LOS ANGELES, OCT. 1 -- An earthquake measuring 6.1 on the Richter scale struck at 7:42 a.m. near the old Los Angeles suburb of Whittier today and sent violent tremors through most of southern California, crumbling walls, setting fires, smashing windows and knocking out power downtown. At least six people were killed and 100 injured.

The sudden shock on a muggy Thursday morning was the strongest earthquake in the area in more than 16 years and the third largest in the 55 years that records have been kept. It was followed by at least 15 aftershocks in three hours, including three of at least Richter magnitude 4.

Striking only 10 miles from downtown Los Angeles and nine miles from a major earthquake study center at the California Institute of Technology in Pasadena, the quake was expected to provide crucial data on new construction methods used since the last major quake and spur preparations for much more destructive temblors anticipated in the near future.

"This is an alarm clock," said Michael Guerin of the California governor's office of emergency services. "This is the little wake-up call to show that those who have been trying to get you to bolt down your water heaters and not use the phones after the quake are serious. This is serious."

Rush-hour traffic into the city came to a halt on the freeways, some of which remained closed while authorities checked the stability of overpass supports. In downtown Los Angeles, office workers stood in the streets and asked each other what would have happened if the quake had been worse.

In Pasadena, some got out just in time. "It started, and my boss was running out the door and I followed him, then the building just exploded," said Dave Adams, who escaped from his automobile repair shop in an old brick building in Pasadena shortly before it collapsed in a cloud of dust.

Five cars were destroyed and a broken pipe sprayed water high into the humid, 90-degree air, but no one was injured. Officials throughout the region expressed relief that the initial quake hit before many people were at work.

Caltech senior seismologist Kate Hutton said the earthquake was centered on the northern edge of the Whittier Narrows Dam Recreational Area -- latitude 34 degrees, 4 minutes, longitude 118 degrees, 5 minutes -- just north of Whittier and about 10 miles east of central Los Angeles.

U.S. Geological Survey geophysicist Lucile Jones estimated that the quake occurred at the northwest end of the Whittier-Elsinore Fault and began with a break in the fault line about 10 kilometers deep.

Although that area of the fault has had several small temblors, its largest previous earthquake occurred in 1929 and is thought to have measured about Richter magnitude 5. At about magnitude 6, today's quake was 10 times as powerful as the 1929 quake, according to the Richter system of seismic measurement, but only one-hundredth the force of the 8-magnitude quake that scientists expect to hit southern California in the next 30 years.

The 1980 census said 7.5 million people reside in the Los Angeles-Long Beach corridor.

Caltech geophysicist Clarence Allen said today's quake should remind Californians "that with all of us waiting in the next 30 years for the big one, we are going to have quakes of this size every four or five years that can be very damaging . . . . Overall, the accumulated damage from those quakes may turn out to be more than from one big quake on the San Andreas Fault."

Since records were first kept in 1932, only two Los Angeles County temblors have exceeded the magnitude of today's quake: the 6.3 Long Beach earthquake in 1933, which killed 120 people, and the 6.6 San Fernando earthquake in 1971, which killed more than 60 people. It is estimated that the 1906 quake that killed 452 people in San Francisco would have measured 8.3 on the Richter scale.

Although Caltech was only nine miles from the epicenter, it had to rely on initial magnitude estimates from the University of California at Berkeley (magnitude 5.8) and the U.S. Geological Survey's earthquake information center in Golden, Colo., (6.1). The strong jolt threw Caltech's sensitive instruments completely off the scale, delaying their own calculation for hours.

Lupe Exposito, a microbiology student at California State University (Los Angeles) was killed by a 10-by-6-foot concrete slab that fell off a university parking structure as she walked under it on her way to an 8 a.m. class. Her sister and a friend walking with her were unhurt.

A power company employe, Antonio Bernal, of Hemet, was working in a 35-foot-deep trench in the Eaton Canyon area of northeast Pasadena when he died of apparent suffocation after being buried in seven feet of dirt, Pasadena Fire Battalion Chief Duncan Baird said.

Juan Herrerra died of head injuries after the quake threw him through a second-story window of his apartment in suburban Maywood. Officials also attributed three fatal heart attacks to the quake.

Bob Dambacher, spokesman for the Los Angeles County coroner's office, said Robert Reilly died during the evacuation of a high-rise building and that a 72-year-old woman who was distraught over damage to her apartment died in the city of Bell. Veronica Lopez, 20, of Azusa, died of an apparent heart attack in Covina, according to a coroner's spokesman.

The Los Angeles Fire Department responded to reports of 67 natural gas leaks, 36 structural fires, 41 heart attacks, 21 stuck elevators, 35 traffic accidents and at least six downed wires in a deluge of calls after the quake. A police spokesman said there had been five arrests for looting of stores with shattered windows, and a special antilooting squad was called in.

At least 100 houses were declared unfit for habitation, and their former residents were told to go to shelters.

Motorists on Interstate 210 through the San Gabriel Valley watched this morning as the San Gabriel Mountain slopes to the north exploded in puffs of smoke and dust, a signal of numerous rock and dirt slides. The earthquake was felt as far away as Las Vegas, San Diego and Santa Barbara.

The quake set off a fire in a small shopping mall southwest of downtown Los Angeles. Flames and smoke poured out of at least seven of the mall's 15 stores.

Almost every house in the Los Angeles area appeared to have suffered at least some minor damage, bottles or glasses falling off shelves, light fixtures crashing to the floor, vases cracking or cracks developing in concrete and wood supports. By 10 a.m., the sidewalks of most Los Angeles area business centers were full of office workers temporarily barred or evacuated from their buildings while structures were checked and cracks investigated.

Trains in the area were stopped for hours while tracks were checked. Burbank's airport closed early in the day when several control tower windows shattered, and Los Angeles International Airport closed briefly while the runway was checked for cracks. Several schools closed, particularly in areas near the east side epicenter.

"My place is a mess," said Sally Murphy, a librarian in Whittier. "I've got cracks in several support beams of my house." Two blocks of Whittier's Greenleaf Avenue looked like a ghost town. A second story brick wall atop Art's Jewelry and Loan and Knight's Flower Shop had tumbled into the street, exposing the living quarters above. Bricks and glass were strewn everywhere, waiting for a city inspection force to make a building-by-building survey.

Three cars lay crushed under bricks from the upper story wall of the Gospel Outreach Christian Center on Bright Avenue.

Jones, the U.S. Geological Survey geophysicist working at Caltech, said several members of the joint federal-university earthquake study team were remarking at a coffee break Wednesday at the unusual lack of large quakes in recent months. Team members carry beepers that are triggered whenever a quake of magnitude 4 or above is detected. Such quakes had not occurred in the broad area of southern California covered by their 280 seismographs in 11 months, the longest lapse in the records.

"We said, 'I wonder what that means?' " Jones said. "But we really didn't know." She said there had been a gap of nine months in 1977 with no magnitude-4 quakes that did not lead to a large quake such as today's. She said this quake might take some stress off that particular fault but would have not appreciable effect on the San Andreas Fault north of Los Angeles, which has not had a major, magnitude-8 quake since 1857.

Seismologists put the chance of a second quake in the 6-point range at 5 percent in the first 24 hours, 1 percent in the next 24 hours.

Two employes of the Federal Emergency Management Agency were in Los Angeles when the quake hit. They were preparing the annual test of the area's plans to cope with a catastrophic earthquake -- one that measures more than 8.5 on the Richter scale.