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One of Southern California’s most dangerous faults caused land on the Orange County coast to sink between 1.5 feet to 3 feet in a matter of seconds during prehistoric earthquakes, according to a new study that suggests the seismic risk is greater than previously believed.
“It’s not just a gradual sinking. This is boom – it would drop. It’s very rapid sinking,” said the lead author of the report, Robert Leeper, a geology graduate student at the University of California, Riverside, who worked on the study as a Cal State Fullerton student and geologist with the U.S. Geological Survey.
The study of the Newport-Inglewood Fault focused on the wetlands of Seal Beach. But the area of sudden dropping could extend to other regions in the same geologic area of the Seal Beach wetlands, which includes the U.S. Naval Weapons Station and the Huntington Harbour neighborhood of Huntington Beach.
Leeper and a team of scientists at Cal State Fullerton had been searching the Seal Beach wetlands for evidence of ancient tsunami. Instead, they found buried organic deposits that they determined to be the prehistoric remains of marsh surfaces, which they say were abruptly dropped by large earthquakes that occurred on the Newport-Inglewood Fault.
Those earthquakes, roughly dated in 50 B.C., A.D. 200 and the year 1450 – give or take a century or two – were all more powerful than the magnitude 6.4 Long Beach earthquake of 1933, which did not cause a sudden drop in the land, Leeper said.
As a result, the observations for the first time suggest that earthquakes as large as magnitudes 6.8 to 7.5 have struck the Newport-Inglewood/Rose Canyon fault system, which stretches from the Westside of Los Angeles through Long Beach and the Orange County coast to downtown San Diego.
The newly discovered earthquakes also suggest that the Newport-Inglewood Fault is more active than previously thought. Scientists had believed the Newport-Inglewood Fault ruptured in a major earthquake once every 2,300 years on average; the latest results show that a major earthquake could come once every 700 years on average, Leeper said.
It’s possible the earthquakes can come more frequently than the average, and can arrive spaced as little as 300 years apart from one another.
If a magnitude 7.5 earthquake did rupture on the Newport-Inglewood/Rose Canyon fault system, such a temblor would bring massive damage throughout Southern California, said seismologist Lucy Jones, who was not affiliated with the study. Such an earthquake would produce 45 times more energy than the 1933 earthquake.
“It’s really clear evidence of three earthquakes on the Newport-Inglewood that are bigger than 1933,” Jones said of the earthquake that killed 120 people. “This is very strong evidence for multiple big earthquakes.”
The idea that the Newport-Inglewood Fault could produce more powerful earthquakes than what happened in 1933 has been growing over the decades. Scientists have come to the consensus that the Newport-Inglewood Fault could link up with the San Diego County coast’s Rose Canyon Fault, producing a theoretical 7.5 earthquake based on the length of the combined fault system.
An earthquake of magnitude 7 on the Newport-Inglewood Fault would hit areas of Los Angeles west of downtown particularly hard.
“If you’re on the Westside of L.A., it’s probably the fastest-moving big earthquake that you’re going to have locally,” Jones said. “A 7 on the Newport-Inglewood is going to do a lot more damage than an 8 on the San Andreas, especially for Los Angeles.”