Aftershock in Upland Measures 4.6 : Earthquake: No injuries or major damage are reported in yet another jolt stemming from the Feb. 28 temblor.

A magnitude 4.6 earthquake that struck just west of Upland at 3:32 p.m. Tuesday was the most recent of literally 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 quake was felt as far away as Pasadena and the Los Angeles Civic Center. It was followed within the next few hours by three more aftershocks, all under magnitude 3, according to Caltech.

Jones said the Tuesday afternoon quake appeared to be on the San Antonio Canyon Fault, a northeast-southwest trending fault that passes just north of Upland. Tuesday’s shocks were in an area of the fault that was not ruptured in the February quake.

“We ruptured one patch of the fault in February,” she said, “and now we’ve extended it into the neighboring patch by a kilometer or two (about 0.6 to 1.2 miles).”

The shocks did not come as a surprise to geologists, who said shortly after the February quake that the number of aftershocks was higher than average and that they were dying out slowly.

Jones said the three early quakes Tuesday prompted her to calculate the probability of larger shocks.

“About an hour before the quake, I calculated that there was a 50-50 chance of a magnitude 4 within the next year,” she said. “I guess I was right.”