Sylmar earthquake
The Sylmar earthquake, which struck on February 9, 1971, at 6:01 a.m., was a significant seismic event that impacted the San Fernando area of Southern California. Measuring 6.6 on the Richter scale, the earthquake originated from a previously unstudied fault located about 12 kilometers (7.5 miles) below the surface, with its epicenter just 10 kilometers (6 miles) from Sylmar. Prior to this event, the region had been largely stable for nearly four centuries, leading to the construction of many older buildings without adequate seismic reinforcement. The quake resulted in the collapse of structures, most notably at the Sylmar Veterans Administration Hospital, where 44 lives were lost.
Approximately 80,000 residents were affected, with widespread destruction of buildings, utility lines, and infrastructure. The earthquake also caused serious damage to several dams in the region, prompting evacuations due to concerns about their stability. Fortunately, the quakes occurred early in the morning, minimizing casualties. In the aftermath, building codes were revised to improve earthquake resilience, leading to a more robust architectural approach in the area. The Sylmar earthquake ultimately resulted in 65 fatalities and over half a billion dollars in damages.
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Subject Terms
Sylmar earthquake
The Event A 6.6 magnitude earthquake also known as the San Fernando earthquake
Date February 9, 1971
Place The San Fernando Valley of Southern California, near the San Gabriel Mountains
The Sylmar earthquake caused sixty-five deaths and more than one thousand serious injuries. Many structures collapsed, particularly old, unreinforced masonry buildings.
At 6:01 a.m. on February 9, 1971, the city of San Fernando, about twenty miles northwest of Los Angeles in Southern California, was struck by a strong earthquake. Also known as the Sylmar earthquake because its epicenter was located about 10 kilometers (6 miles) from the town of Sylmar, the earthquake originated in a previously unstudied fault 12 kilometers (7.5 miles) below the surface.
Prior to 1971, the region had experienced no major earthquakes for almost four hundred years; the last recorded one occurred in Pico Canyon, 6 miles from Sylmar, in 1593. Major structures had been built over the fault, and many older buildings were designed and constructed without proper reinforcement. Another potential threat to Sylmar was the Lower San Fernando Dam upstream from the town. Built between 1912 and 1915, this 142-foot-high earth dam held 15 million tons of water in a reservoir that was 1.6 miles long and 130 feet deep. Approximately eighty thousand people lived in the San Fernando region at the time of the earthquake.
When the earthquake measuring 6.6 on the Richter scale hit, a number of older buildings collapsed. Forty-four people died when two tile masonry buildings at the Sylmar Veterans Administration Hospital were destroyed. Even at Olive View Hospital, housed in a well-engineered, reinforced modern building, serious damage killed three people. Chimneys, masonry walls, fences, about 1,700 mobile homes, and 1,300 buildings were partially or totally destroyed. Landslides and soil liquefaction were widespread. Freeways were heavily damaged, and some bridge spans were destroyed. Luckily, because the quake occurred very early in the morning, few people were on the roads or in the center of town, where dense construction could have resulted in many fatalities.
In some areas, the fault caused vertical displacement as great as 1.5 meters (5 feet) and horizontal movement of about 1.3 meters (4.5 feet), damaging utility lines and causing fires to erupt. The San Fernando Dam was damaged, as were three other dams in the region. The Van Norman Dam, which at the time was the largest provider of water for Los Angeles, suffered sufficient damage to cause concern for its stability and the evacuation of an 80-square-mile area below it. The Hansen Dam also reported minor damage and leaks, resulting in further evacuations. Fortunately, none of the dams collapsed. Had they failed, the loss of life and property would have been greatly increased, well beyond the sixty-five deaths and half billion dollars of damage that the earthquake actually caused.
Impact
Because the area had been considered very stable, many buildings were designed and constructed without proper reinforcement to withstand earthquakes. Following the earthquake, building codes were strengthened. The Los Angeles Dam and Reservoir, built from 1975 to 1976 further up the valley, survived the similar 1994 Northridge earthquake with little damage.
Bibliography
Hough, Susan Elizabeth. Earthshaking Science: What We Know (and Don’t Know) About Earthquakes. Portland, Oreg.: Book News, 2004.
Hough, Susan Elizabeth. Finding Fault in California: An Earthquake Tourist’s Guide. Missoula, Mont.: Mountain Press, 2004.