Los Angeles sits atop up to 6.2 miles of sedimentary basin layers that significantly amplify seismic shaking during earthquakes, according to a 3D subsurface model published in the journal JGR Solid Earth. Researchers deployed 273 temporary seismic stations to map the rock layers beneath the region, revealing precise basin boundaries that direct seismic waves toward vulnerable urban centers like downtown LA and Santa Monica.
Mapping the LA Sedimentary Basin With 3D Seismic Data
Scientists have long understood that Los Angeles rests inside a low geological depression formed over 15 million years of oceanic and mountain sediment accumulation. However, previous geological surveys relied solely on linear arrays crossing single cross-sections of the basin, according to Valeria Villa, a doctoral student at Caltech and co-author of the study.
To capture a comprehensive picture, researchers utilized 273 temporary seismic stations scattered across the metropolitan area. These instruments detected faint, ambient earthquake waves originating from distant tremors that humans cannot feel. By tracking the precise paths and timing of these waves as they passed through subterranean rock layers, the research team constructed the first complete three-dimensional map of the Los Angeles basin.
Did you know? The deepest sediment layers sit directly beneath downtown LA, while the outer edges of the basin thin out to between 0.6 and 2.5 miles of sediment before hitting hard crystalline basement rock.
How Basin Edges Amplify Earthquake Shaking
The newly mapped irregular edges of the basin play a critical role in how seismic energy moves through the city. Geological Survey seismologist who did not participate in the research, these uneven boundaries can focus or direct seismic waves in unpredictable ways.
Cochran pointed to the magnitude 6.7 Northridge earthquake, where Santa Monica suffered heavier damage than expected because seismic waves traveled directly along the basin’s edge. While the map itself does not pinpoint exact damage zones for a future major event, Villa noted that geophysicists can now update hazard simulations to test various scenarios involving regional faults like the San Andreas fault system, which is capable of producing a magnitude 7.8 quake.
Engineering Buildings for Seismic Resonance
Damage during a major Southern California earthquake depends heavily on a phenomenon known as resonance. Resonance occurs when the natural shaking frequency of buildings matches the frequency of the sediment layers underneath them, which intensifies structural destruction.
Understanding the exact depth and geological structure of sediment layers beneath downtown LA allows structural engineers to design safer skyscrapers. According to Cochran, builders can alter the stiffness of a structure and implement other engineering techniques to offset resonance effects once the local geologic profile is known.
Frequently Asked Questions
How deep is the sediment beneath downtown Los Angeles?
The sediment beneath downtown LA reaches depths of up to 6.2 miles, representing the deepest section of the local sedimentary basin.
What caused the Los Angeles sedimentary basin to form?
The basin formed over at least 15 million years, starting as an underwater environment that accumulated oceanic sediments before tectonic forces pushed the area above sea level and mountain erosion added terrestrial sediment layers.
How did researchers create the new 3D underground map?
Researchers deployed 273 temporary seismic stations across Los Angeles to record faint seismic waves from distant, imperceptible earthquakes, using the travel times of those waves to map underground rock layers.
Can this new map predict where the next earthquake will cause damage?
The map alone does not predict specific damage locations, but scientists use the updated data in simulations to model how different faults might impact areas with varying sediment depths and resonance properties.
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