Bay area earthquake swarm edges toward the major Calaveras Fault

By Ross Stein, Ph.D., Temblor

Check your hazard rank

Danville, California, with Mount Diablo in the background.
Danville, California, with Mount Diablo in the background.

During the past week, 75 quakes have rattled the beautiful town of Danville, scene of the beloved Robin Williams movie, “Mrs. Doubtfire.” The quakes are occurring at a depth of 5-7 km (3-4 mi) on a heretofore unmapped fault or set of faults, and exhibit a diversity of mechanisms, some similar to the ‘right-lateral’ (whichever side you are on, the other side moves to the right) Calaveras Fault, and others perhaps related to the ‘blind thrust fault’ that has jacked up Mount Diablo. Although most of the quakes in this week’s swarm are 3-5 km (2-3 mi) from the Calaveras Fault, the swarm has been migrating toward the Calaveras over the past few days.

 

The red quakes struck in the past 24 hours, the green quakes over the past week, making the southward migration obvious.
The red quakes struck in the past 24 hours, the green quakes over the past week, making the southward migration obvious.

 

The Calaveras Fault and lost seismograms

 

With a slip rate of about 15 mm/yr (0.6 in/yr) and a length of about 100 km (60 mi), the Calaveras is highly active and certainly capable of a M7+ earthquake. The fault cuts through the towns of Walnut Creek, San Ramon, Dublin, Pleasanton, Sunol, and Hollister.

The largest historical quake on the Calaveras Fault was a M=6.6 event in 1911 (Doser, 2009), but far too little is known about this large event. At that time, most U.S. seismic observatories were run by scientist priests at Jesuit universities. But by the time these professors began to die in mid-century, their lifelong seismogram archives were—inconceivably and unconscionably—thrown out. Only one 1911 seismogram from the U.S. survived, from the University of St. Louis, which was saved by Prof. John Ebel, former director of the Weston Observatory of Boston College.

Robin Williams in “Mrs. Doubtfire,” working under cover to care for his children. The famous restaurant scene, where Mrs. Doubtfire performs the Heimlich maneuver on Pierce Brosnan, was filmed at a Danville restaurant.
Robin Williams in “Mrs. Doubtfire,” working under cover to care for his children. The famous restaurant scene, where Mrs. Doubtfire performs the Heimlich maneuver on Pierce Brosnan, was filmed at a Danville restaurant.

 

Swarms mean creep

 

Swarms likely light up portions of faults that suddenly begin to creep—or slip at a much higher rate than usual. Some 600 quakes struck San Ramon, also near the Calaveras Fault, in October 2015 (Xue et al, 2018). Another swarm struck the central portion of the Calaveras Fault in April 2016 (http://temblor.net/earthquake-insights/calaveras-535/); neither triggered a larger shock. Most faults do not creep at all, but parts of the San Andreas, Hayward and Calaveras all do. Why faults start and stop creeping is a mystery, but most swarms and creep events do not cascade into larger earthquakes.

Nevertheless, should this swarm penetrate the Calaveras Fault, the chances of a larger shock will climb, and the monitoring vigilance will intensify. If you live or work in the East Bay, this is the time to ask yourself if you are quake ready. This means having an emergency kit and plan, securing your contents, retrofitting an older home, and considering insurance.

Sources

 

USGS
California Geological Survey
Doser, Diane I., Kim B. Olsen, Fred F. Pollitz, Ross S. Stein, and Shinji Toda (2009), The 1911 M∼6.6 Calaveras earthquake: Source parameters and the role of static, viscoelastic, and dynamic Coulomb stress changes imparted by the 1906 San Francisco earthquake, Bull.Seismol. Soc. Amer., 99, 1746–1759, doi: 10.1785/0120080305
Lian Xue, Roland Bürgmann , David R. Shelly, Christopher W. Johnson, Taka’aki Taira (2018), Kinematics of the 2015 San Ramon, California earthquake swarm: Implications for fault zone structure and driving mechanisms, in press, Earth and Planetary Science Letters.

  • John Harvey

    Thanks Ross. I always enjoy your insight.

  • Robert H. Sydnor

    This is an excellent and timely insightful report by Dr. Ross Stein and his team at Temblor. The Hayward Fault and the San Andreas Fault are much better known to the general public in the San Francisco Bay Area.

    However, the “sleeper” is the Calaveras Fault. This swarm of small magnitude earthquakes near Danville is of cogent concern. I used to live in Walnut Creek, so this Temblor report is of vital concern to me.

    The Calaveras Fault is legally zoned as an “Active Fault” under the Alquist-Priolo Earthquake Fault Zoning Act. Members of the public that live in the Danville – San Ramon — Lafayette area can readily download the detailed fault zone maps of the Calaveras Fault from the website of the California Geological Survey.

    If you have site-specific questions about the earthquake ground-motion for your particular homesite in the Diablo Valley – Livermore Valley area, then Temblor can readily perform a seismology analysis for you.

    Robert Hadley Sydnor
    Senior Engineering Geologist (retired after 25 years)
    California Geological Survey

  • Ross Stein

    As of Feb 28, the swarm has slowed, but not stopped, and is not currently migrating toward the Calaveras Fault. Seismologist David Shelly of the USGS wrote me that he thinks these and other Calaveras swarms are not caused by creep, but by fluids lubricating small faults and then squirting through cracks as shocks occur, more or less like a slow-mo fluid diffusion process. None of these presumably hot high-pressure fluids reach the surface, so its hard to confirm this hypothesis, but he may be right.

  • Ross Stein

    I have added an in press reference from Xue et al for the 2015 San Ramon swarm.

  • Ross Stein

    Lian Xue, the U.C Berkeley researcher and lead author in the paper we cite, wrote me, “Both slow slip and fluid diffusion are possible mechanisms for the 2015 swarm. Unfortunately, there are no nearby geodetic observations, such as GPS, that can provide evidence for fault creep. From the fast propagation rate and triggering ability of static stress change by a assumed slow slip event, there is a possibility for slow slip event. “