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 earthquake’s 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 wasn’t even the feared “Big One.” Twenty years ago, no one knew the Northridge fault existed. Thanks to Northridge, the next one won’t 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 Earth’s 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 earthquake’s energy into the sparsely settled mountains north of the Valley.
The 1994 quake wasn’t the first blind thrust to trigger a quake beneath L.A., but it was the most powerful. Northridge swung scientists’ 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.
“When Northridge occurred, people didn’t appreciate these blind thrusts as seismic hazards,” said James Dolan, an earthquake geologist at the University of Southern California in Los Angeles. “Northridge really swung the pendulum back [from the San Andreas] to trying to understand the urban fault system in much more detail,” Dolan told LiveScience’s OurAmazingPlanet. “Although we think these faults aren’t going to generate a magnitude-8 earthquake on the scale of the San Andreas, they are literally underneath your feet.”
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.) They’ve 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.
The digital fault model now allows researchers to model how future earthquakes may damage different parts of Los Angeles — one of the vexing problems of the Northridge earthquake.
“We have a much clearer picture of what we’re up against in terms of the seismic threat facing L.A.,” Dolan said. “In some respects it’s 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.”
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