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A 5.2 earthquake rocked the Sea of Cortez Sunday night, about 61 miles east of San Felipe.
The temblor struck at 10:26 p.m. and was preceded by a rash of moderate quakes that shook the south end of the Salton Sea Sunday, and the largest quakes were felt across Southern California to Arizona.
"What we're seeing is a classic Brawley seismic swarm," USGS seismologist Lucy Jones told City News Service. "We haven't seen one of these since the 1970s, and there was another one back in the 1930s."
Some buildings were evacuated in Brawley on Sunday, a small farm town 115 miles east-northeast of San Diego. "It's pretty bad, we had to evacuate the hotel just for safety," said Rowena Rapoza, office manager at the Best Western Hotel there.
Jones said USGS seismographs and analysis computers were overwhelmed by the rash of rattling that began at sunrise, and reached a crescendo with a magnitude 5.5 quake shortly before 2 p.m. It was followed by more shakers within minutes.
"Our system is choking on so many earthquakes," Jones said. "This area of California is deep soils, and we do not get as precise data as we do over the rest of the state, and that makes our data a little less precise."
Preliminary computerized USGS reports had indicated that three quakes larger than magnitude 5.3 had rattled out from Brawley at 12:30. That was later resolved by seismologists to be two quakes, magnitude 5.3 and 4.9, Jones told CNS.
The quakes were strongly felt at Borrego Springs, in San Diego County about 25 miles west of the epicenter. "We've felt shaking for sure, but electricity has not gone out," said Gwenn Marie, owner of the Borrego Valley Inn.
The quakes were felt over all of San Diego, Riverside and Imperial counties, and in Yuma and La Paz counties in Arizona, according to a USGS registry.
However, there was no damage reported in Riverside County, according to Melody Hendrickson, spokeswoman for the Riverside County Fire Department. A dispatcher at the Riverside County Fire Department's Office of Emergency Services also reported no damage across the county.
Southern California Edison declared an "unusual event" as the quakes were felt in the control room of the San Onofre Nuclear Generating Station's twin reactors. Both of those nuclear piles have been in secure shutdown mode all summer, while federal and state officials work with the utility to discover why a recent upgrade and expansion caused parts of the boilers to shake violently.
SC Edison said the notification was a standard step requited by the federal government.
The ground about 5 miles north-northwest of Brawley began to spasm at sunrise, and Brawley was rocked by a magnitude 3.9 quake at 10:02 a.m., followed by a 3.4 quake about 90 seconds later.
In the three hours after the first earthquakes, an additional 11 quakes struck the same approximate epicenter near the Salton Sea. Quakes with magnitudes of 4.0, 4.0. 4.6 and 4.7 reportedly also hit during the noon hour.
The apparent quake cluster was centered 3 miles north-northwest of Brawley, 16 miles north of El Centro and about 115 miles east-northeast of San Diego. Some of the quakes may have been just east of Brawley.
Jones said the quake swarm was about midway between fault complex on the west side of the Imperial Valley, and the main branch of the San Andreas Fault, which runs from near Palm Springs to enter Mexico just west of Yuma.
"These don't seem to be related to earthquakes on the San Andreas itself, other than in a general way," she said. "It's pretty far away."
Jones says she expected the quake swarm "to continue to bubble along, they're going to get a bunch of 4s and 5s."
By midafternoon, the automated USGS lists were lengthy with reports of dozens of quakes echoing out from within 8 miles of Brawley. Jones has emphasized that many of the reported quakes are duplicates or have unreliable magnitudes, given the nature of the geology of the Brawley area. --Toni McAllister contributed to this report.