Collapsed Northridge Parking Structure. Photo Courtesy of U.S. Geological Survey.
The Northridge earthquake occurred at 4:30 a.m. local time on January 17, 1994. Northridge is located about 30 km northwest of Los Angeles. This earthquake had a 6.9 moment magnitude. The hypocentral depth was 19 km. The duration was about 10 seconds to 20 seconds. The earthquake occurred along a "blind" thrust fault, close to the San Andreas fault. Note that a blind fault is a fault which does not extend to the surface. In other words, it is buried.
The Northridge earthquake was the worst earthquake in the Los Angeles basin since the San Fernando earthquake in 1971, which had a 6.7 magnitude.
The number of fatalities in the Northridge earthquake was 57. About 9000 people were injured. The fact that the earthquake occurred at 4:30 a.m. minimized the death toll.
The Northridge earthquake caused extensive damage to parking structures and freeway overpasses.
For example, a section of the Antelope Valley Freeway collapsed onto the Golden State Freeway south of Newhall. Also, a section of the Santa Monica Freeway in West Los Angeles collapsed.
In addition, a 2500-car parking garage at the California State University at Northridge collapsed. This structure was about 3 km away from the epicenter.
Furthermore, the Northridge earthquake triggered landslides in the Santa Susana Mountains, Santa Monica Mountains, and western San Gabriel Mountains. These landslides blocked roads and damaged water lines. The landslides also damaged homes, particularly in the Pacific Palisades area.
One positive note was that the Olive View Hospital in Sylmar withstood the Northridge earthquake. This hospital had been destroyed by the 1971 San Fernando earthquake. After its collapse, the hospital was rebuilt to a more exacting seismic code.
Fortunately, the Northridge area has arid, dry soil. Thus, there were few significant liquefaction and ground failure effects. In constrast, soil liquefaction caused massive damage in the 1964 Alaskan and the 1989 Loma Prieta earthquakes.
Thousands of aftershocks occurred after the main earthquake. For example, a magnitude 5.9 aftershock occurred about 1 minute after the mainshock. A magnitude 5.6 earthquake occurred 11 hours later. Aftershocks are a concern because they can trigger the collapse of structures weakened by the mainshock.

1. Bruce A. Bolt Earthquakes (Earthquakes, 4th Ed) 1999.
2. M. Levy and M. Salvadori, Why the Earth Quakes, Norton, London, 1995.
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