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6.6 Sylmar quake almost felled a dam 41 years ago today

February 9, 2012 | 11:23 am

Photo: Line of Los Angeles County ambulances and other vehicles caught under collapsed roof of parking structure at Olive View Hospital in 1971. Credit: Boris Yaro / Los Angeles Times

LA-Times-Sylmar-earthquake-front-pageForty-one years ago today, a magnitude 6.6 earthquake ripped through the San Fernando Valley, jolting the region in its first destructive temblor in generations.

The Sylmar quake traumatized Southern California. Forty-seven people died at one site alone, after the San Fernando Veterans Administration Hospital, built in 1925, disintegrated into a pile of rubble. A Times photo of the aftermath shows part of a two-story facade precariously leaning over. Another photo shows rescuers on the shingled roof of a pulverized section, frantically looking for survivors.

Nearby, in Sylmar, the first floor of the newly built six-story Olive View Hospital was pulverized, and three people died. A line of Los Angeles County ambulances was crushed by a parking overhang. One photo appears to show a wing of a multi-story tower tipped on its side.

Photos: Sylmar earthquake on Framework

The destruction revealed a major flaw in concrete buildings throughout California -– that they could collapse like brittle salt crackers in major shaking. Buildings constructed since the '70s were designed to more resilient standards with better reinforcing steel, to keep the structures flexible during shaking, but California still has many potentially vulnerable buildings that have not been evaluated nor retrofitted.

The problem was still evident in building damage during the Northridge earthquake in 1994 and the earthquake in New Zealand in 2011. The collapse of one concrete building that housed a television studio in Christchurch killed 115 people, accounting for about two-thirds of those who died in the entire quake, according to news reports.

According to a detailed Times story in 1996, the Sylmar quake -– also known as the San Fernando earthquake -– could have been even more catastrophic, because the Van Norman dam in Granada Hills almost collapsed and could have resulted in a flood disaster for the San Fernando Valley.

Officials at the time ordered a wide swath of the Valley downstream from the dam evacuated, affecting 80,000 people in sections of North Hills, Van Nuys, Northridge and Lake Balboa -– generally the area east of Balboa Boulevard and west of the 405 Freeway as the valley slopes down to the Sepulveda Flood Control Basin. Authorities furiously tried to lower the dam's water over three days, hoping they could drain the dam before an aftershock could knock the rest of it out.

L.A. was fortunate that the dam was only half full when the quake struck.

The 1971 quake had knocked out the top 30 feet of the dam, leaving the water a mere 6 feet from the top. But if the quake happened a year earlier, when the dam had nearly double the amount of water and the level was 8 feet higher, the force of water rushing over the dam's top could have caused the rest of the dam to crumple.

A UCLA study found that a dam collapse could have killed between 71,600 and 123,400 people. A replacement dam survived the subsequent Northridge earthquake in 1994 with no damage.

Quake damage also forced four freeways to be closed: Interstate 5, Highway 14, the 405 Freeway and the 210 Freeway.


Read 1971 coverage of earthquake

Photos: Sylmar earthquake on Framework

1996 article from the Times: Hospital staffers recall terror

-- Rong-Gong Lin II

Photo: Los Angeles County ambulances and other vehicles were pinned under a collapsed roof of parking structure at Olive View Hospital in 1971. Credit: Boris Yaro / Los Angeles Times