A severe earthquake felt for hundreds of miles struck this small oil and farming community in the western San Joaquin Valley today, collapsing buildings, igniting fires and injuring at least 25 people, authorities said. No deaths were reported.

Fissures emitting hissing steam opened in the earth at Coalinga, cracking highways and preventing rescue vehicles from neighboring towns from reaching the injured.

Doctors aboard military helicopters were followed into the stricken town by more than 200 law enforcement officials.

The quake, which struck about 4:45 p.m. PDT, was felt throughout California from Sacramento to Valencia and as far east as Reno, Nev. More than three dozen aftershocks were reported.

In Golden, Colo., the U.S. Geological Survey's National Earthquake Center estimated the earthquake at 6.1 on the Richter scale, a measure of ground motion as recorded on seismographs. A 6 reading indicates severe damage, while a 7 reading is considered a "major" earthquake capable of widespread, heavy damage.

The University of California seismographic station at Berkeley put the epicenter five miles northeast of Coalinga about 25 miles north of the San Andreas fault. A string of aftershocks was reported, with at least 40 hitting 2.5 on the Richter scale.

As floodlight rescue operations continued into the night, Fresno County Coroner David Hadden said it appeared likely that there were no deaths. Craig Reid, Fresno County public information officer, said at least 25 people were injured, but he added that he was unsure what authorities would find in the rubble.

Navy Lt. Comdr. Rick Ellworth said from police headquarters that three people were hurt critically, two with head injuries and an elderly woman with a broken hip.

"There were buildings collapsed all over and three or four different fires," said Pat Barber, 42, a resident of this town of 7,000 people 200 miles southeast of San Francisco.

Roy Manning, Fresno County's coordinator of emergency services, said many of the injured were taken to hospitals in Monterey, Hanford and Fresno because Coalinga's hospital, like most of the town, had no electricity. He said most of the fires were under control.

Lindsay Daniels, assistant administrative officer for Fresno County, said there was "major structural damage as far as buildings are concerned, fires in the city itself and possible injuries."

Grass and oil fires had been reported burning around the town, surrounded by oil wells, and several fuel tanks ruptured but were not burning, said Lee DeLap, public information officer for the state Department of Forestry office.