'The Big One' Earthquake Will Hit L.A. Harder Than We Thought, Scientists Say

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Mon, Jan 27, 2014 at 7:55 AM
click to enlarge Simulation of a magnitude-7 quake on the San Andreas Fault. Seismic waves radiate outward, then deep into L.A. - STANFORD UNIVERSITY
  • Stanford University
  • Simulation of a magnitude-7 quake on the San Andreas Fault. Seismic waves radiate outward, then deep into L.A.
A  seismology study by scientists from Stanford and MIT, published in the journal Science on Friday, finds that if the Big One hits the San Andreas Fault near Palm Springs, some seismic waves will travel near the path of the 10 Freeway into the heart of Los Angeles, where the city and its suburbs will suffer stronger ground motions than previously believed. Downtown L.A. will endure three times the shaking of surrounding areas, scientists now say. 

The study shows that a "funneling action" of seismic waves will roll straight into the Los Angeles Basin through a 60-mile-long corridor, striking a 13-million population region that stretches from the Santa Monica Mountains to Newport Bay and inland to the basins of the San Gabriel and Santa Ana rivers. The study confirms a 2006 supercomputer simulation that predicted L.A. could endure worse shaking than long feared. Greg Beroza, a professor at School of Earth Sciences of Stanford University who led the study, explains (See VIDEO below):
The waves travel through that corridor towards Los Angeles, essentially guided into the sedimentary basin that underlies Los Angeles. Once they're in that basin, they reverberate; they get amplified. They cause stronger shaking than would otherwise occur.
(Video credit: Stanford University)

Stanford-MIT scientists devised a detailed new computer simulation using data from "omnipresent" waves that continually vibrate beneath Southern California. What they found is not good news for cities that sit atop the vast "sedimentary" L.A. Basin -  a huge rock bowl filled with eons of accumulated sand and sediment whose rim is made up of the San Gabriel, Santa Monica and Santa Ana Mountains, the Hollywood Hills and Palos Verdes Peninsula. Says Beroza:
"Our study indicates that high rises in downtown Los Angeles will get more strongly shaken by future earthquakes on the San Andreas Fault because of the wave-guide and basin effects that funnel waves from the San Andreas Fault into the Los Angeles Basin where the waves are trapped and amplified. What this means is that the threat posed by earthquakes to Los Angeles is higher than it would be otherwise because those earthquakes will shake Los Angeles more strongly."
At their deepest point, the L.A. Basin sediments reach more than 30,000 feet into the earth, where they hit solid rock. Mount Everest, positioned upside down, could fit into this bowl. 

"Those sediments are not compacted, like sand on the beach," says Marine Denolle, first author of the study, who recently received her PhD in geophysics from Stanford. "They're very compliant. So they can move very easily.

The study says:
Our ground motion predictions show strong seismic amplification in the Los Angeles sedimentary basin compared to surrounding areas...[W]e find seismic amplification in downtown Los Angeles with peak amplitudes up to three times larger than in surrounding areas.
click to enlarge Palos Verdes Peninsula and the Los Angeles Basin, with the San Gabriel Mountains in the distance. - DOC SEARLS/FLICKR
  • Doc Searls/Flickr
  • Palos Verdes Peninsula and the Los Angeles Basin, with the San Gabriel Mountains in the distance.

The study used "virtual earthquakes" to predict ground motion. Scientists used data taken of weak vibrations beneath the ground - which are actually generated by the movement of ocean waves and then transmitted into the earth - to construct a major, fake earthquake, according to Denolle. 
"Those waves [are] transferred to the sea bottom. These get transmitted to the crust where we actually have instruments recording," says Denolle. 
Weak ground vibrations, known as "the ambient seismic field" in geology, are present all the time, but they're so mild that they're unnoticeable. Scientists have known about the ambient seismic field for more than 100 years, but viewed them as interference in their efforts to study earthquakes. Now, with this study, scientists made use of them - as proxies for strong seismic waves created by a big earthquake.

See also: The First 15 Minutes After the Big One

Beroza explains, "Even though billions of times weaker than earthquake waves, they interact with the complex geological structure of the crust just the same way."

In the study, the scientists considered 96 simulated ruptures, or "examples of 96 possible earthquake sources" by selecting different epicenters and rupture speeds on a 40-mile segment of the San Andreas Fault from roughly the junction of 10 Freeway and Highway 111 on the north (San Gorgonio Pass) to where the 10 crosses the fault on the south near Coachella. 

Denolle explains in an email: 

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