Magnitude 4.6 Aftershock Jolts Southern California

A magnitude 4.6 earthquake that struck just west of Upland at 3:32 p.m. Tuesday was the most recent of hundreds of aftershocks that have startled Southern Californians after the Feb. 28 Upland earthquake.

The afternoon jolt was preceded by two foreshocks Tuesday, a magnitude 3.3 temblor at 7:12 a.m. and a magnitude 2.6 at 7:19 a.m.

Those, in turn, had been preceded by a magnitude 3.4 shock about three miles farther northwest at 1:47 a.m. Tuesday.

Geologist Lucile Jones of the U.S. Geological Survey in Pasadena said the number of aftershocks from the 5.5-magnitude Feb. 28 earthquake was “in the normal range, but higher than average.”

The greater-than-average aftershock activity in the Upland area does not necessarily indicate that Tuesday afternoon’s quake is the precursor of a larger quake, Jones said. In California, she noted, there is a one-in-20 chance that any earthquake is the foreshock of a larger shock, and that probability holds for this one as well.

Although the afternoon shock startled residents of Upland and Pomona, which suffered the most damage in the Feb. 28 earthquake, there were no reports of significant damage or injuries. Officials surveyed buildings for damage, but none was found except for a few bricks that tumbled from a chimney in the city’s “old district,” Pomona Fire Inspector Ron Gomez said. “There wasn’t any damage other than nerves,” he said.

Upland Assistant City Manager Mike Matlock said he was sitting at his desk in City Hall when the 4.6 aftershock hit.

“It was strange,” he said. “The floor felt like it sank three inches and came back up. It was a hard jolt but not a very long one.”

On Upland’s Main Street, anxious shopkeepers and customers ran out of storefronts.

The 4.6 quake was also felt from Brea to Huntington Beach, but Orange County emergency officials said there were no reports of any significant damage. The temblor was felt most strongly in North County, where police agencies in Fullerton, Brea and La Habra reported dozens of calls from concerned residents.

“It was a strong roll, but we’ve had so many lately, nobody seemed to panic,” said Sally Williams, a shopper in a Placentia supermarket. “A few boxes of laundry soap fell off the shelf. Nothing serious.”