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.
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.
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:
“We do not know which one actually would occur, so we account for different scenarios to look at an average of all possible scenarios.
“When you drop a rock in flat water, you can see circles moving outward on a growing circle. That's exactly what seismic waves do, they move outward like this. Because the Earth is more complex than water, the circle is not perfect.”
Devices placed in the ground for four months along the fault in Palm Springs collected data from the ever-present ground vibrations.
This study confirming the 2006 San Andreas simulation – as well as reports of a new state geological map showing that the separate Hollywood Earthquake Fault runs directly beneath a new luxury complex and under Hollywood's proposed Millennium skyscrapers – prompted one respected geologist to call for major changes at the Los Angeles Department of Building and Safety.
Robert H. Sydnor, an engineering geologist who worked for California Geological Survey for 25 years and is a life member of the Seismological Society of America, said via email:
Recent building permits and recent Environmental Impact Reports issued by the City of Los Angeles do not properly characterize the known problem of amplified earthquake ground-motion in Hollywood and downtown Los Angeles.
I have read 2013 official comments from the LA Planning Department that wrongly state that the earthquake ground-motion in Hollywood is no greater than elsewhere in the city. With the active Hollywood Fault cutting right through Hollywood, plus basin-edge amplification effects in strong-motion seismology, these LA City official comments are egregiously false and scientifically wrong.
It is time for a significant internal change within the Department of Building and Safety of the City of Los Angeles. Building permits for multimillion-dollar high-rise structures are being issued by LA City officials who have no understanding of strong-motion seismology, Holocene-active faults, and geologic hazards. This is a significant disservice in public safety.
Sydnor noted that studies on earthquake ground motion in Santa Monica and Los Angeles have been heavily published since the 1994 Northridge Earthquake, which killed 57 people and left more than 9,000 injured.
Beroza leaves Angelenos with this thought:
“I should note that this [the San Andreas Fault] is not the only earthquake threat that Los Angeles faces. There are earthquake faults within Los Angeles that rupture in earthquakes less frequently than the San Andreas Fault, but when they do, they will shake Los Angeles very strongly.”