Fault system off San Diego, Orange, Los Angeles Counties could produce magnitude 7.3 quake

Study finds rupture of offshore Newport-Inglewood/Rose Canyon fault is possible

7 March 2017

Joint Release

A Scripps research vessel tows a hydrophone array used to collect high-resolution bathymetric to better understand offshore California faults.
Credit: Scripps Institution of Oceanography, UC San Diego.

WASHINGTON, DC — A fault system that runs from San Diego to Los Angeles is capable of producing up to magnitude 7.3 earthquakes if the offshore segments rupture and a 7.4 if the southern onshore segment also ruptures, according to a new study led by Scripps Institution of Oceanography at the University of California San Diego.

The Newport-Inglewood and Rose Canyon faults had been considered separate systems but the study shows that they are actually one continuous fault system running from San Diego Bay to Seal Beach in Orange County, then on land through the Los Angeles basin.

“This system is mostly offshore but never more than four miles from the San Diego, Orange County, and Los Angeles County coast,” said study lead author Valerie Sahakian, who performed the work during her doctorate at Scripps and is now a postdoctoral fellow with the U.S. Geological Survey in Menlo Park, California. “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.”

The new study was accepted for publication in the Journal of Geophysical Research: Solid Earth, a journal of the American Geophysical Union.

In the new study, researchers processed data from previous seismic surveys and supplemented it with high-resolution bathymetric data gathered offshore by Scripps researchers between 2006 and 2009 and seismic surveys conducted aboard former Scripps research vessels New Horizon and Melville in 2013. The disparate data have different resolution scales and depth of penetration providing a “nested survey” of the region. This nested approach allowed the scientists to define the fault architecture at an unprecedented scale and thus to create magnitude estimates with more certainty.

Locations of NIRC fault zone as observed in seismic profiles.
Credit: AGU/Journal of Geophysical Research: Solid Earth.

They identified four segments of the strike-slip fault that are broken up by what geoscientists call stepovers, points where the fault is horizontally offset. Scientists generally consider stepovers wider than three kilometers more likely to inhibit ruptures along entire faults and instead contain them to individual segments – creating smaller earthquakes. Because the stepovers in the Newport-Inglewood/Rose Canyon (NIRC) fault are two kilometers wide or less, the Scripps-led team considers a rupture of all the offshore segments is possible, said Neal Driscoll, a geophysicist at Scripps and co-author of the new study.

The team used two estimation methods to derive the maximum potential a rupture of the entire fault, including one onshore and offshore portions. Both methods yielded estimates between magnitude 6.7 and magnitude 7.3 to 7.4.

The fault system most famously hosted a 6.4-magnitude quake in Long Beach, California that killed 115 people in 1933. Researchers have found evidence of earlier earthquakes of indeterminate size on onshore portions of the fault, finding that at the northern end of the fault system, there have been between three and five ruptures in the last 11,000 years. At the southern end, there is evidence of a quake that took place roughly 400 years ago and little significant activity for 5,000 years before that.

Driscoll has recently collected long sediment cores along the offshore portion of the fault to date previous ruptures along the offshore segments, but the work was not part of this study.

“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.

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Notes for Journalists
This research article is open access. A PDF copy of the article can be downloaded at the following link: http://onlinelibrary.wiley.com/doi/10.1002/2016JB013467/pdf.

Journalists and PIOs may also order a copy of the final paper by emailing a request to Lauren Lipuma at llipuma@agu.org. Please provide your name, the name of your publication, and your phone number.

Neither the paper nor this press release is under embargo.

Title

“Seismic Constraints on the Architecture of the Newport-Inglewood/Rose Canyon Fault: Implications for the Length and Magnitude of Future Earthquake Ruptures”

Authors:
Valerie Sahakian: Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, U.S.A.; now at Earthquake Science Center, U.S. Geological Survey, Menlo Park, California, U.S.A.;

Jayne Bormann: Nevada Seismological Laboratory, University of Nevada Reno, Reno, Nevada, U.S.A.; now at Department of Geological Sciences, California State University-Long Beach, Long Beach, California, U.S.A.;

Neal Driscoll, Alistair Harding: Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, U.S.A;

Graham Kent, Steve Wesnousky: Nevada Seismological Laboratory, University of Nevada Reno, Reno, Nevada, U.S.A.

Contact information for the authors:
Valerie Sahakian: vsahakian@usgs.gov, +1 (650) 329-5153.


AGU Contact:

Lauren Lipuma
+1 (202) 777-7396
llipuma@agu.org

Scripps Institution of Oceanography Contact:
Robert Monroe
+1 (858) 822-4487
rmonroe@ucsd.edu