For the second time in 12 hours, a strong earthquake, this one measuring 6.3 on the Richter scale, jolted the Imperial Valley, injuring three people and doing structural damage in the border town of Calexico today.
The 5:16 a.m. quake was felt as a rolling motion in Los Angeles and San Diego and as far east as Phoenix and Las Vegas.
In Los Angeles and Orange counties, some residents reported being awakened, but others slept through the quake. It was felt more strongly in the Palm Springs area.
The epicenter of the quake, like the 6.0 temblor that struck Monday evening and its numerous aftershocks, was in a lightly populated desert area 10 to 14 miles west and southwest of the small community of Westmorland.
Its distance from populated areas kept damage and injuries down compared to the severe damage caused by the Oct. 1 Whittier Narrows earthquake, which was only about half as strong but was centered in heavily populated parts of the San Gabriel Valley just east of Los Angeles.
Three Injuries Reported
Fire officials in Calexico reported that there was one injury there and that a furniture store wall collapsed on three automobiles. In nearby El Centro, a pregnant woman fell down some stairs and broke her ankle, and the Imperial County Sheriff's Department listed a third injury.
Calexico firemen said they could see several fires burning across the border in Mexicali, Mexico.
A Mexicali official said as many as 650 people were evacuated from buildings believed to have suffered structural damage there, including 500 patients at two hospitals. Many of the evacuees were taken into hotels, and others moved in with friends.
Caltech seismologists said that the main shock was on the Superstition Hills Fault northwest of the Imperial Valley and that a four- to seven-inch horizontal displacement in the ground was found on Imler Road, southwest of Westmorland.
Monday night's quake, which had originally been assessed at 6.2 on the Richter scale but was downgraded on further evaluation, was centered about six miles northeast of the epicenter of today's main shock on what the seismologists said was an unknown fault perpendicular to the Superstition Hills Fault.
'Shocks for Weeks' Expected
They called the two-day episode, which also was marked by 5.4 and 5.5 shocks as well as more than 20 quakes above 3.5, the Superstition Hills sequence. Seismologist Kate Hutton said it was a "very rich aftershock sequence" and cautioned, "They'll be feeling shocks for weeks out there."
At the National Earthquake Information Center in Golden, Colo., spokesman Waverly Person said the sequence is part of a series that could last for some time.
The aftershocks were coming so frequently "that we haven't been able to exactly locate them and compute a magnitude," he said. "They're hitting every minute or minute and a half."
Today, there were numerous power and telephone outages and gas line breaks in an area extending from Westmorland through El Centro and Calexico into Mexicali.
Authorities estimated that 65,000 households and business lost power after today's quake, but 95% of the outage was corrected within 20 minutes, a spokesman for the Imperial Valley Water District said.
Cave-Ins Along Canal
State emergency authorities also said there had been some cave-ins along the All-American Canal, which carries irrigation water from the Colorado River into the rich farmlands of the Imperial Valley, although there were no reports that the flow of water was disrupted.
Bulldozers were sent to repair the damage there.
A number of trailers fell off their foundations, and there was substantial window breakage and grocery damage in the valley.
California 98, paralleling the Mexican border, was temporarily closed due to possible structural damage.
Authorities at the border crossing between Mexicali and Calexico took the precaution of ordering the U.S. entry station closed until a damage assessment could be made, but it reopened 30 minutes later.
The authorities said even lighter damage had been done in Monday night's 6.0 quake. It was mostly limited to darkened traffic lights, cracked windows and items knocked off shelves.
At Southern California Edison's nuclear power generators in San Onofre, officials declared an "unusual event" after today's quake and checked for damage, although they said later that none had been found.
The quake was also felt at the Palo Verde nuclear power station 50 miles west of Phoenix, but again no damage was reported.
Gov. George Deukmejian, in New Mexico for a Republican governors' conference, ordered the state Office of Emergency Services to convene a federal-state damage assessment team, which was expected to arrive in the area by the end of the day.
The Imperial Valley has historically been one of the most seismically active areas of the state. There have been magnitude 6 or higher quakes in 1915, 1940 and 1979, with the strongest, a 7.1 quake that killed seven people, occurring in 1940.