Northridge earthquake
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ΉThe 1994 Northridge earthquake occurred on January 17, 1994 at 4:30:55 AM Pacific Standard Time in the city of Los Angeles, California, falling on Martin Luther King Day in 1994. The earthquake had a "moderate" moment magnitude of 6.7, but the ground acceleration was the highest ever instrumentally recorded in an urban area in North America[1], and it proved to be the most costly earthquake in United States history.
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[edit] The earthquake
The earthquake struck in the San Fernando Valley about 31.75 km (20 mi) northwest of downtown Los Angeles.Its epicenter was first reported as being in the community of Northridge, thus giving the earthquake its moniker, but was later calculated to be in Reseda. The National Geophysical Data Center places the epicenter's geographical coordinates at , which is at the end of Elkwood Street, just east of Baird Avenue. Despite the area's proximity to the San Andreas Fault, the Northridge quake did not occur along this fault, but rather on a previously-undiscovered blind thrust fault.
Damage occurred up to 125 km (85 mi) away, with the most damage in the west San Fernando Valley, the city of Santa Monica, and Simi Valley. seventy-two people were killed, and 9000 were seriously injured. Major freeway damage occurred up to 32 km (20 mi) from the epicenter. Portions of Interstate 10 (the Santa Monica Freeway), Interstate 5 (the Golden State Freeway) and California State Highway 14 (the Antelope Valley Freeway) collapsed and had to be rebuilt. The interchange of Interstate 5 and California State Highway 14 collapsed as it did 23 years ago during the 1971 Sylmar earthquake (it had been rebuilt without improved structural components [2]). California Department of Transportation has been praised for their quick and effective response to the collapse of the highway. They used an incentive technique for contractors in which contractors received $200,000 per day for every day they were ahead of schedule. This incentive method combined with expedition of permits allowed the highway to be completed in 710 days, rather than the predicted time of 1,112 days.[3]
Additional damage occurred about 50 miles south in Anaheim, California at Anaheim Stadium. The upper portions of the structure supporting the giant scoreboard collapsed onto several hundred seats. Fortunately, the stadium was empty at the time of the quake. Despite the fact that this was the only damage of note in the city of Anaheim (and in all of Orange County), the city council received several million dollars of federal aid to repair the slight damage to the stadium -- a decision called into question when the city heavily renovated the entire stadium three years later.
This quake was unusual because the epicenter was within a major metropolitan area. Although several commercial buildings collapsed, loss of life was minimized because of the early morning hour of the quake, and it occurred on the birthdate (observed) of Martin Luther King, Jr.. Also, because of known seismic activity in California, area building codes dictate that buildings incorporate structural design intended to reduce the risk of structural collapse due to earthquakes. As it turned out, one of the few multi-family buildings to collapse in Northridge had not been built to code.
Post-quake investigations revealed that some structural specifications did not perform as well as expected. Because of this building codes were revised. Some structures were not red-tagged until months after the earthquake because damage was not immediately apparent.
The quake produced unusually strong thrust, with accelerations in the range of 1.0 g over a large area. While most of the damage was caused by shaking, some damage was also caused by fire and by ground deformation. In some areas the ground surface was permanently uplifted by up to 50 cm (20 in).
This was the fourth destructive earthquake to occur in California in a span of 23 years. The first was the Mw 6.6 San Fernando (Sylmar) Earthquake, affecting the same area in 1971; the second was the Mw 6.9 (Richter magnitude 7.1), 1989 Loma Prieta earthquake south of San Francisco; the third was the R6.5 Coalinga Earthquake on May 2, 1983. The 1994 event is the most damaging earthquake to strike the United States since the San Francisco earthquake of 1906. In terms of property damage, the earthquake is also one of the worst natural disasters in U.S. history, comparable to Hurricane Andrew in 1992.[citation needed]
Several key hospitals suffered severe structural damage and were rendered unusable after the earthquake. Not only were they unable to serve their local neighborhoods, they had to transfer out their inpatient populations, which further increased the burden on nearby hospitals that were still operational. As a result, the state legislature passed a law requiring all California hospitals to ensure that their acute care units and emergency rooms are in earthquake-proof
[edit] References
- ^ Northridge Earthquake Southern California Earthquake Data Center, Accessed October 6, 2006
- ^ http://www.tfhrc.gov/pubrds/summer94/p94su26.htm
- ^ http://www.fhwa.dot.gov/modiv/lessons.htm
