Northridge earthquake

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The Northridge earthquake occurred on January 17, 1994 at 4:31 AM Pacific Standard Time in the city of Los Angeles, California. The earthquake had a "strong" moment magnitude of 6.7, but the ground acceleration was the highest ever instrumentally recorded in an urban area in North America.^[1] Fifty-seven people died as a result of the earthquake and over 11,000 were injured. In addition, the earthquake caused an estimated $12.5 billion in damage, making it one of the costliest natural disasters in U.S. history.^[2]

[edit] The earthquake

The earthquake struck in the San Fernando Valley about 31 km (20 mi) northwest of downtown Los Angeles near the community of Northridge. The National Geophysical Data Center places the epicenter's geographical coordinates at 34°12′47″N, 118°32′13″W and a depth of 17 km. 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.

The underpass of the 10 Freeway at La Cienega Blvd. This image shows the collapsed section of the freeway.

The freeway "bent" even before the section collapsed.

Kaiser Permanente Building

Collapsed Apartment Building

Street Damage

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. Fifty-seven people died as a result of the earthquake, and over 11,000 were injured including 1,600 that required hospitalization.^[3] 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 State Route 14 (the Antelope Valley Freeway) collapsed and had to be rebuilt. The Newhall Pass interchange of Interstate 5 and State Route 14 collapsed as it had 23 years earlier during the 1971 Sylmar earthquake even though it had been rebuilt without improved structural components.^[4] One life was lost in the Newhall Pass interchange collapse: Los Angeles Police officer Clarence Dean, who fell 40 feet from the severed overpass along with his motorcycle after he failed to observe the damaged overpass in the early morning darkness, and was unable to stop in time to avoid the fall.

Additional damage occurred about 50 miles south in Anaheim, California as the scoreboard at Anaheim Stadium collapsed onto several hundred seats. Fortunately, the stadium was empty at the time of the quake. Although several commercial buildings collapsed, loss of life was minimized because of the early morning hour of the quake, and it occurred on a Federal holiday (Martin Luther King Day). Also, because of known seismic activity in California, area building codes dictate that buildings incorporate structural design intended to withstand earthquakes. However, the damage caused by the earthquake 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 ground accelerations in the range of 1.0 g. Damage was also caused by fire and landslides. The Northridge earthquake was notable for striking almost the same area as the MW 6.6 San Fernando (Sylmar) Earthquake. In terms of property damage, the earthquake is one of the worst natural disasters in U.S. history.^{[citation needed]}. Some estimates of total damage range as high as 18 billion dollars (U.S.).

Most casualties and damage occurred in multi-story wood frame buildings (e.g. the three-story Northridge Meadows apartment building). In particular, buildings with a weak first floor (such as those with parking areas on the bottom) performed poorly. Numerous fires were also caused by broken gas pipes caused by houses shifting off foundations or by unsecured water heaters falling over.^[5] As is common in earthquakes, unreinforced masonry buildings and houses on steep slopes suffered damage. However, school buildings (K-12), which are required to be reinforced against earthquakes, in general survived fairly well.

An unusual effect of the Northridge earthquake was an outbreak of coccidioidomycosis (Valley Fever), a respiratory disease caused by inhaling airborne spores. The number of cases (203) in Ventura County, CA was roughly 10 times the normal rate in the eight weeks following the earthquake and three people died. It is thought that the spores were carried in large clouds of dust created by seismically triggered landslides. Most of the cases occurred immediately downwind of the landslides.^[6]

Eleven hospitals suffered structural damage and were damaged or unusable after the earthquake.^[7]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 structures by January 1, 2005.

The Northridge earthquake led to a number of legislative changes. Due to the large amount lost by insurance companies because of the earthquake, most insurance companies either stopped offering or severely restricted earthquake insurance in California (and elsewhere). In response, the California Legislature created the California Earthquake Authority (CEA), which is a publicly managed but privately funded organization that offers minimal coverage.^[8] A substantial effort was also made to reinforce freeway bridges against seismic shaking and a law requiring water heaters to be properly strapped was passed in 1995.

[edit] Notes

[edit] See also

• Earthquake
• Northridge, Los Angeles, California
• San Fernando Valley

[edit] External links

• Southern California Earthquake Data Center
• USGS Pasadena
• USC Earthquake Engineering-Strong Motion Group
• SAC Steel Project (Study of welded steel failures)