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Fault along Calif. coast could unleash earthquake on scale of San Andreas, study shows

Salvador Hernandez, Los Angeles Times on

Published in News & Features

LOS ANGELES — A fault system running nearly 70 miles along the coast of Los Angeles and Orange counties has the potential to trigger a magnitude 7.8 earthquake, according to a new study that is the latest to highlight the seismic threats facing Southern California.

Known as the Palos Verdes fault zone, the system runs deep beneath the Palos Verdes Peninsula. It previously was thought to be a segmented network of smaller faults, but a closer look by scientists at Harvard University suggests it's a system of interconnected, closely spaced planar fractures stretching from the Santa Monica Bay to the waters off Dana Point.

The analysis determined the fault system, which runs beneath numerous neighborhoods as well as the ports of Long Beach and L.A., has a much larger surface area that could rupture in the same seismic event, making it capable of a far more powerful quake than was previously known.

Scientists found the fault could produce a quake of a magnitude comparable to one from the San Andreas fault. Earlier estimates said the fault zone could generate up to a magnitude 7.4 earthquake, but the new study shows it could produce a quake as strong as 7.8.

The difference may be only a few decimal points, but an earthquake's energy is measured exponentially. According to the U.S. Geological Survey, a magnitude 7.8 quake produces quadruple the energy of a magnitude 7.4.

In a worst-case scenario, the Palos Verdes fault system could unleash a quake that combines the most destructive qualities of the 1994 Northridge earthquake, a magnitude 6.7 temblor, and the 7.1 Ridgecrest quake that struck in 2019, said John H. Shaw, a professor of structural and economic geology at Harvard University and one of three authors of the new study.


The Northridge quake, which killed 57 people, had a devastating combined side-to-side and up-and-down motion that proved especially destructive to structures. That same combined lateral and vertical movement of faults is possible along the Palos Verdes network.

The Ridgecrest quake was a prolonged series along multiple interconnected fault lines, similar to those of the Palos Verdes system.

"Rather than one line in the map, we could see a rupture (series) that could occur in a wide area," Shaw said of a major quake along the Palos Verdes fault zone. Aftershocks would be like falling dominoes, he said.

James Dolan, an Earth sciences professor at USC who reviewed the Harvard report, said the study is "by far the most detailed look we've had of the internal structure and connectivity of the Palos Verdes fault system."


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