Low-level quake swarm flutters through East Bay
By David Perlman
Published Wednesday, October 14, 2015
A swarm of small earthquakes shook the ground Wednesday near San Ramon along the well-known Calaveras Fault, where scientists have recently found links to the more dangerous Hayward Fault in nearby Alameda County.
The quakes began Tuesday night and were continuing throughout the day Wednesday. By midafternoon, the total had reached 30, the U.S. Geological Survey reported. The largest registered a magnitude of only 3.0. Few people in the area felt the temblors, and there were no reports of damage in the heavily populated region of Contra Costa County.
Brad Aagaard, a research geophysicist at the survey's Earthquake Hazards Program in Menlo Park, said swarms of similar magnitude are not uncommon on the Calaveras because the fault is marked by many "structures," or smaller fault branches, that have formed along the two sides of the main fault.
"We tend to see many swarms of small quakes there," he said, "and there is also a significant amount of surface creep in the area."
That same fault saw a swarm of 120 small quakes in the region in 2003. In that series, the largest one registered a magnitude of 4.2. And in 1990, a swarm of 350 small temblors lasted 42 days, with the largest a magnitude 4.4, Aagard said.
Scientists studying traces of the Calaveras and the Hayward Fault have found evidence that the faults are probably linked at depth, which raises the probability that a larger quake could someday strike on either fault.
"Whenever we see swarms like Wednesday's, it does raise the probability of a larger one, but that probability remains very low," Aagaard said. The most recent official estimate of the likelihood that a major quake with a magnitude of 6.7 or larger could strike along the Calaveras in the next 30 years is about 7.4 percent — or 7.4 chances in 100.
The Calaveras Fault is about 76 miles long, and while the surface evidence is somewhat obscure — "there are a lot of hills in the way," as Aagard put it — it appears to run from the Alamo and Danville area on the north to where it joins the much larger San Andreas Fault near Hollister in San Benito County.
People living along the Calaveras Fault have experienced two significant earthquakes there in recent decades: A magnitude 5.6 struck in San Jose's Alum Rock Park on Oct. 30, 2007, and the much larger magnitude 6.2 Morgan Hill quake caused more than $8 million damage around that Santa Clara County town on April 24, 1984.
By chance, Wednesday's small quakes occurred on the same day that all 600 employees at the Geological Survey's headquarters in Menlo Park observed their "Great Shakeout" with a "Drop, Cover and Hold On" drill simulating a truly major quake. All of the survey's buildings were evacuated, and simulated injuries tested employee readiness at an improvised triage center.
It went "very well," an observer reported.http://www.sfgate.com/bayarea/article/Low-level-quake-swarm-flutters-through-East-Bay-6571290.php
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