LOS ANGELES (AP) — Geologists have dug up evidence that two extremely powerful earthquakes struck only 12 miles from what is now downtown Los Angeles within the past 15,000 years, suggesting the region is far more vulnerable than previously believed.
“This is direct evidence that earthquakes in the region have been substantially larger than ever before documented — and suggests they could happen again,” said Charles Rubin, a geologist at Central Washington University and author of the study in Friday’s issue of the journal Science.
The quakes happened along the Sierra Madre Fault, with the more recent one taking place about 10,000 years ago. One had a magnitude of 7.2, the other 7.6, the researchers said.
The scientists found traces of the quakes by digging 18 feet into the ground in Altadena, 12 miles northeast of downtown Los Angeles. Radiocarbon dating of charcoal found during the dig allowed them to estimate when the quakes took place.
“Until our recent excavation we had no clue that an earthquake of a magnitude 7.5 had ever occurred in this region,” Rubin said.
Such quakes are far stronger than the deadly 1994 Northridge quake that rocked Los Angeles, and the location on the Sierra Madre Fault is much closer to millions of people living in the nation’s most populous county. The Northridge quake struck with a magnitude of 6.7, killed 72 people and caused an estimated $40 billion in damage.
A quake of the sort detected by the geologists “would probably be the biggest event that this country has ever experienced in a major urban area,” said Thomas Henyey, director of the earthquake center at the University of Southern California.
“If anyone had any doubt that faults around metro Los Angeles were capable of producing earthquakes much larger than the Northridge quake of 1994, this should erase that doubt,” Dolan said.
Paul Somerville, an engineering seismologist in Pasadena who specializes in seismic designs for critical structures such as dams and bridges, expressed concern for NASA’s Jet Propulsion Laboratory and hospitals in the San Gabriel foothills, as well as freeway bridges.