The 1994 Northridge earthquake occurred on January 17, 1994 at 4:30:55 am Pacific Standard Time in the city of Los Angeles, California. This earthquake was considered moderate, with a moment magnitude of 6.7, but was the most monetarily costly quake in United States history.
The epicenter was in the San Fernando Valley, originally believed to be in the community of Northridge (later shown to actually have occurred within the neighboring community of Reseda), about 32 km (20 mi) northwest of downtown Los Angeles. The National Geophysical Data Center places the epicenter's geographical coordinates at 34.213 N and 118.537 W, which is at the end of Elkwood St., just east of Baird Av.
Damage occurred up to 77 km (52 mi) away, with the most damage in the west San Fernando Valley and the city of Santa Monica. Fifty-seven people were killed, and over 1,500 were seriously injured. Major freeway damage occurred up to 32 km (20 mi) from the epicenter. Portions of Interstate 10 (the Santa Monica Freeway) and Interstate 5 (the Golden State Freeway) collapsed and had to be rebuilt.
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 occured on a federal holiday. 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 had not been built to code.
Through investigation of damaged buildings, it was discovered that structural steel did not perform as well as expected. Because nobody anticipated that steel would fail at the rate that it did, buildings that were not expected to have major damage were red-tagged many months after the quake, when the inspectors finally got to them. This new information on the nature of earthquake damage resulted in even more stringent building codes.
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 third very destructive earthquake to occur in California in 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 1994 event is the most damaging earthquake to strike the United States since the San Francisco earthquake of 1906. In terms of financial loss, the earthquake is also one of the worst natural disasters in U.S. history, comparable to Hurricane Andrew in 1992.
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 structures by January 1, 2005.