Sharp 5.5 Aftershock Jolts Desert a Week After Quake : Temblor: Nerves rattled in Yucca Valley and throughout Southern California, but no damage is reported.

The most active period of earthquakes recorded in California left another calling card Sunday--a magnitude 5.5 aftershock that rattled nerves in the San Bernardino County community of Yucca Valley but caused no reported damage.

The aftershock of the June 28 Landers quake struck at 2:18 p.m. and was felt throughout Southern California. The epicenter was about 35 miles northeast of Yucca Valley. The strong aftershock was a natural consequence of the 7.4 Landers quake, said Jim Mori, a seismologist with the U.S. Geological Survey in Pasadena.

Instruments have recorded nine aftershocks of magnitude 5.0 or greater in the last week, Mori said. He added that quakes of that magnitude can cause substantial damage if centered under a densely populated area. Geologists have recorded about 90 aftershocks of 4.0 or greater since the Landers quake and a 6.5-magnitude quake that struck the Big Bear area about three hours later.

“The surprising thing is the large extent we’re seeing of related earthquake activity,” Mori said. “We don’t exactly understand why this is, but it seems as if the (June 28) earthquake has triggered a lot of earthquake activity in California.”

He said seismologists had not experienced a magnitude 7.4 temblor in Southern California, “so we don’t have anything to compare it to.”

After the latest temblor struck, the Yucca Valley station of the San Bernardino County Sheriff’s Department received calls from rattled residents. “They’d ask: ‘Was that another one?’ ” a sheriff’s dispatcher said. “ ‘Yeah, it was another one,’ I’d say.”

Authorities reported some jangled nerves, but no damage or injuries. Damage from the earlier quakes is estimated at more than $92 million.

“It was kind of a short jolt,” said a medical worker at the Hi-Desert Medical Center in Joshua Tree. “We’ve just learned not to put anything on the walls.”

Mori said residents could expect a few more strong aftershocks before the cycle of quake activity declines. The latest aftershock did not affect the risk of a powerful earthquake along the San Andreas Fault, but the overall increase in seismic activity does have experts concerned, the seismologist said.

“That fault had the potential of a large earthquake any day, even before the Landers earthquake,” Mori said. “With this additional activity there is a slightly heightened concern.

“There’s always a possibility, so you should always be ready.”