Bay Area hit by over 300 earthquakes since November
Since November, more than 300 earthquakes have rattled San Ramon and the surrounding Bay Area in California.
The largest quakes in this series occurred recently, with a magnitude 4.0 on Friday and 3.9 on Saturday, News.Az reports, SFGATE.
Despite public concern, experts emphasize that small earthquakes have occurred in the region before without triggering a major event. “This is a lot of shaking for the people in the San Ramon area to deal with,” said Sarah Minson, a research geophysicist at the U.S. Geological Survey’s Earthquake Science Center in Moffett Field, California.
“It’s quite understandable that this can be incredibly scary and emotionally impactful, even if it’s not likely to be physically damaging or related to any sort of threat of a larger magnitude earthquake.”
This is at least the sixth time a series of minor quakes, known as “swarms,” has struck the area since 1970, including a previous sequence in 2015. Swarms involve many earthquakes in a small region and do not follow the typical aftershock pattern.
The current swarm began on November 9 with a magnitude 3.8 quake, followed mainly by minor tremors, with only 71 of approximately 330 quakes exceeding magnitude 2. By comparison, the 2015 swarm included 89 quakes over magnitude 2 in a two-month period. Residents, however, report that the ongoing shaking remains unsettling.
Unlike areas with geothermal activity, the quakes in San Ramon are likely caused by pressurized fluids moving through Earth’s crust and activating a network of small faults. Minson explained that the convergence of multiple faults in the region, including the nearby Calaveras Fault, has created fluid-filled cracks in the rock, which trigger these minor quakes.
“The process that is making all these little earthquakes is related to very small, complex faults,” Minson said. “These are not the kind of faults that host large-magnitude earthquakes. It’s fundamentally a different process.”
Seismologists would be concerned only if the swarm shifted to a new area, but currently that appears unlikely. “These very small earthquakes are localized. They are only whispering just the littlest bit in the direction of major faults, but they probably have no way of jumping onto one, since they’re in these small, fluid-filled cracks,” Minson said.
However, the Bay Area still faces the risk of a major earthquake. A 2014 study estimated a 72% chance of at least one magnitude 6.7 or larger quake hitting somewhere in the region before 2043. The Hayward Fault, which stretches about 75 miles from San Jose to San Pablo Bay, is considered the most likely to produce a major event. It runs under or near cities including Fremont, Oakland, Berkeley, and Richmond. With the last major quake occurring in 1868, the fault is considered due.
The Calaveras Fault, intersecting the Hayward Fault, is the second-most likely source of a large earthquake. Running from Danville to San Benito County, it passes through or near Dublin, Pleasanton, Sunol, and Morgan Hill. The last magnitude-6+ quake on this fault occurred in 1984.
Experts urge residents to prepare for earthquakes. The USGS provides guidance for assembling emergency kits, and the California Department of Public Health offers tips for general emergency preparedness. Californians can also receive earthquake alerts through tools such as the ShakeAlert app.
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