A powerful fault system that runs from San Diego to Los Angeles could produce a magnitude 7.3 to 7.4 earthquake if the segments rupture, according to an analysis led by researchers at Scripps Institution of Oceanography at the University of California San Diego (UCSD).
The study looks at the Newport-Inglewood fault and the Rose Canyon fault, two systems that had previously been considered separate systems.
However, this new study concludes the systems are actually one continuous fault system which runs from the San Diego Bay to Seal Beach in Orange County, then on land through the Los Angeles Basin. The study appeared in the most recent issue of the American Geophysical Union's Journal of Geophysical Research.
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“This system is mostly offshore but never more than four miles from the San Diego, Orange County, and Los Angeles County coast,” said Valerie Sahakian, who led the study during her doctorate at Scripps. She is now a postdoctoral fellow with the U.S. Geological Survey.
“Even if you have a high 5- or low 6-magnitude earthquake, it can still have a major impact on those regions which are some of the most densely populated in California,” Sahakian explained.
When conducting research, the team processed seismic surveys from the past and added in high-resolution bathymetric data, gathered offshore by Scripps researchers between 2006 and 2009, in addition to seismic surveys conducted aboard former Scripps research vessels.
The combination of those techniques allowed researchers to have a clearer definition of how the fault was built, and make estimates about magnitudes with more certainty.
If the offshore segments erupt, researchers estimated a 7.3 magnitude quake; if the southern onshore segment also ruptures, researchers estimated a magnitude 7.4 quake.
The two methods used to derive the maximum potential for a rupture of the entire fault found estimates between magnitude 6.7 and magnitude 7.3 to 7.4 quakes, according to the study.
In 1933, a magnitude 6.4 quake struck the Long Beach area and killed 115 people. Researchers discovered evidence that quakes happened prior to that in the region, though they could not measure a magnitude.
In the last 11,000 years, there have been three to five ruptures at the north end of the fault system. On the south end, which goes through San Diego, there is evidence a quake happened roughly 400 years ago, but there is little significant activity for the 5,000 years prior.
“Further study is warranted to improve the current understanding of hazard and potential ground shaking posed to urban coastal areas from Tijuana to Los Angeles from the NIRC fault,” the study concludes.