A jolt-and-roll felt around the Bay Area on Friday afternoon jangled plenty of nerves, but scientists say the 4.1-magnitude quake, followed by several smaller ones, was routine in an area notorious for seismic restlessness.
The quake struck at 4:10pm local time with an epicenter placed 10 miles east of Seven Trees or 13 miles east-southeast of the San Jose City Hall. The U.S. Geological Survey (USGS) said the quake originated 4.4 miles beneath the surface and was followed by small aftershocks of magnitude 2.6, 2.1 and 1.1.
Reports from the USGS said the shaking was felt across the South Bay, through San Francisco and as far north as Santa Rosa. Residents to the west in Santa Cruz also reported feeling the quake.
Its epicenter was only 1.5 miles from the center of the infamous 1984 temblor in Morgan Hill, which caused damage totaling more than $10 million. But U.S. Geological Survey scientists do not think it is a precursor to that large quake. They predict a recurrence at that site in another 30 years.
Check Other Live Content News and Tips
"It's not unusual for the Calaveras to have magnitude-4 quakes, which may knock things off shelves but are unlikely to cause any structural damage," said USGS seismologist David Oppenheimer, who is based in Menlo Park.
Around the South Bay and elsewhere, the earthquake jolted more than a few residents, although no one reported any damage.
The quake appears to have occurred on the Calaveras Fault, a major branch of the better known San Andreas Fault. The fault has generated sizable quakes in the past including a magnitude 6.5 in 1911 and a 6.2 in 1984, both of which were centered in the Morgan Hill area. More recently a magnitude 5.6 quake occurred on October 30, 2007 near Alum Rock.
Earthquakes in the San Francisco Bay Region result from strain energy constantly accumulating across the region because of the northwestward motion of the Pacific Plate relative to the North American Plate. The region experienced large and destructive earthquakes in 1838, 1868, 1906, and 1989, and future large earthquakes to relieve this continually accumulating strain are a certainty.
To evaluate the probability of future large earthquakes in the San Francisco Bay Region, the U.S. Geological Survey has established a series of Working Groups on California Earthquake Probabilities (hereafter referred to as WG88, WG90, WG99). Each of these Working Groups has expanded on the work of its predecessors, applying, in turn, the data and methodology available at the time and drawing on input from broad cross-sections of the earth science community.
More on Live Content News and Tips
