L.A.’s biggest quake threat on overlooked part of San Andreas

Scientists have pinpointed a long-overlooked portion of the southern San Andreas fault that they say could pose the most significant earthquake risk for the Greater Los Angeles area — and it’s about 80 years overdue for release.

But there could be a silver lining. If their analysis is right, experts say it’s possible that when a long-predicted and much more devastating earthquake hits, it may not do quite as much damage to the region as some scientists previously feared.

“That’s a significant reduction in risk for L.A. if this is true,” said longtime seismologist Lucy Jones, who was not involved in the study published Wednesday in the journal Science Advances.

The San Andreas fault is a roughly 800-mile fracture that runs much of the length of California and is capable of producing a much-feared, massive temblor known simply as “the Big One.”

The San Andreas also serves as a major marker of the boundary between the Pacific and North American plates. As the plates move past one another, so do the two sides of the fault.

But the fault itself is caught by friction, and as the two sides move, strain builds up until it’s eventually released through earthquakes. The southern San Andreas carries roughly half the strain resulting from the plates’ motion, as much as 25 millimeters (about one inch) per year.

Not every part of the fault carries that strain equally, though. In Southern California, the San Andreas fault system is made up of many smaller “strands,” and it’s difficult for earthquake researchers to identify which parts of the fault system are most at risk of rupture.

Case in point: the bouquet of fault strands — Garnet Hill, Banning and Mission Creek — that crosses the Coachella Valley. Scientists long thought much of the southern San Andreas fault’s slip occurred along the Banning strand and the Garnet Hill strand; the Mission Creek strand, they said, didn’t take much of the strain at all.

But the new findings turn that idea on its head.

Kimberly Blisniuk, an earthquake geologist at San Jose State University, went looking for evidence that earthquakes had caused landforms to move across the surface. She found them at Pushawalla Canyon, a site along the Mission Creek strand in the Little San Bernardino Mountains.

There, right next to the water-carved canyon, she saw a series of three ancient “beheaded channels” — long depressions in the desert that looked like they were once part of the original canyon before earthquakes shoved them aside.

Blisniuk walked the area to get a better look at these telltale signs of ancient rupture. In each of the channels, she and her team dated the ages of rocks and soil.

The oldest channel, which lay about 2 kilometers (more than a mile) away from the current canyon, was roughly 80,000 to 95,000 years old. The second, about 1.3 kilometers (less than a mile) away, was about 70,000 years old; and the third beheaded channel, about 0.7 kilometer (less than half a mile) away, was about 25,000 years old.

Based on these three landmarks, the researchers calculated that the average slip rate for the Mission Creek strand was about 21.6 millimeters (less than an inch) per year. At…