Los Angeles’ seismic risk highlighted by last night’s M=3.6 earthquake

By David Jacobson, Temblor

Check your hazard rank

Last night, a M=3.6 earthquake just north of Santa Monica shook much of greater Los Angeles. While the earthquake registered over 8000 USGS felt reports, no damage was caused by the quake.

 

Last night, at 11:20 p.m. local time, a M=3.6 earthquake struck just north of Santa Monica, and resulted in widespread shaking over Beverly Hills, Hollywood, and greater Los Angeles. Despite its small magnitude, which was not great enough to cause any damage, it did result in over 8,000 felt reports on the USGS website. This is likely because it was centered only 10 km beneath a very densely-populated area. Based on the USGS ShakeMap, only moderate shaking was felt close to the epicenter, with Twitter responses suggesting it was a single jolt.

This Temblor map shows the location of last night’s earthquake just north of Santa Monica within the Santa Monica fault zone.

 

From the focal mechanism produced by the USGS, last night’s earthquake was almost purely compressional in nature. Given its location, it likely occurred on a blind thrust associated with the Santa Monica Fault Zone. The classification of this feature as a blind thrust fault means the fault plane does not reach the surface. Faults like this are partially responsible for uplifting the Santa Monica Mountains and are capable of generating earthquakes similar to, and larger than the 1994 M=6.7 Northridge earthquake. While that earthquake caused billions of dollars worth of damage, a quake along the Santa Monica Mountains would likely cause much greater damage to the Los Angeles Basin, given its proximity to the city.

This USGS ShakeMap shows how strong shaking could be in a M=6.8 earthquake along the Santa Monica fault zone. This scenario event has an epicenter in almost the exact location of last night’s M=3.6 event. This stresses how greater Los Angeles is exposed to a variety of seismic sources.

 

According to the USGS, such an earthquake in almost the exact location as last night’s event could reach M=6.8 (see above). This type of earthquake would result in severe shaking throughout Los Angeles, and could cause catastrophic damage to the country’s second largest city. While some people may view such a scenario as an unlikely event, based on the Global Earthquake Activity rate (GEAR) model, which is available in Temblor, it is actually the type of earthquake all Los Angelenos should prepare for. This model uses global strain rates and historical seismicity to forecast the likely earthquake magnitude in your lifetime anywhere on earth. For this area just north of Santa Monica, the likely magnitude is 6.5-6.75, or almost exactly what the USGS scenario shows.

This Temblor map shows the Global Earthquake Activity Rate (GEAR) model for Southern California. This model highlights how in the location of last night’s earthquake, a M=6.5+ is likely in your lifetime.

 

In response to the numerous earthquake sources around the Los Angeles Basin, in March, the city of Santa Monica adopted the nation’s most extensive retrofitting plan. The Santa Monica City Council unanimously approved the ordinance, which could require that up to 2,000 buildings undergo earthquake safety improvements. The move was made in an attempt to “limit the loss of life and infrastructure” Mayor Ted Winterer said. Therefore, even though last night’s M=3.6 was extremely minor, it allows us to highlight and educate people on what could happen in their own backyard and how city officials are attempting to mitigate potential damage as much as possible.

 

References

USGS

LA Times