Countywide

An aftershock from the strong July 13 offshore earthquake shook parts of San Diego and Orange counties early Tuesday, but police agencies received few phone calls and no damage was reported, officials said. The quake measured 4.2 on the Richter scale, according to the seismic lab at Caltech in Pasadena.

The 2:52 a.m. quake was centered on the same fault line 28 miles southwest of Oceanside where a quake of magnitude 5.3 took place July 13. That quake was the largest ever recorded off San Diego County and resulted in one injury and an estimated $500,000 in minor damages. Each numerical increase on the Richter scale (e.g., from 4 to 5) represents a tenfold increase in energy released.

A quake measuring 4.0 is capable of causing some moderate damage depending on the location of its epicenter. Caltech officials say that aftershocks from the July 13 quake could continue for months. Numerous small quakes have been reported during the 2 1/2 months since the large quake.