5.3 Quake Rocks S.D. and Baja : Seismology: No damage or injuries were reported from the temblor centered east-southeast of Ensenada.

A mild earthquake, centered 51 miles east-southeast of Ensenada, gently rocked San Diego and the Baja region Tuesday morning, but no damage or injuries were reported.

The earthquake, which registered 5.3 on the Richter scale, shook the San Diego region and sparsely populated parts of Mexico at 9:54 a.m., said Bruce Presgrave, a geophysicist with the U.S. Geological Survey National Earthquake Information Center in Boulder, Colo.

At 9:59 a.m., a mild aftershock measuring 2.9 nudged the region. The quake erupted from the San Miguel Fault in Sierra Juarez, a continuation of the mountain range just east of San Diego, said Robert Finn, a spokesman at the California Institute of Technology in Pasadena.

Later in the evening at 11:10 p.m., an earthquake registering 4.4 on the Richter scale and centered 30 miles east of Escondido shook a large area from Riverside to the Mexican border, state officials said. There were no immediate reports of damage nor injury from the moderate shake.

Quakes of 5.3 magnitude are not unusual, shaking Southern California about once a year, said Mark Legg, an earth scientist with Acta Inc., an engineering consulting firm in the Los Angeles area.

“An earthquake of a 5 or greater magnitude can be damaging if it happens in a populated area. In downtown L.A., it would cause several million dollars of damage,” Legg said. “But this (epicenter) was a sparsely populated area,” he said, referring to the morning quake.

At first, however, the big question was whether the quake was a precursor--or a foreshock--to a much larger earthquake. But such a quake is not likely to occur, experts said. The possibility becomes more remote as time passes, said Lucy Jones, a seismologist with the U.S. Geological Survey.

In fact, only 6% of quakes in Southern California and Baja are foreshocks to larger quakes, Jones said. Of those, about a quarter happen within an hour of the initial quake. The odds rapidly drop off after that first hour, Jones said.

“The probability decays with time,” Jones said. “By the time you are half a day away, the risk is greatly diminished.”

Not everyone in San Diego felt the Tuesday morning quake.

“If you are used to monitoring earthquakes of a 7 or 8 magnitude, this was nothing,” Legg said. “It all depends on your perspective.”

Some, however, experienced a gentle rocking. At the county Board of Supervisors’ meeting, the quake occurred just in time to quell a minor dispute between Supervisors George Bailey and Brian Bilbray.

As the room rocked, Supervisor John MacDonald quipped, “Well, your opinions are earth-moving.”