Southern extension of San Andreas Fault lights up in a seismic swarm

By David Jacobson, Volkan Sevilgen, Ross Stein Temblor

Check your seismic hazard rank

Over the past several days, a swarm of earthquakes has been rumbling the Brawley Seismic Zone, which is the southernmost extension of the San Andreas Fault. As of 11 a.m. on Tuesday January 3, there had been 80 M=2+ earthquakes in three days. Most of these were small, though the largest, a M=3.9, generated strong shaking according to the USGS, and was felt throughout the town.

Regional map of the Brawley Seismic Zone. The active swarm is located south of the San Andreas Fault, and just north of the Imperial Fault.


A seismic swarm is a series of earthquakes, none of which can be identified as the main shock. What this means is that there is no clear decay rate in the number of earthquakes. Swarms are more common in volcanic areas and geothermal sites, both of which are suffused with steam. Swarms also occur along creeping fault sections such as on the San Andreas between Hollister and Parkfield. In general, we think swarms accompany rapid fault creep. Take a look at the time series plot below.

Time series of the active Brawley seismic swarm. From this graph, it is seen that there was a small lead-up, with no clear mainshock.


Geothermal favorability of Southern California (DeAngelo and Williams, 2010), and the Brawley Seismic Zone. Warmer colors correspond to higher favorability, and stars represent active geothermal sites. This current swarm is located in an area of very high geothermal favorability, where the crust is very thin and under geothermal production.


Earthquakes in this portion of California are extremely common as the area, known as the Brawley Seismic Zone, sits between the San Andreas, and Imperial faults. In fact, the Temblor Seismic Hazard Rank is 99 out of 100. The area last saw a large swarm in 2012, when more than 300 quakes shook the region. In 2012, the largest earthquake reached M=5.5. Additionally, in both 1940 M=6.9 and 1979 M=6.5 Imperial Valley Fault earthquakes came very close to rupturing this portion of the seismic zone.

Accurate relocation (by the so-called ‘double-difference’ method) by Egill Hauksson of Caltech reveals that most of the recent swarms in the Brawley Seismic Zone are left-lateral, perpendicular or ‘conjugate’ to the right-lateral character of the San Andreas to the north, and the Imperial Fault to the south.


Relocated Brawley Seismic Zone seismicity, and the associated quake activity with time, from Barbour et al (2016). The remarkable dominance of left-lateral, northeast-striking streaks is clear.


Readers may recognize this area, as it is approximately 45 km southeast of where the September 2016 Bombay Beach Swarm took place, which triggered a one-week earthquake advisory. That alert was issued since it was determined that there was an increased probability of a M=7+ earthquake along the San Andreas Fault.

For this swarm, no earthquake advisory has been issued, perhaps because it is far from Bombay Beach. Nevertheless, if anything should change, we will update you.


Southern California Seismic Network (SCSN)
Southern California Seismic Network Bulletin on 2012 Brawley Swarm Click here for bulletin
Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. Click here for paper
Andrew J. Barbour, Eileen L. Evans, Stephen H. Hickman, Mariana Eneva, Subsidence rates at the southern Salton Sea consistent with reservoir depletion, J. Geophys. Res., 121, 5308-5327, doi: 10.1002/2016JB012903 Click here for paper
DeAngelo, J., and Williams, C., Identified Moderate and High Temperature Geothermal Systems of the Western United States including AK and HI, U.S. Geological Survey, Downloadable GIS Data
The Oklahoman