1994 Northridge Earthquake: The Quake That Shook Los Angeles

The Northridge earthquake struck the San Fernando Valley of Los Angeles at 4:30 a.m. on 17 January 1994. The magnitude 6.7 quake killed 57 people, injured more than 8,700, brought down sections of three freeways and damaged tens of thousands of buildings. It was one of the costliest natural disasters in US history and revealed hidden weaknesses in modern construction that engineers had believed to be safe.

A hidden fault

Southern California is crossed by many faults linked to the San Andreas system. The Northridge earthquake did not occur on any of the well-known surface faults. It ruptured a blind thrust fault, one that slopes beneath the surface without breaking through it, about 18 km below the ground.

Because the fault was hidden, it had not been mapped as a significant hazard. When it slipped, the ground above was pushed upward by tens of centimetres and the Santa Susana Mountains rose slightly. The epicentre was in Reseda, a short distance from the community of Northridge that gave the quake its name.

The shaking

The shaking lasted only about 10 to 20 seconds, but it was exceptionally strong. An instrument in Tarzana recorded a horizontal acceleration of about 1.8 times the force of gravity, one of the highest values ever measured. Because the quake struck before dawn on a public holiday, Martin Luther King Jr. Day, roads and offices were nearly empty, which almost certainly saved many lives.

The worst single disaster occurred at the Northridge Meadows apartment complex, where the ground floor of a three-storey building collapsed and killed 16 residents. Buildings with weak ground floors, such as apartments built over open parking spaces, failed across the valley.

Collapsed freeways and buildings

Elevated freeways that had been built before modern seismic standards failed in several places. Sections of the Santa Monica Freeway (Interstate 10), one of the busiest roads in the country, fell onto the streets below. The interchange between Interstate 5 and State Route 14 collapsed for the second time in its history, having also been damaged in the San Fernando earthquake of 1971; a police officer riding a motorcycle in the dark was killed when he drove off the broken edge.

A parking structure at California State University, Northridge, collapsed, as did a large car park at the Northridge Fashion Center shopping mall. Several hospitals were damaged and had to evacuate patients. Broken gas lines caused fires, and water mains burst, while power failed across much of the Los Angeles area in the first hours.

Response and recovery

Governor Pete Wilson and Mayor Richard Riordan coordinated the state and city response, and President Bill Clinton visited the area two days after the quake. Federal assistance for housing repairs, public infrastructure and loans eventually ran to many billions of dollars, among the largest relief efforts for any US disaster up to that time.

The rebuilding of the Santa Monica Freeway became a symbol of recovery. The state offered the contractor large bonuses for early completion, and the freeway reopened on 12 April 1994, less than three months after the earthquake and well ahead of schedule.

Lessons for engineering and insurance

The most surprising discovery came months later. Inspectors found that welded connections in more than 100 steel-frame buildings had cracked, although the buildings looked undamaged from outside. This led to a large research programme by the US government and engineering bodies and to new guidelines for designing steel moment frames.

California passed a law in 1994 requiring hospitals to meet stricter seismic standards. Insured losses of about 15 billion US dollars led many insurers to stop selling home policies in the state, so in 1996 California created the California Earthquake Authority, a publicly managed but privately funded earthquake insurer. Much later, in 2015, Los Angeles required owners to strengthen thousands of soft-storey apartment buildings like Northridge Meadows.

By the numbers

Timeline

• 1971, 9 February: The San Fernando earthquake damages the same region.
• 1994, 17 January, 4:30 a.m.: The magnitude 6.7 Northridge earthquake strikes.
• 1994, 17 January: Fires, freeway collapses and building failures across the San Fernando Valley and west Los Angeles.
• 1994, 19 January: President Clinton visits the damaged area.
• 1994, 12 April: The rebuilt Santa Monica Freeway reopens.
• 1996: The California Earthquake Authority is created.
• 2015, October: Los Angeles adopts a mandatory retrofit ordinance for soft-storey buildings.

Why it matters

Northridge struck a region that thought itself well prepared after the Loma Prieta earthquake of 1989, and it showed that hidden faults and hidden flaws in modern buildings could still cause enormous losses. Exactly one year later the Kobe earthquake in Japan delivered a similar lesson on a far deadlier scale. Together they reshaped earthquake engineering, much as the San Francisco earthquake of 1906 had done for an earlier generation.