Wednesday, February 10, 2021

Sylmar Earthquake of 1971 Still Rattles

I was 9-years-old when the Sylmar earthquake woke up Southern California Feb. 9, 1971 --- and I still remember it clearly. Even in Orange County it caused structural damage. Our house in the City of Orange had some cracks in the walls afterwards. And on that same day, my fourth grade class had a field trip planned to Los Angeles (I think to the tar pits, but I can't remember.) All the kids lining up before classes at 8:00am were chattering on about how their families also felt it. And while there've been stronger earthquakes in California since then (Lomo Prieta in 1989 and the Northridge quake in 1994), apparently it's the Sylmar quake that still resonates the strongest in the geological scientific community.

Also interesting, at the L.A. Times piece below, is that apparently back in the 1970s, the state actually had some good and farsighted leadership who passed legislation that did some good things to protect the state's residents from future temblors. (Must've not been so many Dems in Sacramento back then, for one thing.)

In any case, this is fascinating, "50 years ago, the Sylmar earthquake shook L.A., and nothing’s been the same since":

How close Los Angeles came to what would have been — many times over— the deadliest disaster in U.S. history remains a matter of historical conjecture.

When the Sylmar earthquake rumbled through Los Angeles 50 years ago, on Feb. 9, 1971, the top of the earthen Lower Van Norman Dam melted into the reservoir. No one knows exactly what kept the dam near Granada Hills from collapsing. Was it the number of feet of earthen wall that remained? Was it the duration of the quake, since a few more seconds might have shaken loose the rest of the dam face, unleashing a torrent on tens of thousands of homes below?

That the dam survived has rendered those questions a subject for scientific inquiry rather than the annals of catastrophe.

But what might have been remains part of the mystique that sustains the Sylmar earthquake — formally, the San Fernando earthquake — as the keystone in the long arc of seismic knowledge and the practice of earthquake safety. The quake might not have been the Big One, but it still managed to wake California up to a danger that was largely unrecognized. The modern era of earthquake awareness and preparedness is deeply rooted in Sylmar.

Before then, earthquakes were either removed in time — 1906 in San Francisco, 1933 in Long Beach — or physically distant —1964 in Anchorage.

The 6.6 magnitude earthquake that struck the northeast San Fernando Valley seconds after 6 a.m. not only woke up the city but fixated the nation’s budding seismic community as none had before.

“Los Angeles was the city of the future,” said geophysicist Susan Hough of the U.S. Geological Survey’s Earthquake Sciences Center. “You had the space-age LAX. You have this modern glistening city and all of a sudden hospitals are being knocked down. It really got people’s attention in many ways.”

The indelible images of Sylmar were the hospitals.

At the Veterans Administration Hospital in Sylmar, two buildings dating to the 1920s collapsed and several others were severely damaged, causing 49 of the 64 deaths attributed to the disaster.

Less costly in lives, yet more startling to engineers and scientists, was the partial collapse of the 4-month-old Olive View Medical Center. Elevator towers tumbled, and the second floor of the 50-bed psychiatric unit collapsed onto the first. Three died there.

No less shocking was the collapse of the soaring, nearly completed overpass from the new Antelope Valley Freeway (Highway 14) to the Golden State Freeway (Interstate 5) in Newhall Pass and portions of the Foothill Freeway (I-210) interchange, where two men in a pickup were killed.

“There were some structures that people thought were safe that turned out not to be,” Hough said.

The hospital buildings and the freeways, all made of concrete, proved unable to roll with the earthquake’s punches.

“We as an engineering community learned from that, that just having strength was not enough,” said Jonathan Stewart, professor of civil and environmental engineering at UCLA. “You had to have ductility” — the ability to stretch. “The [building] code would essentially produce nonductile concrete buildings.”

Another revelation was the damage to single-family homes, at the time thought to be resilient enough to ride out moderate quakes. They proved helpless when the fault rupture reached the surface, a phenomenon that had not previously occurred in an urban earthquake.

“It would go through people’s lawns, it would go through homes,” said Tim Dawson, engineering geologist with the California Geological Survey. “It would torque the buildings. That was the recognition of that earthquake, ‘Oh, we shouldn’t be building on top of faults that can rupture the surface.’”

For the seismic community, the near debacle of the Lower Van Norman Dam, causing no loss of life but forcing the evacuation of 80,000 people, was the most frightening lesson.

“This was a big one because people started to realize you could have killed 100,000 people if that dam had cut loose,” said acting state geologist Steve Bohlen.

Luck may have played a part. The water level had been lowered 10 feet in 1967 after an evaluation had raised doubt about its stability.

“It was very close,” Bohlen said. “Had the shaking gone on for maybe another five seconds or 10, it could have been horrific. It galvanized both the state and the federal government.”

Still more at that top link, including photos and video.