in studies, such as the pumping of groundwater, local surface geology, and precipitation levels.

By making use of a new statistical technique, Howell and his team were able to "break down the noisy signals to isolate a simple vertical motion pattern that curiously straddled the San Andreas fault," he said.

In doing so, added co-author Bridget Smith-Konter, associate professor at UHM, the team was able "to confirm that the buildup of century-long earthquake cycle forces within the crust" were responsible for the pattern.

The San Andreas fault is considered California's most dangerous fault line, but the last major event along it came in 1906, when San Francisco was hit by a 7.8-magnitude earthquake. In the southern part of the fault, though, it's been about a century and a half since a big earthquake, and for areas south of Los Angeles, three centuries have passed.

Smithsonian Magazine noted in 2015 that stresses along the fault have built up over the years.

"We think Southern California is locked and loaded, that the stresses have really built up," US Geological Survey seismologist Ned Field told the Smithsonian at the time, "and when things start unleashing, they could unleash for years."