ALHAMBRA, CALIF., OCT. 4 -- A long overdue major aftershock from Thursday's strong earthquake jolted awake several million southern Californians early today, creating a short-lived wave of renewed panic and adding significantly to the $108 million in damage from Thursday's quake.

Seismologists at the California Institute of Technology measured the 3:59 a.m. earthquake at about magnitude 5.5, centered seven miles beneath the eastern Los Angeles County communities of Alhambra and San Gabriel. It was by far the largest of more than 100 significant aftershocks following Thursday's 7:42 a.m. earthquake, which at 6.1 on the Richter scale was the strongest to hit the Los Angeles area in 16 years.

"It just woke me up and I wondered what was happening. It is frightening when it's in the middle of the night," said Linda Sotello, assistant manager of a candy store on Alhambra's Main Street. Once the sun rose she went to survey her shop's broken front window, one of several that left glass scattered this morning on sidewalks in commercial areas in Pasadena and San Gabriel as well as Alhambra.

Michael Guerin, coordinator with the governor's Office of Emergency Services, said broken water and gas mains in some communities would add a "significant" amount to the earthquake damage. His office today estimated at least $100 million in private and $8 million in government property loss before today's aftershock.

The unwelcome jolt knocked books off shelves, collapsed already weakened walls and chimneys, set off thousands of automobile burglar alarms and left many Los Angeles area residents standing on street corners, red-eyed and groggy as they faced another day of temperatures soaring over 100 degrees. "It's shake and bake in Southern California," declared one caller to radio station KNX-AM.

Authorities blamed at least one fatal heart attack on the early morning quake, centered about nine miles northeast of downtown Los Angeles. Several hospitals reported treating cuts and bruises. Rock slides and falling concrete forced officials to close temporarily some roads and freeways. Thursday's earthquake had produced at least six deaths -- three from heart attacks -- and more than 100 injuries.

Caltech senior seismologist Kate Hutton said major aftershocks usually occur within a day of the original temblor, so the nearly three-day delay was a surprise. The largest aftershock before this morning's occurred within an hour of Thursday's quake and registered 4.4 on the Richter scale, which meant it produced only one-tenth the ground motion of this morning's jolt.

Today's 5.5-magnitude aftershock was the 22nd measuring more than 3.0 on the Richter scale since Thursday morning. It was followed by aftershocks of 3.0 magnitude at 4:09 a.m. and 4:56 a.m. and a 3.6-magnitude aftershock at 7:05 a.m.

Thursday's earthquake and aftershocks all originated in a hitherto unknown underground extension of the Whittier Fault. Until now the fault was thought to end in the suburb of Whittier, population 70,000. But Thursday's quake and today's major aftershock occurred some distance northwest of Whittier, the apparent result of an underground, nearly horizontal extension of the fault that was undetected until now.

Whittier, celebrating the 100th anniversary of its founding as a small Quaker community, suffered extensive damage to homes and shops in Thursday's quake and was further shaken this morning. A water main broke beneath a major intersection, causing a section of pavement to collapse.

In San Gabriel, within a mile of the quake's suspected epicenter, a 30-foot bell tower at the old San Gabriel Civic Auditorium collapsed, a police spokesman said. The prized example of mission-style architecture suffered extensive damage Thursday and was expected to be pulled down.

Churches in affected areas appeared to be full this morning. "We just changed the location," said Philip Lee, part of a predominantly Chinese-American protestant congregation in Alhambra. The group held services in an American Legion hall while its building was inspected for structural damage.