Six earthquakes of magnitude 3.0 or higher struck the East Bay on Sunday morning, part of a swarm of dozens of small earthquakes that could go on for hours or even days, according to seismology experts.
Earthquake swarm strikes SF Bay Area, may last awhile
East Bay Tremors Continue: San Ramon Area Rattled by Swarm of Earthquakes
A series of earthquakes has kept residents in California’s East Bay on edge, with a notable swarm of tremors centered around San Ramon. The seismic activity, which began on Sunday morning, included at least six earthquakes registering magnitude 3.0 or higher, part of a larger and ongoing pattern of dozens of smaller seismic events.
Experts are characterizing this as an earthquake “swarm,” a phenomenon not uncommon in this seismically active region. These swarms, which can persist for hours or even days, are a reminder of the dynamic geological forces at play beneath California.
Understanding the Tremors
The largest of the recorded Sunday quakes, a magnitude 3.8, struck near San Ramon at approximately 9:38 a.m., according to the U.S. Geological Survey (USGS). Residents in nearby communities such as Danville, Dublin, Emeryville, and San Ramon reported feeling the ground shake, with descriptions ranging from “jolts” to a “short rattle.”
“It literally felt like the whole house just shook,” recounted Amelia Boaz, a resident of Danville, who was at home when the tremors hit. Her experience of her dog becoming agitated and pictures rattling on the walls was echoed by others who felt the approximately five-second-long shaking.
State Geologist Jeremy Lancaster, head of the California Geological Survey, and Harold Tobin, a professor at the University of Washington and director of the Pacific Northwest Seismic Network, explained that such swarms are often attributed to the strain caused by interconnected fault lines. They also noted that the movement of fluids within the Earth’s crust can sometimes contribute to these seismic events.
“California is earthquake country, and there is always a very small possibility of a larger, potentially damaging earthquake,” Lancaster stated via email. He advised, “If you feel earthquake shaking or receive an earthquake alert, drop down to the ground, take cover underneath a sturdy object, and hold on until all the shaking has stopped.”
By Sunday midday, according to Tobin, at least 25 earthquakes below magnitude 3.0 had been recorded as part of the swarm, in addition to the larger events. The seismic activity continued into Monday morning, with a 2.6-magnitude earthquake registered around 10 a.m.
The Calaveras Fault: A Major Player
The recent earthquakes are occurring on the northern segment of the Calaveras Fault, a significant branch of the San Andreas Fault system. This fault extends approximately 90 miles from San Ramon southward to about 20 miles south of Hollister in San Benito County. Geologists consider both the San Andreas and Hayward faults to be primary sources of significant seismic hazard in the region, with the Calaveras Fault having the long-term potential to produce a magnitude 6.5 to 7.0 earthquake.
While earthquake swarms have been observed in the San Ramon area in past decades, the current sequence has prompted varied reactions from residents. Edison and Vanjie Mendoza, attending Mass at St. Isidore church in Danville, described the jolt as a “shake of joy” during the homily. Originally from the Philippines, they are accustomed to seismic activity and found this particular tremor to feel like “bouncing,” not causing significant concern.
“That kind of earthquake, I don’t think it will have another aftershock. It’s just a split second. We felt it, but it’s not that strong,” Vanjie Mendoza commented later that afternoon.
For others, the experience was more unsettling. James Young, preparing for his day at home in Dublin, felt a series of quick shakes he described as a “continuous wiggle.” He expressed concern about potential damage, noting, “I was worried that the gas lines were going to break or something. It felt stronger than what they said it was, to be honest.” His lingering apprehension was palpable as he observed families enjoying a pleasant afternoon in San Ramon’s Central Park. “Every time I feel a small one, I think, ‘Is that going to make a big one later?’ It’s a little bit unnerving to be on top of the fault line.”
A group of families at San Ramon’s Central Park shared their experiences. Anant Agarwal recounted his 7-year-old daughter’s distress at their Dublin home, while another man mentioned their Christmas tree shaking. In contrast, Geetanjali Gupta found her children were excited by the tremor, which she felt was “fairly strong” due to the epicenter’s proximity. “They were jumping,” she said, and eagerly asked, “‘Mom, did you feel it?'”
A Broader Seismic Context
The current swarm follows a relatively active autumn in Berkeley, which experienced several small quakes in September and October near the UC campus. This heightened seismic activity underscores the ongoing geological processes shaping the region.
It’s important to note that while earthquake swarms themselves are not typically cause for alarm, scientists universally acknowledge that any earthquake, regardless of magnitude, carries a small possibility of being followed by a larger, more damaging event within approximately a week. This underscores the importance of maintaining preparedness for seismic events.
The USGS has reported over 200 seismic events in the San Ramon area in the span of a week at one point, highlighting the sustained nature of this swarm. As California remains a globally recognized hub for seismic research, data from these events will contribute to a deeper understanding of fault behavior and improved earthquake early warning systems.
A magnitude 2.8 earthquake was also registered on Sunday morning near Patterson, approximately 50 miles southeast in Stanislaus County, indicating broader seismic activity in the region.
ARTICOL ORIGINAL:
Vanjie and Edison Mendoza said they felt one of Sunday morning’s East Bay earthquakes while they were attending church in Danville.
The largest so far was a 3.8 magnitude quake that struck around San Ramon at 9:38 a.m., according to the U.S. Geological Survey.
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Residents of Danville, Dublin, Emeryville and San Ramon reported feeling jolts and a short rattle.
Amelia Boaz was in the middle of a conversation at her grandmother’s one-story house in Danville when the pictures on the walls suddenly started shaking. Her dog darted around the house. The quake lasted about five seconds, she said, and “literally felt like the whole house just shook.”
Boaz, a San Ramon resident who was heading into work at Salt & Straw on Sunday afternoon, said she’s used to earthquakes as a frequent feature of life in California.
“But I’d never felt one that heavily before,” she said.
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The San Ramon area occasionally experiences Swarms of earthquakes, seismology experts say.
The swarm could go on for hours or even days, according to State Geologist Jeremy Lancaster, the head of the California Geological Survey, and Harold Tobin, a University of Washington professor who directs the Pacific Northwest Seismic Network.
At least 25 earthquakes of less than 3.0 magnitude occurred as part of the swarm by around 11 a.m. Sunday, in addition to the larger ones, Tobin said. The small quakes were still going as of Monday morning, with a 2.6-magnitude earthquake recorded around 10 a.m.
The earthquakes have occurred on the northern end of the Calaveras Fault, Tobin said, adding that it has the long-term potential for producing a quake between 6.5 and 7.0 magnitude. The fault is a major branch of the San Andreas Fault system and extends about 90 miles from San Ramon to about 20 miles south of Hollister in San Benito County. The San Andreas and Hayward faults present higher overall hazard, Tobin said.
Swarms of earthquakes sometimes occur in the area, Tobin said, citing a few swarms in the past decade. They are attributed to the strain in the area caused by several fault lines linking up, he said, and may also be caused by fluid in Earth’s crust moving around. There is a small chance of a larger earthquake to follow over the time frame of about a week, but Tobin emphasized that this was not something to be alarmed about.
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This is fairly standard across earthquake science: After each small earthquake, scientists say there is a tiny chance of a larger quake following over the next week.
A swarm of small earthquakes that struck the San Ramon area Sunday could go on for days, seismology experts say.
One earthquake struck while Edison and Vanjie Mendoza were listening to the homily during Mass at St. Isidore in Danville, sending a jolt through the pews — and even up to the altar.
“The priest announced this is a ‘shake of joy,’” Vanjie Mendoza said with a chuckle.
Originally from the Philippines, the Mendozas said they are familiar with earthquakes. This one felt like “bouncing,” they said. But by early afternoon, the couple were not too concerned about aftershocks while enjoying the pleasant afternoon at a shady park table in San Ramon.
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“That kind of earthquake, I don’t think it will have another aftershock. It’s just a split second. We felt it, but it’s not that strong,” Vanjie Mendoza said.
James Young was getting ready for the day in the bathroom at his home in Dublin when he felt a sequence of shakes in quick succession. It felt “like a continuous wiggle,” he said.
“I was worried that the gas lines were going to break or something,” he said. “It felt stronger than what they said it was, to be honest.”
By midday, Young was having coffee outside at the City Center Bishop Ranch in San Ramon, where families picnicked and children ran through a small fountain. But the morning earthquake wasn’t far from his mind.
“Every time I feel a small one, I think, ‘Is that going to make a big one later? ’” Young said. “It’s a little bit unnerving to be on top of the fault line.”
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From left: Dublin residents Anant Agarwal, Geetanjali Gupta, Neha Agarwal and Pooja Dubey recall the morning’s jolts while visiting San Ramon Central Park.
A group of families gathered at Central Park in San Ramon on Sunday afternoon expressed a range of feelings about the morning’s earthquake. Anant Agarwal said his 7-year-old daughter started crying when the quake shook their Dublin home. Another man said the family’s Christmas tree shook and his youngest child crawled under the table. But Geetanjali Gupta said her children were excited by the shaking, which she said felt “fairly strong” because of the proximity of the epicenter.
“They were jumping,” Gupta said, and asking, “‘Mom, did you feel it?’”
Lancaster, of the California Geological Survey, noted by email that “California is earthquake country and there is always a very small possibility of a larger, potentially damaging earthquake. If you feel earthquake shaking or receive an earthquake alert, drop down to the ground, take cover underneath a sturdy object, and hold on until all the shaking has stopped.”
The quake swarm comes after an active fall in Berkeley, which was rattled in September and October by a few small quakes centered in the vicinity of the UC campus.
A 2.8 magnitude earthquake also struck Sunday morning near Patterson, which lies roughly 50 miles to the southeast in Stanislaus County.