New study finds Los Angeles and San Diego are at risk from M=7.3+ offshore earthquakes

By David Jacobson, Temblor

Check your seismic hazard rank

 

A new study by researchers at the Scripps Institution of Oceanography, the Nevada Seismological Laboratory, and USGS has revealed that the Newport-Inglewood/Rose Canyon fault zone, which extends offshore from San Diego to Los Angeles, is capable of rupturing in M=7.3+ earthquakes. Such a quake could cause significant damage to some of the most densely populated areas in California.

 

The Newport-Inglewood/Rose Canyon fault zone is a strike-slip system in Southern California, and is part of the Pacific-North American plate boundary. Prior to this study, the Newport-Inglewood and Rose Canyon faults were believed to be separate systems. However, using a combination of seismic surveys, and high-resolution bathymetric data, the researchers determined that they are actually a continuous fault system, posing an even greater hazard to millions of people. By using these two types of data, which differ in their resolution and depth of penetration, the team was able to define the fault structure in great detail, which could then be used to calculate earthquake magnitudes.

The Newport-Inglewood/Rose Canyon fault zone exhibits a mainly continuous fault trace over four distinct segments. These segments are separated by what geologists term as stepovers, which are where a fault is horizontally offset. Most scientists believe that stepovers at least 3 km wide are likely to prevent through-going ruptures. However, along the Newport-Inglewood/Rose Canyon fault zone, none of the three stepovers exceed 2 km in width, leading the team to believe that an earthquake could rupture the entire length of the fault zone in M=7.3+ events. This was based historical ruptures worldwide which have been shown to rupture through stepovers of this width.

 

While the likelihood of a full rupture sits at 30-40%, lead author Valerie Sahakian, who is now a postdoctoral fellow at the USGS in Menlo Park, says that a smaller earthquake in the magnitude 5 or 6 range could still have a significant impact on the region. Such an earthquake happened in 1933, when the M=6.4 Long Beach earthquake killed 113 people.

Overall, this new study provides additional evidence showing the risk offshore faults pose to Southern California. Because the Newport-Inglewood/Rose Canyon fault is never more than four miles from the coast, significant shaking from an earthquake could have great impacts to the cities of Los Angeles and San Diego. Therefore, further research is needed to improve the seismic hazard models for the Southern California coast.

 

References
Sahakian, V., J. Bormann, N. Driscoll, A. Harding, G. Kent, and S. Wesnousky (2017), Seismic Constraints on the Architecture of the Newport-Inglewood/Rose Canyon Fault: Implications for the Length and Magnitude of Future Earthquake Ruptures, J. Geophys. Res. Solid Earth, 122, doi:10.1002/2016JB013467. Link

Scripps Institution of Oceanography press release (Fault System off San Diego, Orange, Los Angeles Counties Could Produce Magnitude 7.3 Quake) – Link