The 1994 Northridge earthquake affected Greater Los Angeles, California, United States, on January 17, 1994, at 04:30:55 PST. The epicenter of the moment 6.7 () blind thrust earthquake was beneath the San Fernando Valley. The shock lasted approximately 8 seconds and achieved a peak ground acceleration of over 1.7 g. It is the largest recorded earthquake in the area's history, slightly surpassing the 6.6 1971 San Fernando earthquake. Shaking was felt as far away as San Diego, California; Turlock, California; Las Vegas, Nevada; Richfield, Utah; Phoenix, Arizona; and Ensenada, Baja California, Mexico. Sixty people died and more than 9,000 were injured. In addition, property damage was estimated to be $13–50 billion, making it among the costliest natural disasters in U.S. history.
Geology The epicenter region of the earthquake was located in the San Fernando Valley, about northwest of downtown Los Angeles. The United States Geological Survey (USGS) placed the hypocenter's geographical coordinates at and at a depth of . Measuring 6.7, it was the largest earthquake recorded in the Los Angeles area since the 1971 San Fernando earthquake ( 6.7). However, unlike the Northridge earthquake, the San Fernando shock occurred on a north–northeast dipping thrust fault beneath the San Gabriel Mountains. Motion during the Northridge event occurred along a south–southwest dipping blind thrust fault beneath the valley. The rupture propagated upward and northwestward along the fault plane for eight seconds at speeds of ~ per second. The fault planes that produced both earthquakes run parallel to each other. Several other faults experienced minor rupture during the main shock and other ruptures occurred during large aftershocks, or triggered events. The previously undiscovered fault that ruptured in 1994 was subsequently named the Northridge Blind Thrust Fault or Pico Thrust Fault.
Thrust faulting in the northern Los Angeles basin occurs in response to north–south crustal shortening. The fault responsible for the earthquake represented a component of a larger system of faults within the "Big Bend" stepover of the San Andreas Fault. Crustal compression occurs along this bend of the San Andreas Fault. The Transverse Range and Los Angeles basin hosts east–west striking thrust faults and folds that accommodate over of the compressive motion annually. This zone comprises north and south dipping faults that run subparallel to each other. Most of these faults are buried structures and only a handful reach the surface.
Aftershocks
Seismic observatories recorded 3,000 aftershocks greater than magnitude 1.5 in the three weeks following the mainshock. By comparing previous aftershock sequences in California against a decay rate equation, the Northridge aftershock activity was highly energetic but decayed marginally more quickly than average. The strongest aftershock, measuring 5.9, was recorded one minute after the mainshock. These aftershocks were distributed in three zones; the first zone illuminated a fault structure dipping 35° to 45° at a depth range of . Another aftershock zone at shallower depth suggested diffuse tectonic behavior of an anticline above. The third aftershock cluster was in the Santa Susana Mountains after a 5.6 aftershock 11 hours after the mainshock. That event and its aftershocks were distributed along a strike and vertically within the crust, likely associated with faults that did not rupture during the mainshock.
Ground effects The earthquake caused uplift across including the San Fernando Valley. Global Positioning System observations indicate the maximum uplift was . Direct observations at Oat Mountain suggest the Santa Susana Mountains were thrusted up . Across the northern valley area, uplift was as much as . The neighborhood of Northridge was raised . The USGS described the surface above the zone of greatest fault displacement as compressed into an "asymmetric dome-shaped uplift". There was also of horizontal deformation which was the largest of its kind measured after the mainshock.
The USGS recorded over 11,000 landslides within ; most were recorded throughout a area that comprised the Santa Susana Mountains and ridges north of the Santa Clara River Valley. On average, these documented landslides were under , although a significant number of them were larger than . Rotational block slides were thought to be in the tens or hundreds, including a handful measuring over 100,000 m3. The largest block slide measured . A majority of them occurred in the weak Tertiary to Pleistocene sediments and were previous landslides that became reactivated.
Strong ground motion
Main shock Mercalli intensities MMI Locations Granada Hills, Los Angeles (I-10)Northridge, Santa Monica, Sherman Oaks San Fernando, Hollywood, ResedaChatsworth, Universal City, Van Nuys Agoura Hills, Beverly Hills, Burbank, Century CityGlendale, Downtown, Culver City Anaheim, Arcadia, Compton, Oxnard, Palmdale Bullhead City, Chino, Inglewood, Long Beach, Westmorland Barstow, Calexico, El Centro, San Diego Parker, Las Vegas, Fresno, Santa Maria A seismometer at the Cedar Hill Nursery in Tarzana observed the greatest horizontal peak ground acceleration (pga) at 1.82 g, south of the epicenter. The recording was one of the largest observed during an earthquake at the time, yet buildings in the immediate vicinity only sustained broken windows and cracked walls and driveways. It garnered international scientific interest among those who visited the ranch in the aftermath, who installed additional instruments to record the aftershocks. The USGS Strong Motion Instrumentation Program ruled out technical malfunction and ground amplification effects, stating that the abnormally high pga recorded by a rock-bolted seismometer was authentic. The same site also recorded an unusually high pga (0.65 g) during a 1987 earthquake, though no abnormal readings were found during earthquakes in 1991 and 1992. USGS seismologist Rufus Catchings suggested the intense shaking was localised and most of the city was unharmed. A possible contributing factor for the effect is the local soil condition and topography. The maximum vertical pga recorded at the same station was 1.18 g.
Several sites in the valley also recorded pga exceeding 1.0 g. There were over 200 recordings of pga exceeding 0.01 g, and the horizontal component pga was larger than average for a reverse-mechanism event. A possible explanation for the high pga is due to the large seismic moment release within a small fault rupture. A peak ground velocity (pgv) of per second was measured at a station in Granada Hills. At the Sylmar county hospital, north–northeast of the epicenter, the pgv in the horizontal component was per second. South of the hospital, the Los Angeles Department of Water and Power Rinaldi Receiving Station recorded a pgv exceeding per second.
Damage and fatalities
Damage occurred up to away although it was concentrated in the west San Fernando Valley, and the cities and neighborhoods of Santa Monica, Hollywood, Simi Valley, and Santa Clarita. The Historic Egyptian Theater in Hollywood was red-tagged and closed as was the Capital Theater in Glendale due to structural damage. The exact number of fatalities is unknown, with sources estimating the number to be 60 or "over 60", to 72,Reich, K. Study raises Northridge quake death toll to 72. Los Angeles Times December 20, 1995 where most estimates fall around 60. The "official" death toll was placed at 57; 33 people died immediately or within a few days from injuries sustained, and many died from indirect causes, such as stress-induced cardiac events. Some counts factor in related events such as a man's suicide possibly inspired by the loss of his business in the disaster. More than 8,700 were injured including 1,600 who required hospitalization. Actress Iris Adrian died in September 1994 from complications of a broken hip she suffered in the earthquake.
Sixteen people were killed as a result of the collapse of the Northridge Meadows apartment complex. The Northridge Fashion Center and California State University, Northridge also sustained very heavy damage – most notably the collapse of parking structures. The earthquake also gained worldwide attention because of damage to the vast freeway network, which serves millions of commuters every day. The most notable was to the Santa Monica Freeway, Interstate 10, known as the busiest freeway in the United States, congesting nearby surface roads for three months while the freeway was repaired. Farther north, the Newhall Pass interchange of Interstate 5 (the Golden State Freeway) and State Route 14 (the Antelope Valley Freeway) collapsed as it had 23 years earlier in the 1971 San Fernando earthquake, even though it had been rebuilt with minor improvements to the structural components. LAPD motorcycle officer Clarence Wayne Dean died because of the collapse of the Newhall Pass interchange, falling 40 feet from the damaged connector from southbound 14 to southbound I-5. He likely did not realize until too late in the early morning darkness that the elevated roadway had collapsed. The rebuilt interchange was renamed in his honor a year later.
Additional damage occurred about southeast in the city of Anaheim, located in Orange County, as the scoreboard at Anaheim Stadium collapsed onto several hundred seats. Most casualties and damage occurred in multi-story wood-frame buildings (such as the three-story Northridge Meadows apartment building). In particular, buildings with an unstable first floor (such as those with parking areas on the bottom) performed poorly. Numerous fires were also caused by broken gas lines from houses shifting off their foundations or unsecured water heaters tumbling. Slope side homes supported on one side by a concrete foundation and stilts on the other were also prone to collapse such as in Sherman Oaks where nine stilt homes crashed down a slope, killing four. In the San Fernando Valley, several underground gas and water lines were severed, resulting in some streets experiencing simultaneous fires and floods. Damage to the system resulted in water pressure dropping to zero in some areas; this predictably affected success in fighting subsequent fires. Five days later, it was estimated that between 40,000 and 60,000 customers were still without public water service.
Valley fever outbreak An unusual effect of the Northridge earthquake was an outbreak of coccidioidomycosis (Valley fever) in Ventura County. This respiratory disease is caused by inhaling airborne spores of the fungus. The 203 cases reported, of which three resulted in fatalities, constituted roughly ten times the normal rate in the initial eight weeks. This was the first report of such an outbreak following an earthquake, and it is believed that the spores were carried in large clouds of dust created by seismically triggered landslides.Schneider, E., Hajjeh, R. A., Der Spiegel, R. A., Jibson, R. W., Harp, E. L., Marshall, G. A., ... Werner, S. B. (1997). A coccidioidomycosis outbreak following the Northridge, Calif, earthquake. Jama, 277(11), 904-908. Most of the cases occurred immediately downwind of the landslides.
Facilities and infrastructure affected Hospitals Eleven hospitals suffered structural damage and were damaged or rendered unusable. Not only were they unable to serve their local neighborhoods, but they also 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 enacted Senate Bill 1953 to amend the Hospital Facilities Seismic Safety Act to require all hospitals in California to ensure that their acute care units and emergency rooms would be in earthquake-resistant buildings by January 1, 2005. Most were unable to meet this deadline and belatedly came into compliance during the 2010s.
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