While earthquakes aren’t necessarily surprising in California, the 6.1 magnitude quake that hit Napa County on Sunday morning was a reminder to local officials across the Golden State and in other known quake zones that there’s still a lot of uncertainty to understanding the full extent of local seismic risk.
While the United States Geological Survey reported on Twitter on Sunday morning that there was no definitive determination of the “causative fault,” one USGS seismologist said that it was possible that the quake was centered on the Franklin Fault, which “has not ruptured for at least 1.6 million years,” according to the San Francisco Chronicle. Later in the day, the USGS indicated that the quake's origins may have been on the "little known" Browns Valley section of the West Napa fault, according to the Chronicle.
Early reports indicated that damage was localized and not widespread across the larger region. But damage was significant in parts of southern Napa County. In the city of Napa, officials reported major damage at the historic courthouse, library and some commercial buildings plus approximately 50 gas main breaks, 30 water main breaks and power outages. There were more than 100 reported injuries but no fatalities.
While major fault lines in California—like the well-known San Andreas—have been studied for decades, there are hundreds of other faults that are less understood by seismologists.
The 6.7 magnitude Northridge earthquake centered in Los Angeles’ San Fernando Valley in 1994 had its origins on a previously unknown blind-thrust fault. This past March, a widely-felt quake in the L.A. area that didn’t do any damage “surprised seismologists because it was the strongest to hit directly under the Santa Monica Mountains in the 80 years ‘since we started recording earthquakes in Southern California,’” Caltech seismologist Egill Hauksson told the Los Angeles Times.
The state has a lot of catch-up work to do to understand localized seismic risks.
According to a report in the L.A. Times from July:
California aggressively mapped faults from the 1970s through the early 1990s. A series of budget cuts slowed the effort dramatically over the next decade, and no new fault zoning occurred between 2004 and 2011. The California Geological Survey restarted the effort again using its existing budget, but progress has been slow.
Under the previous budgetary conditions, the state had been completing just one seismic map every year.
California law prohibits new construction on top of active fault lines—defined as those that have shown seismic activity in the previous 11,000 years—that could generate quakes with surface rupturing. Buildings must be offset from active fault lines by 50 feet, according Southern California Public Radio.
In Los Angeles, the state has been busy mapping the Hollywood Fault, which cuts through some of the most dense parts of the nation’s second-largest city. There have been no significant seismic events recorded on that fault in historic times—the last rupture was at least 7,000 years ago—but with more development planned along the fault zone, the state has been assessing the seismic risk.
An analysis by the L.A. Times showed that preliminary California Geological Survey maps of the Hollywood Fault and an adjacent fault in the San Gabriel Valley showed that those faults cut through 1,500 developed properties.
Developers have disputed the state’s preliminary map, but state geologist John Parrish said, according to the L.A. Times: “We aren’t going to be moving the fault lines or the traces without some real evidence.”
Editor's note: This post has been updated with additional information.