SAN FERNANDO — It was not only the biggest quake in San Fernando Valley history at the time, but it was the first strong one in the Valley since 1893.
In fact, the area where the fault broke had been the locale of only 10 earthquakes of magnitude 3.0 or greater in the previous 37 years.
But at 42 seconds after 6 a.m. on a Tuesday morning 25 years ago this week, Feb. 9, 1971, the Sylmar-San Fernando earthquake hit the Valley and other parts of Greater Los Angeles for 12 seconds, killing 64, injuring 2,543 and doing $553 million in damage.
Scientists now believe what they call simply the San Fernando earthquake was a magnitude 6.7, identical in strength to the Northridge quake.
"San Fernando and Northridge have to be related somehow, but we don't understand the process well enough to know why they were 23 years apart," said Thomas Heaton, a Caltech professor of geology.
And Lucy Jones of the U.S. Geological Survey commented, "San Fernando clearly nudged the Northridge plane toward having an earthquake. It didn't happen for 23 years, but it could have happened almost immediately as an aftershock.
"For some reason, Northridge wasn't quite ready to go, and we were fortunate it wasn't, because had Northridge occurred right away, then it's pretty clear we would have killed a lot more people."
The San Fernando quake, in fact, could have been a catastrophe instead of just a costly disaster. That conclusion arises from its most striking episode: the near-collapse of the lower dam at the Van Norman reservoir.
The 1,100-foot dam held 3.6 billion gallons of water on the morning of Feb. 9, 1971, but it was only half full; the water level was 36 feet below the lip.
The top 30 feet of the edifice crumbled, leaving the water only six feet from the top and fresh chunks of earth falling off with each aftershock.
With leading seismologists of the day warning there could be a much heavier aftershock at any time, authorities ordered 80,000 people living below the dam in an area bounded by the San Diego Freeway on the east, Victory Boulevard on the south, Balboa Boulevard on the west and Rinaldi Street on the north to evacuate.
The evacuation lasted three days while engineers tried furiously to pump water from the dam through a 24-inch hole cut in an aqueduct pipe and another in the dam face. They succeeded in lowering the water level by about three feet a day.
A year to the day before the quake, the dam held 6.5 billion gallons of water, and its level was eight to 10 feet higher than the level to which the top of the dam slumped in the quake. Engineers recognized that had such a vast quantity of water spilled over the top, the entire dam would have quickly been washed away.
Later, a UCLA study estimated that collapse of the dam would have brought flooding that could have killed between 71,600 and 123,400 people.
Clarence Allen, Caltech professor emeritus of seismology, said the toll might not have been that high, but a dam failure would have been "a disaster unique in American history."
"It was an awful close call," Allen said. "A big aftershock may have let the thing go. I was there. I can remember as if it were yesterday, the water six feet from the top."
It remains a pungent memory too to Tammy Williams Poppen, then a 12-year-old girl living in the evacuation zone in Granada Hills.
Trying to find her three cats, which had disappeared when the quake struck, she suddenly heard loudspeakers telling everyone in the neighborhood to get out.
" 'Dad,' I yelled, 'they're telling us we have to leave,' " said Tammy, now manager of a downtown Los Angeles credit union. " 'No, they can't be,' he said. But then he heard the loudspeakers, too."
Three days later, the Williams family returned to their home and found their cats, safe and sound.
The Van Norman Dam experience led to an inspection of dams across the state and to the rebuilding or retrofit of many.
As for the new dam that was constructed on the site, Allen notes happily, "It came through the Northridge quake without a scratch."
The results were not as fortunate at the San Fernando Veterans Administration Hospital, where 47 of the 64 fatalities attributed to the San Fernando quake occurred.
Built with reinforced concrete structural frames in 1925, before seismic protection was written into building codes, the sides of two big buildings collapsed, trapping many of the patients.
Eventually, the hospital's laundry and chapel were the only buildings among 50 at the site that were not demolished.
Frank Carbonara, 68, survived in the rubble 58 hours before he was rescued.
At first, he recalled after he was safe, "I thought I was dead. . . . I closed my eyes and waited, said a prayer God would forgive my sins. Nobody lives forever. But after a while, I noticed I was still breathing."
Another hospital that was hard hit was Olive View in Sylmar, where a six-story building opened only a few weeks before collapsed, killing three.