Northridge earthquake
The Northridge earthquake struck at 4:30 a.m. on January 17, 1994, with a magnitude of 6.7, centered approximately twenty miles northwest of downtown Los Angeles, within the San Fernando Valley. This significant seismic event was caused by a previously unknown blind thrust fault within the Oak Ridge fault system, rather than the well-known San Andreas fault. The quake lasted approximately twenty seconds in some areas, causing extensive damage particularly to wood-framed buildings and freeway overpasses, and resulting in the collapse of structures such as a four-level parking facility at California State University, Northridge. Despite the destruction, the death toll was relatively low at fifty-seven, attributed in part to the early morning timing of the quake, which limited the number of people on the roads and in buildings. In response, emergency shelters were established by the American Red Cross for those displaced. The earthquake prompted significant changes in building codes and policies around emergency facility construction, marking an important shift in earthquake preparedness in California. The detailed data collected from the event has since contributed to advancements in seismology and engineering practices.
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Northridge earthquake
The Event A 6.7 magnitude earthquake strikes a densely populated area in Southern California, resulting in fifty-seven deaths
Date January 17, 1994
Place The San Fernando Valley of Los Angeles
The Northridge earthquake was among the largest quakes ever to occur directly beneath a major urban area in the United States. Building design and reinforcement programs were credited with preventing catastrophic loss of life. Many structures in the region, including freeway bridges, had been designed using standards implemented after a 1971 earthquake within the San Fernando Valley. However, economic losses were estimated to have exceeded $20 billion, making the quake the costliest in U.S. history at that time.
Located twenty miles northwest of downtown Los Angeles, the community of Northridge lies within the suburban San Fernando Valley. At 4:30 a.m. on the morning of January 17, 1994, an earthquake with a Richter scale magnitude of 6.7 shook the region, causing widespread damage, especially to wood-framed buildings and freeway overpasses. The National Geophysical Data Center estimated the quake’s epicenter at ten miles below the ground surface. Although located in the vicinity of the San Andreas fault, the quake occurred along a previously unknown blind thrust fault within the Oak Ridge fault system. Thirty-eight accelerographs positioned throughout Southern California were used to measure movement associated with the quake. For a period of eight seconds, the rupture moved upward and northwest along the fault plane at two miles per second. At the surface, vertical movements lifted structures off their foundations while horizontal accelerations shifted walls laterally. With shaking lasting twenty seconds in some areas, the quake caused deformation within the Earth’s crust across an area of more than fifteen hundred square miles. The quake’s tremendous force caused the Santa Susana Mountains and much of the San Fernando Valley to be pushed upward more than a foot. Hundreds of aftershocks continued for months, with the largest recorded at 4.0 on the Richter scale.
Damage
Among buildings suffering total collapse was a four-level parking facility at California State University, Northridge. Inside buildings, severe shaking damaged sprinkler pipes, interior partitions, ceilings, and air-handling systems. Throughout the region, about one hundred buildings designed to withstand severe ground motion experienced failure of their steel frames or reinforced concrete.
In contrast to other seismic events, the legacy of the Northridge earthquake was not calamitous damage but that more severe destruction and loss of life had been adverted. Experience with prior earthquakes had prompted building codes and the reinforcement of existing structures in order to reduce damage and threats to building occupants. Despite close proximity to the quake’s epicenter, many buildings experienced minimal damage. However, the quake caused severe damage to large wood-framed buildings, including many apartment complexes. Damage to streets was considerable in some areas, especially in western parts of the San Fernando Valley and within the cities of Simi Valley, Sherman Oaks, and Santa Monica. Significant damage to freeways occurred within twenty miles of the epicenter, affecting sections of Interstates 5 and 10 and State Route 14. The interchange connecting Interstate 5 and State Route 14 located between Newhall and San Fernando experienced a complete collapse.
A total of 170 freeway bridges experienced damage that affected traffic for months following the quake. In addition to damage to structures, the quake caused landslides that destroyed homes and utility lines and blocked roads and streams. In contrast to other large earthquakes such as the 1964 Alaska and 1989 Loma Prieta quakes, liquefaction and ground failure did not cause massive destruction. This was due to the relatively arid climate and dry soil in the Northridge area.
Human Toll
The death toll for the quake included fifty-seven persons, with eleven thousand others injured. A majority of persons killed were in multifloor wood-frame structures. The early morning timing of the quake contributed to the low number of persons killed. Had the quake occurred just hours later, the loss of life on freeways would have been considerably higher. Immediately following the quake, more than nine thousand homes and businesses were without power. Responding to the disaster, the American Red Cross established forty-seven shelters for persons displaced from their homes.
Impact
Information collected from the Northridge earthquake represents among the most detailed data sets on shaking intensity ever recorded in the United States. Scientific research based on data collected during the quake led to new building codes for the construction of steel-framed buildings. The quake also affected policy making. Damage to local hospitals prompted the California state legislature to pass a law requiring emergency and acute care facilities to be built to higher standards. As a result of enormous recovery payouts, many insurance companies discontinued earthquake coverage for property owners. To address this problem, the California Earthquake Authority was created as a publicly managed but privately funded agency offering minimal earthquake coverage.
Bibliography
Bolin, Robert C., and Lois Stanford. The Northridge Earthquake: Vulnerability and Disaster. New York: Routledge, 1998. A look at the effects of urbanization, population movement, and other sociodemographic factors on the vulnerability of Southern California to major disasters.
Bolt, Bruce A. Earthquakes. New York: W. H. Freeman, 1999. Provides a well-illustrated reference about earthquakes, with topics ranging from ground acceleration to earthquake forecasting. Several examples are provided from quakes, including Northridge in 1994 and Kōbe in 1995.
Hough, Susan E. Finding Fault with California: An Earthquake Tourist’s Guide. Missoula, Mont.: Mountain Press, 2004. A guidebook to major faults and earthquakes in the state of California. In addition to explaining characteristics of faults, the book provides profiles of geologists and the methods they use to study seismic activity.
Palm, Risa I., and Michael E. Hodgson. After a California Earthquake: Attitude and Behavior Change. Chicago: University of Chicago Press, 1992. Through twenty-five hundred interviews, the authors examine the attitudes of home owners regarding vulnerability to a major earthquake.
Stein, Seth, and Michael Wysession. An Introduction to Seismology, Earthquakes, and Earth Structure. Malden, Mass.: Blackwell, 2003. Provides an excellent reference for understanding the role of plate tectonics and seismic waves and their relationship to earthquakes.