Scientists Warn That a 7.4 Magnitude Earthquake Could Reach From Los Angeles to San Diego


March 9, 2017

Such an earthquake would be 30 times more powerful than the magnitude 6.4 earthquake that caused the 1933 Long Beach earthquake, which killed 120 people.


Via Los Angeles Times

This map of earthquake faults shows the general route of the Newport-Inglewood/Rose Canyon fault system, which extends from San Diego along the coast to Huntington Beach, Long Beach and into the Westside of Los Angeles. (California Geological Survey / Google Maps)


The discovery of missing links between earthquake faults shows how a magnitude 7.4 earthquake could rupture in the same temblor underneath Los Angeles, Orange and San Diego counties, a new study finds.

Such an earthquake would be 30 times more powerful than the magnitude 6.4 earthquake that caused the 1933 Long Beach earthquake, which killed 120 people.

But to get to a 7.4, the earthquake would not only have to again rupture the Newport-Inglewood fault in Los Angeles and Orange counties. The temblor would also have to jolt the adjacent Rose Canyon fault system, which runs all the way through downtown San Diego and hasn’t ruptured since roughly 1650.

“These two fault zones are actually one continuous fault zone,” said Valerie Sahakian, the study’s lead author, who wrote it while working on her doctorate at Scripps Institution of Oceanography at UC San Diego. Sahakian is now a research geophysicist with the U.S. Geological Survey.

In the past, scientists reported gaps between the two fault systems of as much as 3 miles apart. But the latest study shows the gaps are actually less than 1¼ miles apart.

“That kind of characterizes it as one continuous fault zone, as opposed to two different, distinct fault systems,” Sahakian said, making it far easier for an earthquake to keep shaking land as it races down a longer fault, widening the seismic reach of the temblor.

There had already been consensus among scientists over the last three decades that the fault systems were actually one, said Caltech seismologist Egill Hauksson, who was not involved with this study. “We now have real evidence that this is the case,” Hauksson said.

The difficulty in proving it was caused by the location of the gap — under the Pacific Ocean between Newport Beach and La Jolla. Drawing a better map meant trying to figure out where the fault was underwater.

So Scripps researchers hopped aboard boats, and in the fall of 2013 spent more than 100 days at sea collecting data. They created an image of what the earth looks like under the seafloor to estimate where the fault lies.

To do so, they used a technique kind of similar to how submarines use sonar or bats use echoes to see.

From the ship, scientists towed a machine that generates acoustic waves that bounce off the seafloor and deeper underground layers and returns to the ship, giving the data the scientists need to produce a better map of where the faults actually are located. The researchers also used previously collected data to perfect their new map.

The study was published online Tuesday in the Journal of Geophysical Research.

Work on this fault is a reminder that major earthquakes can strike Orange and San Diego counties, regions that have not undergone catastrophic seismic damage in recent generations.


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