By David Jacobson, Temblor
Just before 10:30 p.m. local time last night (1 May 2017), a M=3.1 earthquake struck just offshore of Santa Monica, California. This quake was widely felt in the western Los Angeles County beachfront town, and at 8 a.m. this morning, nearly 2,500 people reported feeling the quake. However, many more likely felt it as Santa Monica is home to over 90,000 people. While only light shaking was felt close to the epicenter, it gives us an opportunity to look at the seismic hazard of this part of Southern California.
According to data from the USGS, this quake occurred at a depth of approximately 10 km on a fault that has both left-lateral strike-slip and reverse motion. Based on the location of yesterday’s quake, slip likely took place along the Anacapa-Dume Fault, which is part of the Santa Monica Fault Zone in the Transverse Ranges (Dolan, Sieh, and Rockwell, 2000). The Anacapa-Dume Fault is an east-west trending, north-dipping fault that extends from Santa Monica to offshore of Point Dume in Malibu. In fact, the largest earthquake to ever occur along the Malibu coastal region, a M=5.3 in 1973, is believed to have been along the Anacapa-Dume Fault. While a repeat of such an event would not lead to severe damage, there is the potential for multiple faults in the region to rupture simultaneously in M=7+ earthquakes.
The main fault in this part of Southern California that is often called upon to rupture with the Anacapa-Dume Fault is the Santa Monica Fault, which is only 1 km to the north. Like the Anacapa-Dume Fault, the Santa Monica Fault is a moderately-dipping fault with components of both strike-slip and reverse motion. However, the Santa Monica Fault cuts through densely-populated areas, and has a history of M=6.9-7.0 earthquakes. Therefore, simultaneous rupture along the Santa Monica and Anacapa-Dume faults would generate a M=7+ earthquake which would have devastating effects on the region. Not only could shaking damage infrastructure, but a large portions of Santa Monica and West Los Angeles are susceptible to liquefaction.
Because of the earthquake hazards in the region, the city of Santa Monica recently adopted the nation’s most extensive retrofitting plan. At the end of March, 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 yesterday’s M=3.1 was extremely minor, it highlights an area susceptible to large earthquakes, and allowed us to show how city officials are attempting to mitigate potential damage as much as possible.
James F. Dolan, Kerry Sieh, and Thomas K. Rockwell, Late Quaternary activity and seismic potential of the Santa Monica fault system, Los Angeles, California, GSA Bulletin; October 2000; v. 112; no. 10; p. 1559–1581.