Newly identified fault could set off 7.4 earthquake in Southern California

Two earthquake faults known for decades as potential threats to Southern California have been re-evaluated by scientists, who on Tuesday said the pair actually are a single fault that is capable of more damage than previously believed.

The re-identified fault, which includes the Newport-Inglewood and Rose Canyon lines, runs between Los Angeles and San Diego and could set off a 7.4-magnitude quake, according to a report published Tuesday in the Journal of Geophysical Research by scientists from UC San Diego and Scripps Institution of Oceanography.

In their report, the scientists noted the fault “poses a significant hazard to coastal Southern California” because it runs close to some of the most densely populated parts of the country.

“In the past, a lot of people maybe just weren’t concerned about faults that are offshore,” said the study’s lead author, Valerie Sahakian, now a postdoctoral fellow at the Earthquake Science Center in Menlo Park.

She said that while much attention has been paid to the San Andreas fault, which has more movement and runs through much of California, people are less vigilant about many other potentially powerful fault lines that cover Southern California and could “pose a significant threat to other communities.”

A 7.4 earthquake would be significantly bigger than any recent quake to hit the region.

The Northridge quake in 1994 measured at a magnitude of 6.7. It killed 57 people and caused more than $20 billion in damage. The El Mayor-Cucapha quake that hit Baja California, Mexico, in 2010 was measured as a magnitude-7.2 event and displaced 35,000 people.

“That’s pretty big,” Sahakian said, of the potential quake that could be generated by the new Newport/Rose fault. “That would be a situation, depending on how close you are to it and what type of soil you’re on; that could be pretty heavy shaking.”

The Newport/Rose fault runs offshore between San Diego Bay and Newport Beach. Between Newport Beach and Seal Beach, it runs underground, parallel to the coast, before moving inland to Culver City.

Though most of the fault is offshore, it’s never more than 4 miles from the coastline. The fault is broken into four main strands separated by three so-called stepovers, or horizontal breaks that are less than 2 kilometers wide.

News of the newly identified fault – and the potential danger it poses – didn’t faze at least one emergency-preparation expert.

“Regardless of any new discoveries, we live in a seismic zone; we should all be prepared for earthquakes. There’s no denying that. We are in earthquake country,” said Donna Boston, director of emergency management for Orange County.

“The hard part is getting people to do it. It’s the same things we keep drilling home every time: Have a kit. Have a plan. Be informed. Those are the critical pieces.”

Scientists determined that the Newport/Rose fault was a single line by re-examining existing data and running new seismic tests. Those tests identified the so-called “architecture” of the single fault and provided detail that made it possible to re-estimate the fault’s power potential with greater certainty.

With their new data, earthquake researchers think there’s a greater likelihood that different parts of the fault could rupture at the same time, but they did not say if any specific stretch of the Newport/Rose fault is due for a temblor.

Also, because of the formation of the sea floor, scientists don’t think the Newport/Rose fault is likely to trigger a significant tsunami.

The last major earthquake on the Newport/Rose fault was the 1933 magnitude 6.4 earthquake in Long Beach. That event killed 115 people and spurred changes in, among other things, school construction.

Researchers found that there have been between three and five earthquakes along the northern part of the fault line in the last 11,000 years. There is evidence the southern end of the fault generated a significant quake about 400 years ago.

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