California’s earthquake drought will eventually end, with bigger temblors happening more frequently

California is in an earthquake drought.

It has been almost five years since the state experienced its last earthquake of magnitude 6 or stronger - in Napa. Southern California felt its last big quake on Easter Sunday 2010, and that shaker was actually centered across the border, causing the most damage in Mexicali.

Experts know this calm period will eventually end, with destructive results. They just don’t know when this well-documented geological pattern will shift.

“Earthquake rates are quite variable: We have a decade or two where we don’t have many earthquakes, and people expect that’s what California is always like,” said Elizabeth Cochran, seismologist with the U.S. Geological Survey. Eventually, “we’re going to dramatically see a change in earthquake rates.”

Scientists have been warning California of seismic dangers, from the Big One on the San Andreas fault to other catastrophes that could come from lesser-?known faults, like the Hayward and Newport-?Inglewood.

One reason for the urgent alerts: Memories of a truly destructive quake in many urban areas have faded. And with that, some fear, the urgency of ?pushing seismic safety.

Consider how much quieter California has been in regard to earthquakes of magnitude 6 or greater:

In the last 25 years, there have been 11 such temblors statewide. In the preceding generation, there were 32.

In the past quarter-?century, there have been three such earthquakes that shook California’s 10 southernmost counties. In the prior generation, there were nine, including the Sylmar temblor of 1971 (magnitude 6.6) and Northridge of 1994 (magnitude 6.7).

The greater Bay Area has been particularly quiet. Since the great 1906 earthquake destroyed much of San Francisco and largely demolished Santa Rosa and Sebastopol, killing more than 100 people, there have been only three earthquakes of magnitude 6 or greater. But in the 75 years before that catastrophe, there were 14, according to geophysicist Ross Stein.

The greatest earthquake threat to Sonoma County lies along the Rodgers Creek Fault, which cuts north-south and follows a path through central Santa Rosa, Windsor and eastern Healdsburg. Seismologists have concluded the fault is linked to the East Bay’s Hayward Fault and estimated the probability of a magnitude 6.7 or greater quake along the combined fault system at 33% between 2014 and 2043.

Earthquakes must happen at some point to relieve the immense tectonic forces that are pushing part of the state northwest toward Alaska and the rest southeast toward Mexico. On average, there’s about 12 feet of movement per century along the 730-mile San Andreas fault system between Point Arena in Mendocino County and the Mexican border, Rockwell said, but there has been no significant movement along this primary boundary of faults between the Pacific and North American plates in a century.

There may be periods “where things get kind of all locked up and no earthquakes happen for a while. You store a lot of strain in the Earth’s crust,” said Tom Jordan, USC professor of geophysics. “Once it gets going, it’s like a set of dominoes.”

When a time of elevated seismic activity will come is hard to say. Some thought the 1989 magnitude 6.9 Loma Prieta earthquake, which killed 63 people, would mark the end of decades of seismic quiescence in the Bay Area. Instead, the lack of extreme shaking has largely persisted, save for the Napa temblor of 2014.

Scientists have been particularly focused on the lack of major seismic activity on several California faults that as a group produce the most frequent earthquakes on the plate boundary - the San Andreas, San Jacinto and Hayward. They are the ones seen as most likely to cause trouble in our lifetime.

Yet they’ve been exceedingly quiet. A USGS analysis to be published in the journal Seismological Research Letters Wednesday finds that the century between 1919 and 2018 is in all odds the only 100-year period in the past 1,000 years where there have been no earthquakes strong enough to break the ground on those three faults.

Ordinarily, there are roughly three to four of these earthquakes on these faults every 100 years.

“The next century is unlikely to be as quiet as this one. It’s hard to beat,” said USGS seismologist Glenn Biasi, the lead author of the study.

Earthquake scientists have been buzzing for years about California’s hiatus in supersized earthquakes, thinking the chances of such a 100-year gap between ground-shattering seismic events to be improbable. Scientists focus on earthquakes that literally break the ground along the main plate boundary because they’re the ones that actually do the job of relieving centuries of tectonic strain. A famous example of the ground breaking was during the great 1906 earthquake that destroyed much of San Francisco. At Point Reyes in Marin County, a fence that intersected the fault was suddenly cut in two, separated on each side by the San Andreas by 18 feet.

Some quakes don’t do that job. The 1989 magnitude 6.9 Loma Prieta earthquake, while near the San Andreas fault, actually occurred on a neighboring sub-parallel strand, according to USGS research geologist Kate Scharer, a coauthor of the USGS study. Earthquakes of 1992 in Landers and 1999 in Hector Mine broke faults at the surface, but didn’t do it at the main plate boundary, Biasi said.

Experts say it’s important that Californians are psychologically prepared for the prospect of a new era of earthquakes. That means not only having an emergency plan and supplies, said McBride, now a social scientist for the USGS’ earthquake early warning system, but mentally imagining what that will mean - not just putting a flashlight and shoes next to the bed, for instance, but understanding that will mean dealing with broken glass on the floor and a power outage that may last weeks or more, and planning for how your family will get through it.