Scientists have built a new 3D map of the ground beneath Los Angeles, and it shows why some parts of the city will shake harder than others in a major earthquake. The map reveals that downtown LA sits on up to 6.2 miles of sediment, which could amplify shaking during “The Big One.”
The research comes from Valeria Villa, a doctoral student at Caltech, whose team used data from 273 temporary seismic stations placed around the city. Those stations caught faint earthquake waves from distant quakes that humans couldn’t feel, letting researchers build a detailed picture of the rock layers below.
What the Map Shows
The Los Angeles basin is a low spot where sediment has piled up for at least 15 million years. It started underwater, so the lowest layers are oceanic sediments. Later, as tectonic forces pushed the basin onto dry land, more sediment arrived from nearby mountains.
Previous surveys only looked at narrow slices of the basin. This map covers the whole thing in three dimensions. The edges of the basin are thinner — just 0.6 to 2.5 miles of sediment before the hard rock underneath — while the center, under downtown, holds the deepest layers.
Why It Matters
Knowing the shape of the basin’s edges is crucial, according to Elizabeth Cochran, a U.S. Geological Survey seismologist who was not involved in the study. Irregular edges can focus or direct earthquake waves, and that happened in the 1994 Northridge earthquake.
Santa Monica suffered more shaking damage than expected because the seismic waves traveled along the basin edge in a way that concentrated the force. The new map shows the basin’s shape in far more detail, which should help scientists predict where similar focusing might happen next time.
Designing Safer Buildings
The map doesn’t tell geophysicists exactly where shaking will be worst in a big LA earthquake. But it gives them the foundation to run updated models and simulations. The damage depends on many factors, including which fault causes the quake — Los Angeles is surrounded by many, including the famous San Andreas, which could produce a magnitude 7.8 quake.
One key factor is resonance. When buildings shake in time with the sediments below them, the damage is worse. A better understanding of those sediment layers will help researchers ensure buildings are designed to avoid that match.
“You can, for example, change the stiffness of your building and do other things to offset this effect,” Cochran said. “What is important is that you know what the ground is like, and what the larger geologic structure is like, underneath the region where you are building the building to know that this could be an issue.”
Hard Numbers
- Map published: Aug. 4 in JGR Solid Earth
- Lead author: Valeria Villa, Caltech doctoral student
- Co-author: Robert W. Clayton
- Seismic stations: 273 temporary stations around LA
- Downtown sediment depth: Up to 6.2 miles (10 kilometers)
- Basin age: At least 15 million years
- Northridge earthquake: Magnitude 6.7, 1994
- San Andreas potential quake: Magnitude 7.8
Timeline of the Research
| Date | Event |
|---|---|
| Aug. 4 | Paper published in JGR Solid Earth |
| 1994 | Northridge earthquake strikes |
The map is a tool, not a prediction. But the detail it provides means scientists now know exactly what they’re working with — and that knowledge can guide the buildings that sit on top of it.
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