The last major earthquake on the San Andreas Fault in the Coachella Valley was more than 300 years ago, a geologist told visitors to the Cajon Pass on Saturday.
Residents of Redlands, Colton and Riverside joined staff from the San Bernardino County Museum to explore a "treasure trove" of geologic and fossilized history in Cajon Pass, about 80 miles west of Palm Desert.
Kathleen Springer, the museum's senior curator of geological sciences, and Eric Scott, curator of paleontology, shared years of field research and academic study to reveal hidden evidence of prehistoric life and landform movement in the heavily-industrialized transportation and utility corridor.
The mountain-building force of two continental-sized tectonic plates on the San Andreas Fault, and a time-frame stretching back millions of years, make the Cajon Pass an open book into the past - as well as the future, Springer and Scott said.
"The San Andreas Fault is an 800-mile-long fault," Springer said, standing on a rock-and-mortar wall at Blue Cut, below Interstate 15 and across Cajon Creek from Union Pacific railroad tracks. "It starts in the Salton Sea and goes all the way to Mendocino County. It's a huge fault.
"It's a right-lateral fault because the Pacific Plate is moving north and west relative to Kansas and the rest of the North American Plate," Springer said. "But it's got a kink in it. It's got a left bend in it that's about a hundred miles long, and when you put a bend in a right-lateral fault, to the left, the only thing the plates can do is compress. . . . So this gigantic bend has literally uplifted the Transverse Ranges.
"It's not just the San Bernardinos and San Gabriels, it's the entire Transverse Ranges that run oblique to the San Andreas Fault," Springer said. "They start in the Eagle Mountains, in Joshua Tree National Park, and they go all the way to the Channel Islands. It's a huge uplift in this compressional zone."
Springer's intro was part of an eight-hour tour, by van and on foot, to numerous features in Cajon Pass including Lost Lake, a natural reservoir created by the San Andreas, fossilized rhinoceros teeth and bones, decapitated alluvial fans that slope towards Victorville and end abruptly near Cajon Summit, and the massive, tilting sandstone Mormon Rocks near State Route 138.
The scientists discussed how paleontology, the study of prehistoric life, has aided modern-day geologists in their pursuit of understanding earthquake history and hazards.
Springer underscored several times that seismic events of the past that helped form the Transverse Ranges and the Cajon Pass are likely to occur again in our lifetimes.
"This is one of the most critical lifeline spots in all of California," Springer said of Cajon Pass, standing on a picnic table at Lost Lake. "When we fast-forward and talk about earthquakes, we're talking about the San Andreas Fault, I talk a lot about earthquake awareness in the context of this geology stuff, and I talk a lot about the Great California Shakeout. . . .
"This is the premise: When the Earth starts to move, the Shakeout is a hypothetical 7.8 Magnitude earthquake that breaks from the Salton Sea, and ruptures for a hundred and eighty miles, to Lake Hughes in Los Angeles County.
"It ruptures right here where you're standing," Springer said. "This is the rupture zone. You're in the zone of rupture. It can happen right now. So there hasn't been a large earthquake in Southern California since 1857. Where we started our trip today in Redlands, south of there, from San Bernardino south to the Salton Sea, there has not been a major earthquake since 1680.
"On average a large earthquake occurs about once every 150 years," Springer said. "You're all smart people. Do the math for me."
Springer also pointed out that communities like Redlands and Loma Linda are equally if not more vulnerable to violent shaking during future earthquakes because they are built on slopes and sediment created by earlier land movements.
For more information about earthquake hazards and the Great California Shakeout, visit www.shakeout.org/california.
The San Bernardino County Museum is at 2024 Orange Tree Lane in Redlands. For more information about the museum and its programs, visit www.sbcounty.gov/museum.