Dozens of Earthquakes Rattle South Imperial County

More than three dozen earthquakes, including one of magnitude 4.3, rocked southern Imperial County on Wednesday afternoon, but there were no reports of injury or damage.

During the past week, the area around Fillmore in Ventura County also has been hit by a swarm of small quakes, all less than magnitude 3.0--too small to be felt by most people or to cause damage. Seismologists said the rash of temblors is unlikely to trigger a larger shock.

The Imperial County quakes began with one that has been assigned a preliminary magnitude of 4.2 and that hit at noon about seven miles northeast of El Centro, said Kate Hutton, a seismologist at Caltech.

Within three hours it was followed by more than three dozen aftershocks, including 14 above magnitude 3.0. The strongest quake in the series, with a preliminary magnitude of 4.3, struck at 2:49 p.m., seismologists said.

“It’s been quaking all day. We’ve been rattling and moving,” said Oscar Arteaga, 31, an office manager at the Desert Eye Institute in El Centro. “But we’re used to them, because we get them all the time.”

The Imperial Valley, between the Arizona border and San Diego County, is California’s most active seismic region, with quakes produced by the same forces that slowly split Baja California from mainland Mexico to create the Gulf of California.

The Ventura County swarm of quakes hit the vicinity of the Oak Ridge fault system, which runs more than 50 miles from Piru to southeast of Ventura. Some seismologists believe that the deadly 1997 Northridge earthquake was triggered on the system’s southern end.

Series of similarly sized quakes rarely lead to anything bigger, said Lucy Jones, scientist in charge of the U.S. Geological Survey’s Pasadena office, .

“Once you have a swarm going, you’re more likely to trigger others,” she said. “But you’re most likely to trigger it immediately and within the magnitude [of previous quakes]. . . . If anything, it shows you have lots of little ones so you don’t get the big one.”

Fillmore was Ventura County’s hardest-hit area in the Northridge quake. Officials reported 40 minor injuries and $200 million in damage, including the collapse of the landmark Fillmore Hotel.

Another series of quakes is rattling the Obsidian Butte area southeast of the Salton Sea. That swarm has produced three quakes of magnitude 3.0 and higher since June 6 and has been occurring for 11 years, Jones said.