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Northridge Earthquake - January 17, 1994

"At 4:31 A.M. local time, Monday, January 17, 1994 the Northridge earthquake struck the San Fernando Valley region of Southern California with a moment magnitude measured at 6.7 and focal depth of 19 km. The earthquake was centered 32 km west-northwest of Los Angeles along a south-dipping, blind thrust fault. Little if any surface faulting was produced. The earthquake resulted in 57 deaths, more than 5,000 injuries, and structural damage including instances of partial or complete structural collapse. Estimates of more than $20 billion in property damage make this earthquake the costliest seismic disaster in U.S. history. The earthquake touched off considerable scientific and engineering investigation." - http://nisee.berkeley.edu/northridge/


Earthquake Hazard - quoted directly from http://www.rms.com/Reports/northridgeeq_retro.pdf

The Northridge earthquake highlighted the importance of blind thrust faults, in particular contributing to hazard in and around Los Angeles. During the 1980s, seismic source models for California had come to assume that the principal contribution to hazard came from the surface mapped faults, even though two previous major earthquakes: the 1983 Coalinga Earthquake and the 1987 Whittier Narrows Earthquake had occurred on hidden subsurface structures. While the distribution of the compressional faulting caused by the "big bend" in the San Andreas Fault in the northern portion of the Los Angeles basin was generally recognized, there was no overall understanding of the extent or seismic potential of blind thrust faults in this region.

Post-Northridge a comprehensive research mission was initiated to search for similar active faults using geophysical surveying techniques and deep borehole information. Within a year, a complex series of "blind thrust" faults had been identified underlying the Los Angeles Basin and San Fernando Valley regions as well as in other areas of California, such as the Central coast region, the Bakersfield region, and the Santa Clara Valley.

The identification of the blind thrust faults resulted in major changes to the seismotectonic understanding of active earthquake sources and earthquake recurrence rates in Southern California. In 1995, the Southern California Earthquake Center (SCEC), composed of a consortium of California research universities, released a landmark study, proposing a completely new hazard model for the region that incorporated all those newly identified subsurface faults. In 1996, the U.S. Geological Survey showed that blind thrusts contribute (on average) about 15% of the seismic hazard in the region (when measured in terms of the ground motion that has a 10% probability of exceedence in 50 years) The exact configuration of these buried faults has continued to be debated and around Los Angeles the extent of the faults has been reduced relative to the initial maps constructed in 1995. The latest consensus on the location, extent, activities, and maximum magnitudes of the blind thrusts in California was included in the development of the National Seismic Hazard Maps, released by the USGS in 2002-2003.


Freeway Damage - quoted directly from http://ntl.bts.gov/lib/jpodocs/repts_te/13775.html


I-5 Damage

The most severe damage caused by the Northridge earthquake was on I-5. I-5, the main north/south artery in Southern California connecting the Los Angeles basin to Northern California, had collapsed at both the interchange with SR-014 (which connects the cities of Lancaster and Palmdale with Los Angeles) and on top of Old Road at the Gavin Canyon underpasses. I-5 also suffered damage north of the I-5/SR-14 interchange, effectively closing the main highway link over the mountains. Figure 4 on the next page shows the damage at the Gavin Canyon interchange.



Figure 4. I-5 Damage at Gavin Canyon
Figure 4. I-5 Damage at Gavin Canyon


Figure 5. Damage at I-5/SR-14 Interchange
Figure 5. Damage at I-5/SR-14 Interchange


SR-14 Damage

The connector at the I-5/14 interchange in Sylmar collapsed. This connector was the only freeway link over the mountains to Lancaster and Palmdale. Except for the extensive damage at the interchange, Route 14 to the north was unaffected. Figure 5, on the previous page, shows an aerial view of the damage at the I-5/SR-14 interchange.


I-10 Damage

The Santa Monica freeway, a highly traveled east/west corridor, was destroyed at four overpasses: La Cienega, Venice, Washington and Fairfax Streets. Structural damage to buildings, roads, and utilities also occurred in the I-10 corridor connecting Los Angeles and Santa Monica, with the most severe damage in Northridge. Figure 6 shows the damage at I-10 and La Cienga Boulevard.


Figure 6. Damage at I-10/La Cienga Boulevard
Figure 6. Damage at I-10/La Cienga Boulevard


SR-118 Damage

SR-118, just north of Northridge, the earthquake epicenter, had sustained extensive damage. The eastbound roadway had collapsed completely at two separate places in the Granada Hills area over the intersections of Gothic Avenue and San Fernando Mission Boulevard. Additional damage over Balboa Boulevard and other areas along SR-118 closed the entire section of highway between I-405 and I-210 in both directions. At all these locations, closures were immediate and no freeway traffic was able to pass through the damaged zones.


Significant Incidents - quoted directly from http://www.ci.la.ca.us/lafd/eq.htm

One of the most significant incidents in terms of life loss occurred at a 3-story, 120 unit apartment complex located at 9565 Reseda Boulevard. Task Forces 70 and 73 were first to arrive on the scene and reported the collapse of a 3-story apartment building. The first floor was crushed in the collapse, reduced to a crawl space of two to three feet. Many of those that perished were buried under collapsed walls and the ceiling.

Ultimately involved in the rescue operation were seven L.A. City Fire companies, an L.A. City Fire USAR team, and USAR teams from L.A. County, Riverside County, Orange County and the State Office of Emergency Services.Several residents self-evacuated prior to and shortly after the arrival of fire companies. Twenty-five to thirty residents were assisted out of the structure by firefighters. Eight were evacuated by firefighters and USAR teams and then were transported to area hospitals in conditions ranging from serious to critical.

To further assist with locating trapped victims, L.A.P.D. search dogs were brought in. Unfortunately, all that remained trapped had perished. Sixteen fatalities in all resulted from the collapse of this building.

Another significant incident occurred at the Northridge Fashion Mall. A street sweeper that was working on the first floor of a 3-story parking structure was trapped in his pick-up truck when the entire parking structure collapsed. An L.A. City Fire USAR team was at the scene but was redirected to the incident occurring on Reseda Boulevard due to the number of lives involved. Light Force 89, Engine 1, Heavy Utility 56 and a Gas Company Crew continued the rescue effort. As the incident progressed an L.A. County USAR team was directed to the location to assist. Firefighters assisted by the Gas Company crew utilized the 'Jaws of Life', hydraulic jacks, sledge hammers, saws, and the Gas Company's jack hammer to reach the man who was pinned and in critical condition. Aqueous Film Forming Foam had to be flowed down under the crushed pick-up truck, to prevent the ignition of leaking fuel.

It took firefighters six to seven hours to successfully extricate the man. He was transported to the U.C.L.A. Medical Center in critical condition by Air Ambulance. His condition has since been upgraded to serious.

A significant loss of property resulted from structure fires that occurred in three separate mobile home parks. These three fires simultaneously broke out immediately following the earthquake. The cause of these fires appears to have been the rupture of natural gas valves and/or mains.Twenty to thirty mobile homes were lost at the mobile home park located at Cobalt and Rinaldi. Twenty-three mobile homes were lost at the park located near the intersection of Olden and Ralston. Sixty-seven mobile homes were lost at the park located at Glen Oaks and Foothill. Arriving firefighters encountered fire hydrants with little or no pressure. To combat the fires, relay operations and water shuttles were implemented. Firefighters evacuated all residents and no serious injuries were reported.

Also significant were the ruptures of a large natural gas main and a water main at the intersection of Balboa and Rinaldi. These ruptures occurred immediately following the earthquake. Subsequent to the rupture, the natural gas main ignited, and five single family homes were consumed in the ensuing fire. Area residents were evacuated by arriving firefighters and no serious injuries were reported.

At approximately 0520 hours, Engine 97 responded to an incident at 3999 Sunswept Drive in the Studio City area. Upon arriving on scene, Engine 97 reported the complete collapse of a three and four story single family dwelling which was located on a hillside. Two of the four residents of the home had successfully extricated themselves from the structure prior to the arrival of firefighters. One resident was killed in the collapse and the fourth resident was trapped and in need of extrication. Due to the precarious position of the collapsed structure on the hillside and the continued after shocks, firefighters initially were unable to successfully rescue the still entrapped resident. By 0900 hours, firefighters from Engines 78 and 97, along with several L.A.P.D. Officers, had successfully effected the extraction of the trapped resident. The woman had only sustained minor injuries in the collapse of the home.


Lessons Learned From Steel Buildings - quoted directly from http://nisee.berkeley.edu/northridge/mahin.html


Every earthquake provides new lessons for the earthquake engineering profession. The widespread damage to welded steel moment resisting frame systems was one of the major overall lessons of the Northridge earthquake. The brittle nature of the fractures detected in numerous welded steel beam to column connections, essentially invalidated historic design approaches and code provisions based on "ductile" structural response.

The most commonly observed damage occurred in or near the welded joint of a girder bottom flange to the supporting column flange; complete brittle fractures occurred in many cases. Damage was so severe in some buildings that all of the moment resisting connections at one or more floors failed, or significant permanent lateral displacements occurred. In one case, damage was so severe the building was demolished, and several buildings were evacuated.

Thus far, more than 150 damaged buildings have been identified, including hospitals and other health care facilities, government, civic and private offices, cultural and educational facilities, residential structures, and commercial and industrial buildings. Damage occurred in new as well as old buildings; in tall as well as in short structures. While inadequate workmanship was believed to play a role in the damage observed in some structures, most damaged buildings are believed to be constructed consistent with modern codes and standards of practice, The effect of these observations has been a loss of confidence in the procedures used in the past to design and construct welded connections in steel moment frames, and a concern that existing structures incorporating these connections may not be sufficiently safe.

A particularly disconcerting aspect of this damage is that it often occurred without accompanying distress to architectural finishes and cladding. As a result, reconnaissance reports immediately following the Northridge earthquake often cited the apparent excellent behavior of steel frame buildings. However, severe damage found in buildings under construction at the time of the earthquake, and detailed investigations of WSMF buildings which suffered increasing amounts of damage during aftershocks, quickly identified the true performance.

Current professional judgment is that the historic practices used for the design and construction of WSMF connections do not provide adequate reliability and safety, and should not continue to be used in the construction of new buildings intended to resist earthquake ground shaking through inelastic behavior. As a consequence, pre-qualified connection details and design methods contained in the major U.S. building codes have been rescinded, and emergency code provisions stipulate that new designs be substantiated by testing or test-verified calculations.