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Great Wall Institute: The Process of the Great Wall of Los Angeles

San Fernando Earthquake (1971)

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The San Fernando earthquake occurred on February 9, 1971, at 6:00:41 am with a strong ground motion duration of about 12 seconds as recorded by seismometers, although the whole event was reported to have lasted about 60 seconds. The origin of faulting was located five miles north of the San Fernando Valley. Considerable damage was seen in localized portions of the valley and also in the foothills of the San Gabriel Mountains above the fault block. The fault that was responsible for the movement was not one that had been considered a threat, and this highlighted the urgency to identify other similar faults in the Los Angeles area.
This earthquake occurred on the San Fernando fault zone, a zone of thrust faulting which broke the surface in the Sylmar-San Fernando Area. The total surface rupture was roughly 19 km (12 miles) long. The maximum slip was up to 2 meters (6 feet). Source Red lines show the fault active during the earthquake, meaning that is the plane along which the two blocks moved in a thrust fault motion (the motion showed in the following slide).
Two fallen structurally separated stair towers and the collapsed basement at Olive View Hospital after the San Fernando earthquake in February 1971. The earthquake was the first disaster in the United States to happen after the Disaster Relief Act of 1970, which directed federal agencies to provide assistance to state and local governments. At the time of the earthquake, FEMA did not exist. The epicenter of the quake was about 8.7 miles north of San Fernando in a sparsely populated area of the San Gabriel Mountains. It was 5.6 miles deep and generally felt over approximately 80,000 square miles of California, Nevada and Arizona.
Aerial view of damage at Sylmar Veterans Administration Hospital, April-May 1971. For the public, perhaps the most important outcomes of the 1971 San Fernando event were the laws and changes to building codes that were put into place to make buildings safer during major earthquakes. Because the damage during the quake had been so horrible, one of the first changes was the adoption of new seismic standards for hospitals. Other changes took a bit longer. During the quake, the San Fernando Fault actually came to the surface of the earth and tore through people's houses. Prior to the event there was nothing to prevent builders from constructing homes and businesses directly on top of active fault lines. But as Jones notes, there are two types of damage associated with earthquakes. "The damage from shaking can be stopped by building stronger buildings," she says. "The danger from the fault can't be stopped because the fault itself is moving."
Damage to the Lower Van Norman Dam. Both the Upper and Lower Van Norman Dams were severely damaged as a result of the earthquake. The lower dam was very close to breaching, and approximately 80,000 people were evacuated for four days while the water level in the reservoir was lowered. Some canals in the area of the dams were damaged and not usable, and dikes experienced slumping but these did not present a hazard. The damage at the lower dam consisted of a landslide that dislocated a section of the embankment. The earthen lip of the dam fell into the reservoir and brought with it the concrete lining, while what remained of the dam was just 5 feet above the water level.
The San Fernando Veterans Administration Hospital in the aftermath of the 1971 San Fernando Earthquake. Loss of life that was directly attributable to the earthquake amounted to 58 (a number of heart attacks and other health-related deaths were not included in this figure). Most deaths occurred at the Veterans and Olive View hospital complexes, and the rest were located at private residences, the highway overpass collapses, and a ceiling collapse at the Midnight Mission in downtown Los Angeles.
Collapsed overpass at intersection of Sierra Highway and Golden State Freeway. Substantial disruption to about 10 miles of freeways in the northern San Fernando Valley took place, with most of the damage occurring at the Foothill Freeway / Golden State Freeway interchange, and along a five-mile stretch of Interstate 210. On Interstate 5, the most significant damage was between the Newhall Pass interchange on the north end and the I-5 / I-405 interchange in the south, where subsidence at the bridge approaches and cracking and buckling of the roadway made it unusable.
From left: Duane L. Georgeson, Jules Bergman examine damage in the aftermath of the Sylmar Earthquake / 1971 San Fernando earthquake, in the Walt Disney Television series episode "The Violent Earth." Important note about depicting a fault: Shallow crevasses can form during earthquake-induced landslides, lateral spreads, or from other types of ground failures, but faults do not open up during an earthquake. An earthquake occurs when two blocks of the earth’s crust slide past one another after having been stuck together in one place for a long time, because of friction on the fault, while the rest of the crust away from the edges has been slowly moving. If a fault could open up, no earthquake would occur in the first place because there would be no friction locking the two blocks together.
From left: F. Beach Leighton, Jules Bergman examine fault line trench after the Sylmar Earthquake / 1971 San Fernando earthquake, in the Walt Disney Television series episode "The Violent Earth." When geologists want to study an active earthquake fault, they often rely on a trenching study. They dig a long trench across the active trace of the fault, perhaps as much as 10 feet deep, or deeper if money and conditions allow. They brace the walls, if needed, to prevent cave-ins, then they climb down into the trench and carefully map its walls, looking for signs of past earthquakes and ground movement.