Also, without fine sediments, trenching is nearly impossible, so scientists have not been able to establish a chronology of past earthquakes. This is particularly difficult for the San Jacinto. Unfortunately, at Cajon Pass, the San Andreas and the San Jacinto faults are hidden at the bottom of valleys filled with large boulders drained from the San Gabriel Mountains, making it difficult to pin down exactly where to dig. Paleoseismologists can measure 14C in tiny charcoal samples from the sediments to determine when the quakes occurred. Large trenches, dug across faults, can expose sediment layers offset by movement along a fault during ancient earthquakes. Los Angeles as large black circle.ĭigging for ancient earthquakes in a challenging settingĮarthquake scientists hunt for evidence of past earthquakes in the dirt. The named points indicate sites with paleoseismic trenches along the faults. Simplified map of the San Andreas (SAF), San Jacinto (SJF), and Lytle Creek Ridge (LCRF) faults. But until now, scientists had not found any direct geologic evidence of such a leap.Ī new study, recently published in Geology, has uncovered a history of these quakes, highlighting the prevalence of this hazard in Southern California. Theoretically, an earthquake on one fault could grow to a larger temblor if the rupture jumps through the pass onto the other. In this mountain divide, these major faults come within 2.5 kilometers of one another. candidate, UC Davis Rodriguez Padilla, A.M., 2022, North of Los Angeles, Faults Share Earthquakes, Temblor, Īt the base of the San Bernardino Mountains, Interstate Highway 15 steps over the San Jacinto Fault as it climbs northward into Cajon Pass, where the road sharply turns and crosses the San Andreas Fault.
The San Andreas and the San Jacinto faults have simultaneously ruptured in at least three earthquakes over the past 2000 years.īy Alba M.
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