25 years after the 6.7
Northridge: the science of the quake and what awaits California
On this day 25 years ago, a “big one” rattled Southern California. Billions of dollars in direct damage, thousands injured and dozens lost their lives as a result of the 6.7-magnitude Northridge earthquake that lasted about 20 seconds.
Since that time, experts and the general public have learned much from seismic incidents, but is it enough for what awaits?
Tom Heaton, professor of engineering seismology at the California Institute of Technology, said changes and advances in technology, “puts us in a better spot than we were 25 years ago.”
But the question still remains, “When does the next bus come?” he said. “Right now, the bus (or earthquake) is late and it’s going to get here, but I can’t tell you when.”
Here’s a breakdown of how far we’ve come and what lies ahead:
New faults
Whether an expert on earthquakes or not, Californians have heard about the San Andreas fault. What many didn’t know in 1994, including seismologists, was that the Northridge temblor occurred on a hidden fault.
“These types of faults are called ‘blind thrust faults,’” said Vincent
Devlahovich, an environmental sciences professor at College of the Canyons. “We were always mapping faults at the surface, where you could see them and know it was a fault. Since ’94, we’ve mapped out thousands and thousands of blind thrusts all throughout Los Angeles County.”
The Santa Clarita Valley sits only about 15 miles from the San Andreas fault, said Devlahovich. “There’s two places where it’s close. It’s close right in north Palmdale at Avenue S,” and, “at the (Interstate) 5 freeway, right at the top of the grade (near) Frasier Park.”
Infrastructure
What does it mean now that experts know the location of several more faults? Devlahovich said this can improve the way buildings are constructed.
“This means we can build things stronger that are on top of faults,” he said. High-rise structures model those built in Japan, where the building does not entirely connect to the bedrock. “They make them so they can move. When you have motion, then it absorbs a lot of the seismic energy and allows the building to survive.”
Perhaps one of the major lessons of the Northridge earthquake, Heaton said, was the damage to welded steel frame structures. In some cases, buildings were completely demolished or evacuated as a result of connection failure or “significant permanent lateral displacements,” according to a report by the
University of California, Berkeley. Today, with changes to building codes, structures are five times less likely to collapse than steel buildings prior to the 1994 jolt, said Heaton.
The Northridge earthquake was the last in a series of back-to-back quakes. A 6.6 quake in Sylmar in 1971, which killed 64 people, taught engineers that concrete buildings constructed about 60 years ago had serious flaws when it came to columns.
“Those in the ’50s and ’60s misunderstood how to connect steel with concrete, meaning those were susceptible to pancake falling — one falling on top of the other,” said Heaton. This issue reappeared in 1994, including with the collapse of Interstate 5 connector, which killed L.A. police officer Clarence W. Dean. The motorcycle officer was on his way to help with earthquake response, still in the early-morning darkness, when he rode his motorcycle off the collapsed overpass.
Heaton said there has been slow and challenging progress on safety standards, including the required retrofitting of existing buildings.
Technology
When the Northridge quake struck, outdated technology affected efficiency.
Scientists struggled to quickly share information about the epicenter and size of the ’94 temblor. Even more so in 1971, when first responders took about three hours to respond to the Veterans Hospital in Sylmar due to poor communication and off-scale seismographic systems.
For all of Southern California, only three global positioning system stations measuring the Earth’s plates were available when the Northridge quake happened. Today, more than 250 stations are part of the SoCal Integrated GPS Network, allowing scientists to update fault maps and improve the probability of future incidents.
While early-warning technology is not new, Heaton said the public can now get a warning when shaking is about to occur seconds before it happens — something not “nearly possible in 1994.”
At the end of 2018, the city of L.A. launched the early-warning app ShakeAlertLA. App users would receive a push alert seconds before a quake of 5.0 magnitude or greater, should one occur anywhere in L.A. County.
‘The big one’
The San Andreas fault, known as a system or zone with several major numerous branches, is the one everyone connects to “the big one.”
The exact date remains unknown, but Devlahovich said the expected event, “which hasn’t happened for over 150 years, is going to be in the mid-7s (magnitude). That one is going to make Northridge look like a circus or a party.”
The Santa Clarita Valley’s highest risk area is near the Castaic Reservoir, said Devlahovich. “That’s really scary. The dam that holds our water is over 300 feet tall, and could suffer major failure with shaking.”
The best advice, he and Heaton agree, is to have an emergency plan and “drop, cover and hold on.”