Los Angeles, 1971: The Dam That Failed After The Earthquake Stopped

The Lower Van Norman Dam was still standing when the 1971 San Fernando earthquake stopped shaking it. Somewhere between twenty and thirty seconds later — after the ground had already gone quiet — the water-side slope and the top thirty feet of the dam slid into the reservoir. Behind what was left stood 3.6 billion gallons of water, about five feet below the broken edge. Eighty thousand people were evacuated from the valley below. This video explains why the dam failed after the earthquake was over, and why the five feet that saved the valley was not strength. We cover the seismoscope records that date the slide to after the shaking ended, the arithmetic behind the five feet, why the technical literature credits a reservoir that was "relatively low for the season" rather than the structure, and what USGS geologists later found buried in the ground nearby — a fault scarp about 200 years old, higher than the ones 1971 made. Part of an ongoing Earth In Ruin series on disasters whose real mechanism is stranger than the headline. Topics covered: 1971 San Fernando earthquake, Sylmar earthquake 1971, Lower Van Norman Dam failure explained, why the Van Norman Dam failed after the shaking stopped, delayed slope failure, soil liquefaction explained, hydraulic fill dam failure, 80,000 evacuated Los Angeles 1971, Upper Van Norman Dam damage, Los Angeles Aqueduct, Harvey Van Norman, St. Francis Dam, Los Angeles Dam replacement, Granada Hills fault, fault scarp San Fernando Valley 00:00 Twelve seconds, and then quiet 01:26 Twenty to thirty seconds after it ended 02:37 Five feet above 3.6 billion gallons 03:48 What everyone gets wrong about the margin 04:32 The arithmetic 05:22 Why the water was that far down 06:24 "Relatively low for the season" 08:02 The dam that was fine the day before 09:20 The assumption nobody wrote down 10:12 The material that stopped being material 11:08 This does not indict a dam 11:56 The name on the dam 12:52 The outlet tower on the bottom of the lake 14:35 Sandbags on the scarp 15:54 Why they did not just empty it 17:32 The second dam 19:14 What the records changed 20:50 Looking for ground to build on 23:17 The scarp that was already there 25:25 Luck against something ordinary Earth In Ruin investigates natural disasters, geological threats and extreme weather — the hidden mechanisms that turn ordinary ground into catastrophe. Based on publicly available information, including U.S. Geological Survey and U.S. Army Corps of Engineers material and peer-reviewed research. Where sources disagree or figures remain uncertain, the narration says so. Subscribe for a new investigation into the disasters most people only think they understand. @earthinruin #EarthInRuin #NaturalDisasters #DisasterDocumentary