Two mild earthquakes rumbled through Southern California Saturday, one centered near Pasadena and measuring 3.2 on the Richter scale and a 3.9 temblor that shook the Palm Springs area. There were no reports of injuries or damage in either quake.
1988 is still in its infancy but already about 100 earthquakes, of the 11,000 estimated to hit yearly, have jostled Southern California since 12:01 a.m. on Jan. 1.
Residents of the San Gabriel Valley were shaken about 3:52 a.m. by a temblor centered six miles south-southeast of Pasadena. The nearly 1 million people who crowded Pasadena streets for the Rose Parade the day before just missed out on another Southern California tradition. "It would have shaken up all those visitors," said Steve Bryant, seismic analyst at the Caltech's Seismological Laboratory.
Few Phone Calls
Caltech seismologists reported only about a dozen calls after the temblor, Bryant said.
Some hours later, at 11:40 a.m. Saturday, a quake of 3.9 magnitude rattled the Yucca Valley area, about 25 miles north-northeast of Palm Springs, said Caltech spokesman Robert Finn. San Bernardino County Sheriff's Department officials said they only received a few calls about the quake.
The San Gabriel Valley quake was the most recent of about 500 aftershocks which have followed the 5.9 temblor on Oct. 1 centered near Whittier, Bryant said. But it was the first one measuring over 3.0 on the Richter scale since November. About 35 aftershocks measuring 3.0 or more have occurred since the Oct. 1 quake, which killed three people and caused more than $200 million in damage.
Aftershocks can still be felt for more than a year after a quake has struck, although most occur in the first few months after a quake, Bryant said. As time goes on, "the majority of the aftershocks are very small," he said.
The number of aftershocks after the Whittier temblor has been relatively low compared to other earthquakes such as the 6.2 and 6.5 quakes in the Imperial Valley on Nov. 23 and 24, the seismic analyst said.
"The Oct. 1 quake died out fairly quickly," Bryant said. "There were on the order of a few hundred immediately following in October."
Experts are not certain what geological features determine the number of aftershocks. "There's not a single factor that seems to indicate why you have large number of aftershocks or a small number," he said.
Saturday's 3:52 a.m. aftershock, though not the first temblor to jolt the area this year, was the first significant quake of 1988. The first quake struck Jan. 1 at 12:33 a.m. in the Big Bear area. That quake was recorded but has not yet been assigned an exact Richter scale measurement.
"There isn't a day in Southern California that we don't have some earthquakes," Bryant said. "In fact, I don't think we've ever had a day with just one. The least I've seen on a day was about 12."