John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
Supercomputers reveal a natural channel in the earth that could bring potent shaking to the megacity.
When the next major earthquake comes to Southern California – and experts even years ago were saying "The Big One" is long overdue – the shaking could be much more powerful than initially predicted. That's especially bad news for Los Angeles, a city ridden with obsolete concrete buildings that could easily collapse during a ground-slamming quake.
Scientists at Stanford and elsewhere have made a new assessment of the region's seismic vulnerability using supercomputers and something mysterious called the "ambient seismic field." This field, to put it simply, is composed of super-weak waves constantly thrumming through the earth due to the motion of the ocean. By tapping into the field, the researchers were able to simulate how a rupture originating from the San Andreas Fault would send vibrations through a natural channel of sediment that leads right to L.A.'s doorstep. There energy would pool, amplify, and likely cause severe destruction.
Scientists have predicted the existence of L.A.'s self-dooming wave funnel since 2006, but the Stanford team says this is the first time the effect has been confirmed. You can watch the video below for computer demonstrations of the predicted rush of energy, and here's a screenshot from it showing waves headed through the San Bernardino Valley toward L.A.:
When the seismic energy reaches the city, you can see in this supercomputer image where it's likely to reach peak heights of violence. Note the areas of higher-velocity shaking (yellow and red) concentrated in the metropolitan region:
(Kim Olsen / San Diego State University, Amit Chourasia / San Diego Supercomputer Center, and Southern California Earthquake Center)
As to this heavily localized shaking, the scientists explain:
The virtual earthquake approach also predicts that seismic waves will become further amplified when they reach Los Angeles because the city sits atop a large sedimentary basin. To understand why this occurs, study coauthor Eric Dunham, an assistant professor of geophysics at Stanford, said to imagine taking a block of plastic foam, cutting out a bowl-shaped hole in the middle, and filling the cavity with gelatin. In this analogy, the plastic foam is a stand-in for rocks, while the gelatin is like sediments, or dirt. "The gelatin is floppier and a lot more compliant. If you shake the whole thing, you're going to get some motion in the Styrofoam, but most of what you're going to see is the basin oscillating," Dunham said.
As a result, the scientists say, Los Angeles could be at risk for stronger, and more variable, ground motion if a large earthquake – magnitude 7.0 or greater – were to occur along the southern San Andreas Fault, near the Salton Sea.
Whether this new assessment will push the city to force reform upon the owners of unsafe properties is unknown. Right now, more than 1,000 concrete structures could collapse during a major quake, including homes and offices, according to a recent investigation by the Los Angeles Times. Even an ultra-conservative amount of temblor-induced mayhem would destroy up to 50 buildings and expose "thousands to injury or death," warns the paper.
The region's Northridge earthquake of 1994, for what it's worth, killed 57 people, injured more than 5,000, and caused roughly $20 billion in damages. For an indication of SoCal's ongoing seismic peppiness, check out the very active LA QuakeBot.
Top image: A damaged medical building in Los Angeles after the Northridge quake of January 17, 1994. (Gary B. Edstrom / Wikipedia)