Quake could worsen seismic strain

Does a good-size earthquake help relieve pent-up seismic stress? Does that postpone the day of reckoning when the Big One finally arrives?

That was the question some in California were asking hopefully in the wake of the July 4 magnitude 6.4 earthquake that rattled the region.

The answers, like so many when it comes to seismology, are unsettling.

It’s wishful thinking to imagine that, as a rule, earthquakes “relieve” seismic stress, said seismologist Lucy Jones.

In fact, generally speaking, earthquakes actually increase the risk of future quakes.

The reality is coming into focus as Southern California experienced its largest earthquake in nearly two decades, ending a quiet period in the state’s seismic history.

Does an earthquake immediately relieve seismic stress, forestalling a future big quake?

No. Think about what generally happens after a decent earthquake. Aftershocks. Lots and lots of aftershocks.

That’s what’s going on right now in the area around the Fourth of July magnitude 6.4 earthquake in the Mojave Desert close to Ridgecrest, a town of 29,000 notable for being a pit stop for Mammoth-bound skiers from L.A.

But couldn’t relieving seismic stress in one part of the state restart the earthquake clock elsewhere?

No. One part of California, west of the San Andreas, is

constantly moving northwest, toward Alaska, relative to the other side of the Golden State, which is headed toward Mexico.

These immense forces are what generated the state’s mountains, from the ranges seen in the Los Angeles Basin to the hills lining the ridges of the Bay Area. There’s a reason earthquake faults are often alongside hills and mountains.

“If you see mountains in California, that means something is moving up those mountains faster than erosion is wearing them down,” Jones said last year. “Basically, when you see mountains, think earthquakes in California.”

It’s also why California has been home to lucrative deposits of oil. It’s the reason there are springs in the desert giving rise to places like Palm Springs.

There is no avoiding, eventually, big earthquakes being unleashed on faults somewhere in this state. We just don’t know exactly when or where they will happen.

But just as they have happened in centuries and millennia past, they will happen again.

Why are there quakes in California?

Think about the San Andreas fault. It’s a huge fault — more than 800 miles long. Just the southern San Andreas fault, between Monterey County to close to the Mexican border, is capable of producing a magnitude 8.2 quake.

Relatively speaking, places on the southwest side of the San Andreas — such as Los Angeles, San Diego and Santa Barbara — are inching toward Alaska. Places on the other side of the fault, such as Sacramento and the Mojave Desert, are sliding toward Mexico.

But in places along faults like the San Andreas, the land on both sides of the fault is locked, even as land farther away continues to move. Eventually, the San Andreas — as well as other faults throughout California — will have to rupture to relieve mounting tectonic strain.

“Plate tectonics hasn’t suddenly stopped; it is still pushing Los Angeles toward San Francisco at the same rate your fingernails grow — about 1.5 inches each year .... Their motion cannot be stopped any more than we could turn off the sun,” Jones wrote in her recent book, “The Big Ones: How Natural Disasters Have Shaped Us (and What We Can Do About Them).”

The San Andreas is particularly feared because, in some sections, it will move for many feet almost instantaneously. A famous example was during the 1906 magnitude 7.8 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.

A similar size earthquake on the San Andreas fault rupturing through the Palm Springs area would shatter the ground. If a couple had the misfortune of holding hands across the fault in a remote part of the desert near Desert Hot Springs when the Big One hits, they’d suddenly be separated by as much as 30 feet — almost the entire length of a city bus, U.S. Geological Survey research geophysicist Kate Scharer said in 2017.

What about just the area that was hit by the Independence Day quake? Is that area now relieved of quake strain?

It actually probably made things worse for parts of some faults in that region, said earthquake scientists Ross Stein and Volkan Sevilgen, writing on their blog at Temblor.net.

Parts of several other faults — in remote areas of California — were actually “brought closer to failure” by the quake, they wrote.

Some areas hit by Thursday’s quake likely became loaded with more seismic strain after two previous temblors — the 1872 magnitude 7.6 Owens Valley and the 1992 magnitude 7.2 Landers earthquakes.

In an interview Friday, Stein said it looks pretty clear that the Garlock fault and the northern part of the Blackwater fault were stressed from the Independence Day quake. The Garlock fault is probably the more worrisome of the two — at a length of 175 miles, it’s mathematically capable of an earthquake with a magnitude in the high 7s.

Fortunately for the most populous areas of California, the Garlock fault is in a remote location — along the northern boundary of the Mojave Desert, reducing the potential effect of a major quake on it, Stein said. There are also questions as to whether the Garlock fault is a “zombie” fault that could be dead.

What does an era of earthquakes look like?

Sometimes, a moderate quake — after its series of aftershocks — can lead to a period of seismic quiet. Other times, it can usher in a new era of temblors.

For instance, in the 75 years before the 1906 San Francisco earthquake, there were 14 earthquakes of magnitude 6 or greater in the Bay Area, Stein has previously said in an interview. (Since the 1906 quake — a magnitude 7.8 — there have been only three.)

Angelenos might remember what is now known as the earthquake storm of the 1980s and ’90s. As tallied by Caltech seismologist Egill Hauksson, it began with the Whittier Narrows temblor in 1987 (magnitude 5.9), which killed eight. That was followed by Pasadena in 1988 (4.9); Montebello in 1989 (4.4); Upland in 1990 (5.2); Sierra Madre in 1991 (5.8), which killed one; and Northridge in 1994, which killed at least 57 people.

The 1800s were also an active time for earthquakes in California.

In 1800, a magnitude 7.2 quake hit on the San Jacinto fault east of Temecula. Then, in 1812, the San Andreas and San Jacinto faults moved in a magnitude 7.5 earthquake through the sites of present-day cities including San Bernardino, Rialto, Loma Linda, Yucaipa and Highland, and brought down Mission San Juan Capistrano’s Great Stone Church, killing more than 40 people inside.

Then, in 1857, the southern San Andreas saw extreme shaking on both sides of the fault all the way from Monterey County to Los Angeles and San Bernardino counties in a magnitude 7.8 quake.

What’s an example of a moderate quake actually coming before something far worse?

On March 9, 2011, a magnitude 7.3 earthquake off the east coast of Japan was followed two days later by a magnitude 9 earthquake. Some people, lulled into complacency, ignored protocol and failed to immediately evacuate before the catastrophic tsunami hit.

Closer to home, the main shock of the last truly great earthquake in Southern California — at 8:24 a.m. on Jan. 9, 1857 — was preceded an hour earlier in the Monterey County area by a magnitude 5.6 earthquake, and a magnitude 6.1 earthquake an hour before that.