Signs of past California ‘mega-quakes’ show danger of the Big One
HOT SPRINGS, Calif.—As Interstate 10 snakes through the mountains and toward the golf courses, housing tracts and resorts of the Coachella Valley, it crosses the dusty slopes of the San Gorgonio Pass.
The pass is best known for the spinning wind turbines that line it. But for geologists, the narrow desert canyon is something of a canary in the coal mine for what they expect will be a major earthquake coming from the San Andreas Fault.
The pass sits at a key geological point, separating the low desert from the Inland Empire, and, beyond that, the Los Angeles Basin.
Through it runs an essential aqueduct that feeds Southern California water from the Colorado River as well as vital transportation links. It’s also the path for crucial power transmission lines.
California earthquake experts believe what happens at the San Gorgonio Pass during a major rupture of the San Andreas Fault could have wide-ranging implications for the region and beyond.
They worry a huge quake could sever lifelines at the pass for weeks or months, cutting Southern California off from major highway and rail routes as well as sources of power, oil and gas. Southern California’s cities are surrounded by mountains, making access through narrow passes like the San Gorgonio essential.
Experts have also expressed grave concerns about the Cajon Pass, where Interstate 15 and key electric and fuel lines run. Other problem spots are the Tejon Pass, through which Interstate 5 passes, and the Palmdale area, through which the California Aqueduct crosses.
One of the most dire scenarios geologists have studied is a quake that begins at the Salton Sea. Such a quake would be particularly dangerous because the fault’s shape points shaking energy toward Los Angeles.
Southern California has not seen an earthquake like this since humans started recording history here. But the geological evidence of such quakes is all around us. Signs of megaquakes In Desert Hot Springs, hints of the mighty San Andreas Fault lie all over: The rise of mountains that created the Coachella Valley. The oases and palm trees— made possible only because earthquakes pulverized rocks that allowed springs to burst to the surface.
A geologist’s trained eye can even spot exactly where the fault is located. In one exposed cliff, USGS research geologist Kate Scharer showed how one side of a hill has moved northward and skyward compared with the right side—and the gouge in the hillside between them was the fault.
Farther away, Scharer described how an old lower canyon was severed from the upper canyon and its ancient source of water. Direction matters There’s a reason why this particular scenario vexes scientists:
An earthquake arriving from this direction would point cataclysmic shaking directly into the heart of L.A., a kind of disaster that has not been seen since humans began recording history in California. Shaking could last for as long as three minutes.
In a magnitude 8.2 scenario, the earthquake would begin at the Salton Sea, and then—like a big rig driving on a freeway— speed up the San Andreas Fault toward Los Angeles County.
“It’s shooting all of that energy straight into the L.A. Basin,” Scharer said.
Why a quake that begins so far away matters
An earthquake that begins more than 100 miles from L.A. might seem like something you might not worry about.
But a magnitude 8.2 earthquake is no ordinary earthquake.
The traditional image of an earthquake might be to show the epicenter—the point at which the earthquake begins.
But that doesn’t tell the whole story.
A better representation of a large earthquake would show how the earthquake travels up the fault. And this becomes more important for large earthquakes, which require an incredible amount of area in which the sides of the fault move against each other.
So, according to seismologist Lucy Jones, if a San Andreas earthquake began at the Salton Sea and …
ended at Mount San Gorgonio, it would be a 7.3 earthquake.
stopped at the Cajon Pass, it would be a magnitude 7.6 or 7.7 seismic event.
traveled up to Lake Hughes, the earthquake would clock in at 7.8.
and “if it goes all the way from the way from the Salton Sea to near Paso Robles, we’d get an 8.2. So that’s probably the biggest we can have,” Jones said.
“I think it’s going to go all the way to Paso Robles,” Jones said of the next Big One.
Jones cited a recent study by Scharer that found that earthquakes happen at the San Andreas around the Grapevine on average every 100 years. It has been 160 years since the last major earthquake on that section of the fault. Hope for L.A. Here in the Coachella Valley and across the West Coast, scientists have been busy installing new seismic equipment as they construct an earthquake early warning system, which could give places like L.A. seconds—or even a minute or more—of warning before the shaking waves arrive from an earthquake.
The project, however, is in danger of losing funding. President Donald Trump’s proposed budget suggests ending federal funding for the early warning system. Southern California’s elected officials in Congress have voiced support for continuing funding of the project.