Magnitude 4.8 Quake Rattles the Southland

A magnitude 4.8 earthquake centered three miles northeast of Yorba Linda early Tuesday was the strongest temblor to hit the Orange County-Los Angeles metropolitan area in more than five years, scientists said.

The earthquake struck at 12:08 a.m. and was strong enough to wake some residents in Orange County, the nearby Inland Empire and eastern Los Angeles County, although no major damage or injuries were reported.

Scientists at Caltech and the U.S. Geological Survey said it was the strongest quake in the basin since two Northridge earthquake aftershocks of magnitude 5.1 and 4.9 near Simi Valley on April 26 and 27, 1997.

The scientists upgraded the Yorba Linda quake from their initial assessment of magnitude 4.6 to 4.8 on Tuesday afternoon.

Such recalculations are common as more seismic data are reviewed.

That such a comparatively mild shaker was the strongest in five years demonstrates just how seismically quiet Southern California has been for what in human terms is an extended period but in geological terms is just a flicker of time.

Normally, a 4.8 would be strong enough to cause some minor damage, such as breakage of goods in stores. But the epicenter of Tuesday’s quake was close to the San Bernardino County line in an area not heavily populated.

The quake was not too many miles from the epicenter of the Oct. 1, 1987, Whittier Narrows earthquake, but, according to Caltech seismologists Egill Hauksson and Kate Hutton, it was on a different fault system and was a strike-slip, or horizontal, rather than a thrust quake.

So, the two quakes had no seismic relation.

Tuesday’s quake occurred at a depth of a little more than four miles and was felt from Victorville in the north to Escondido in the south and west into Los Angeles. It was followed by 24 aftershocks in 13 hours, a comparatively large but not unusual number.

Several of the aftershocks were in the high magnitude 2 range--between 2.6 and 2.9--and were felt by nearby residents.

Dave Gruchow, assistant city manager of Yorba Linda, said he was trying to fall asleep when he felt the main shock.

Gruchow said quakes have occurred regularly in the area because of its proximity to the Whittier fault system. He took the main shock in stride, but then was struck by the number of aftershocks and called scientists for information.

At the Richard Nixon Library & Birthplace in Yorba Linda, the quake knocked some pictures askew, but that was the only evidence of the jolt.

Seismologists Hauksson and Hutton said the main shock was preceded by two foreshocks, one a 2.6 at 9:50 p.m. Monday and the other a minuscule 1.3 at 10:23 p.m.

Foreshocks occur, on average, in only one of 20 California earthquakes, according to studies by U.S. Geological Survey scientist Lucy Jones and others.

Hauksson and Hutton said the quakes were near the Whittier fault, which is known as having one of the fastest slip rates in Southern California, at 2 to 3 millimeters, or about a 10th of an inch, per year.

The huge San Andreas fault, however, slips at 10 times that rate.

The two scientists said that Tuesday’s quakes occurred on a small fault adjacent to the Whittier fault.

The last quake greater than magnitude 4 in the basin was a 4.2 near Compton on Oct. 28, 2001.

The Jan. 17, 1994, Northridge quake--magnitude 6.7--was about 90 times more powerful than Tuesday’s.