By Ross Stein, Ph.D., Temblor
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 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.
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.
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.