Northridge Earthquake: Is Los Angeles Safer 20 Years Later?
By Becky Oskin, LiveScience.com, January 17, 2014
Roaring like a freight train from hell, the Northridge earthquake threw sleeping Angelenos from their beds at 4:31 a.m. on Jan. 17, 1994.
The earthquakeâs shaking was stronger than the force of gravity, lifting furniture off the floor and buildings off their foundations. Los Angeles firefighters watched their massive fire trucks hop across a station garage in time with the seismic waves.
At least 57 people died and nearly 9,000 people were injured. Some 82,000 buildings were damaged or destroyed. Seven freeway bridges collapsed. With more than $40 billion in property and economic losses, Northridge was one of the most expensive natural disasters in U.S. history.
California geologists say their big cities are overdue for another devastating earthquake. No major earthquake has hit one of the major urban areas since Northridge--and at a magnitude-6.7, it wasnât even the feared âBig One.â Twenty years ago, no one knew the Northridge fault existed. Thanks to Northridge, the next one wonât be such a surprise. The big question is: Are Californians ready?
The Northridge fault is a so-called blind thrust, a fault buried 11 miles (17 kilometers) deep in Earthâs crust. Leaving no telltale signs on the surface, geologists had no idea the fault angled beneath the San Fernando Valley, veering northward from Reseda up toward Chatsworth. That angle saved much of Los Angeles 20 years ago, directing the earthquakeâs energy into the sparsely settled mountains north of the Valley.
The 1994 quake wasnât the first blind thrust to trigger a quake beneath L.A., but it was the most powerful. Northridge swung scientistsâ attention back toward mapping earthquake hazards directly underneath the Los Angeles area, instead of focusing on the sleeping giant next door, the San Andreas Fault.
âWhen Northridge occurred, people didnât appreciate these blind thrusts as seismic hazards,â said James Dolan, an earthquake geologist at the University of Southern California in Los Angeles. âNorthridge really swung the pendulum back [from the San Andreas] to trying to understand the urban fault system in much more detail,â Dolan told LiveScienceâs OurAmazingPlanet. âAlthough we think these faults arenât going to generate a magnitude-8 earthquake on the scale of the San Andreas, they are literally underneath your feet.â
In the past two decades, researchers have done their best to find every fault fracturing Southern California west of the San Andreas Fault. (The eastern part of the state still hides some mysteries, scientists think.) Theyâve also gauged the history of past earthquakes on these faults, when possible, and studied how each fault influences others nearby.
The result looks like a CT scan of the Earth, in which faults are the bones, linked together in a complex framework.
âWe have a much clearer picture of what weâre up against in terms of the seismic threat facing L.A.,â Dolan said. âIn some respects itâs somewhat worse than what we thought 20 years ago, [because] we know that first of all there are big blind thrust faults underlying most of metropolitan L.A. and we know that at a number of occasions in the past these have ruptured to generate very large magnitude earthquakes.â
Before the sun rose on Jan. 17, 1994, the San Fernando Valley looked like a scene from an apocalyptic disaster movie. Dust in the air glowed as more than 1,000 fires burned in the darkness of a city without power. But on the other side of the Santa Monica Mountains, the southern border of the Valley, the city of Santa Monica also suffered a surprising amount of damage. And some pockets in the Valley were left mostly untouched, while others were devastated.
The Northridge earthquake clearly showed how the different soils in an area, and the underlying geologic structures, can magnify an earthquakeâs strength. In Santa Monica, a lens-shaped fault beneath the mountains focused the seismic waves toward the city the way a magnifying glass can concentrate the sunâs rays. In the Valley, the earthquake waves ping-ponged back and forth, bouncing off underground structures and occasionally meeting and intensifying. The strongest ground motion ever recorded in a California earthquake was in the city of Tarzana. The sedimentary layers under Tarzana focused and strengthened the shaking until it was nearly twice that of Earthâs gravity.
Californiaâs urban earthquake faults have been remarkably quiet for the past 20 years. During this time, scientists and emergency planners sought to prepare for the next earthquake disaster. The state runs an annual earthquake drill, the Great ShakeOut, and has installed the framework for an early earthquake warning system similar to Japanâs.
Scientists have simulated a magnitude-7.8 earthquake on the southern San Andreas Fault, which they say is about 150 years overdue for a major temblor. A quake such as that would kill 1,800 people and injure 53,000--relatively minor loss of life in a metropolis of millions, though still a major disaster in the United States.
The big concern is economic loss. Californiaâs seismic building code is meant to preserve life, not preserve buildings, and 50 percent of the Los Angelesâs structures may be uninhabitable after a San Andreas temblor.
âIf you get a large earthquake, weâre probably expecting to see damage in many buildings, even in modern buildings,â said James Wight, a structural engineer at the University of Michigan. âThey are not designed to go damage-free,â he told LiveScienceâs OurAmazingPlanet.
The cityâs old, brittle water pipes would break, along with its water supply lines from outside, such as the California Aqueduct. With no water for weeks or months, Los Angeles could suffer severe, long-lasting economic losses, similar to New Orleans after Hurricane Katrina, said Lucy Jones, a seismologist with the U.S. Geological Survey.
âAt best, it will take us six months to get our water back,â Jones told the Los Angeles City Council this week.
âWhat is at stake is really our whole urban society,â Jones said. âEvery death in an earthquake is horrible, but people are far more likely to die on our freeways. We are more worried about how society will function afterward.â
