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After a magnitude 4.3 earthquake rattled the East Bay Tuesday afternoon, a magnitude 3.5 aftershock struck. Seismologists also recorded a much smaller 1.1.
Experts say it’s normal for these weak aftershocks to follow a moderate quake, and there’s also a slight chance of an even bigger one.
“There’s about a 1 in 20 chance something larger will follow in the same region in the next week,” says Ole Kaven, a seismologist with the United States Geological Survey. “It’s based on the entire catalog of earthquakes and the relationships between earthquakes and aftershocks. These types of statistics really work for California seismicity.”
A 5 percent chance might sound slim but over the long Fourth of July holiday weekend in Southern California the scenario of a significant quake being followed by an even larger quake occurred.
A magnitude 6.4 temblor rocked the remote high-desert of Southern California near Ridgecrest in Kern County on July 4. The next day a magnitude 7.1 quake gave the region an even bigger jolt. The July 5 event was felt as far north as Sacramento, as far east as Las Vegas and as far south as Mexico.
“Today’s event is a gentle reminder for us locals that we live in earthquake country,” says Kaven. “The Ridgecrest incident was a not-so-gentle reminder. We can and will experience significant magnitude earthquakes in the Bay Area and people need to be ready.”
The magnitude 4.3 quake struck at 1:11 p.m. with a depth of 7.46 miles and an epicenter in the East Bay, about 7.5 miles east of Blackhawk and 17 miles southeast of Concord, according to the United States Geological Survey.
“I felt it,” says Aimee Grove who lives in the East Bay. “But it just felt like a single jolt, as if something had slammed against the side of our house.”
The USGS reports the 4.3 quake occurred on the Greenville Fault, which Kaven says is the eastern most fault in the San Francisco–San Andreas Fault system.
“There has seen quite a bit of historic seismicity in the past 20 years,” Kaven says. “There have about 20 or so magnitude 3 earthquakes in that vicinity, and in 1980 there was a magnitude 5.4 and 5.8 in the Livermore area also attributed to the Greenville Fault.”
He says in coming days seismologists will be “monitoring the Greenville Fault and other faults nearby.”
Amy Graff is a news producer for SFGATE. Email her at email@example.com.
This post was originally posted at https://www.sfgate.com/bayarea/article/1-in-20-chance-even-bigger-earthquake-Bay-Area-SF-14100460.php.