1994 Northridge earthquake

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The Northridge earthquake occurred on January 17, 1994 at 4:31 AM Pacific Standard Time in Reseda, a neighborhood 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] Seventy-two people died as a result of the earthquake with more than 12,000 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 20 miles (31 km) northwest of downtown Los Angeles near the community of Northridge. The actual epicenter of the quake was in Reseda, near the intersection of Reseda Blvd. and Saticoy St. However, it took several days to pinpoint the epicenter with accuracy, and in the meantime the media had already dubbed it "The Northridge Earthquake." The name stuck, in part due to the extensive damage and loss of life in 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 (10.56mi). 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.

[edit] Damage and fatalities

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

The freeway "bent" just before the section collapsed

Kaiser Permanente building

Collapsed apartment building

Street damage

Damage occurred up to 85 miles (125 km) away, with the most damage in the west San Fernando Valley, and the cities of Santa Monica, Simi Valley and Santa Clarita. Seventy-two people died as a result of the earthquake, and more than 7,000 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 with improved structural components.^[4] One life was lost in the Newhall Pass interchange collapse: LAPD motorcycle officer Clarence W. Dean fell 40 feet from the damaged connector from southbound 14 to southbound I-5 along with his motorcycle. Because of the early morning darkness, he was unaware that the elevated roadway beneath him had dropped, and was unable to stop in time to avoid the fall and died instantly. When the interchange was rebuilt again one year later, it was renamed the Clarence Wayne Dean Memorial Interchange in his honor.

Additional damage occurred about 50 miles south in Anaheim as the scoreboard at Anaheim Stadium collapsed onto several hundred seats. Fortunately, the stadium was empty at the time of the quake due to the time of day the earthquake occurred. Although several commercial buildings also 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, Jr. 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 M_W 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 $25 billion.

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] In the San Fernando Valley, several undergound gas and water mains were severed, and there were some streets where there were fires burning through floods. 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.

[edit] Base isolation

Before the Northridge earthquake occurrence, a heavy damping mechanism sometimes incorporated in vibration control technologies and, particularly, in base isolation devices was considered a valuable source of suppressing vibrations thus enhancing a building's seismic performance. However, for the very pliant systems such as base isolated structures, with a relatively low bearing stiffness but with an high damping, the so-called "damping force" turned out the main pushing force at this strong earthquake [1]. The finding created a theoretical ground in earthquake engineering for a damping-disengaged base isolation technology called Earthquake Protector [2].

[edit] Valley Fever

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 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]

[edit] Hospitals

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.

[edit] Television production stoppages

The quake stopped production of movies and TV shows filming in the area at the time, most notably The Simpsons season 6's first two episodes "Bart of Darkness" and "Lisa's Rival".

[edit] Changes

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] External links

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