EL CENTRO, Calif. - More than three dozen earthquakes, including a magnitude-4.3 temblor, rocked southern Imperial County on Wednesday, but there were no reports of injury or damage.
The first quake, with a preliminary magnitude of 4.2, hit at noon about 7 miles northeast of El Centro, said Kate Hutton, a seismologist at the California Institute of Technology in Pasadena.
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.
Four more temblors, all centered about 5 miles south-southeast of Brawley, struck Wednesday night. A magnitude-3.4 quake at 9:08 p.m. was followed in the next 18 minutes by three shakers of magnitudes 3.6, 3.2 and 3.4, respectively, Hutton 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.''
A dispatcher for the Imperial County Sheriff's Department in El Centro said there were no reports of injuries or damage.
The Imperial Valley, between the Arizona liner 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.
''We felt a whole mess of them especially because we're in an old building, said Maricela Belasco, 46, a data entry clerk at El Centro Motors in El Centro. ''Everybody's talking about it.''
Wednesday's shaking was taking place in the Brawley seismic zone, an area that experienced thousands of small quakes in the 1930s and 1970s, said Lucy Jones, chief scientist in charge of the U.S. Geological Survey's Pasadena office.
''This is an area that tends not to go to real big ones, but tends to have lots of them when they happen,'' she said. ''I am not sitting around biting my fingernails worrying about a 6.5 coming through.''
For unknown reasons, the zone has been relatively quiet since 1981 although other parts of the county remained active.
''What had been the primary source of earthquakes in Southern California stopped,'' she said. ''If you looked in the '90s, you could not find the Brawley seismic zone. In the early '80s, there were a ton of papers being written about them.''
The zone is between the mighty San Andreas Fault and the Imperial Fault, which was responsible for a magnitude-6.9 quake in 1940 and a 6.5 quake in 1979.
But with only a few hours of frequent shaking, it is too early to say that the area is entering a prolonged period of shaking.
''I would expect us to continue to have them until they stop happening,'' Jones said. ''I know it's disingenuous, but that's really the state of our knowledge. Nobody's going to be surprised if we get more.''
On the Net: Recent California earthquakes: http://pasadena.wr.usgs.gov/recenteqs/
