High-Resolution 3D Map Reveals Subsurface Risks for Los Angeles

Researchers have produced the most detailed three-dimensional map of the geological structure beneath Los Angeles, uncovering a fragile foundation that poses significant risks during seismic events. The model indicates that downtown Los Angeles rests on as much as 6.2 miles (10 kilometers) of sediment, a layer that is likely to intensify ground shaking from any major earthquake in the region.

While the general location of the sedimentary basin has been known for some time, this new study offers unprecedented clarity regarding its shape and boundaries. Elizabeth Cochran, a seismologist with the U.S. Geological Survey who was not involved in the research, noted that the enhanced detail allows scientists to better understand how irregular basin edges can focus or direct seismic waves.

Cochran pointed to the 1994 Northridge earthquake as a case in point, explaining that Santa Monica suffered greater damage than anticipated because seismic waves traveled along the basin’s edge. The Los Angeles basin itself is a depositional low spot that has accumulated sediment for at least 15 million years, beginning with oceanic deposits before tectonic shifts brought the area above sea level, allowing mountain-derived sediments to accumulate.

Valeria Villa, a doctoral student at Caltech and co-author of the study published Aug. 4 in JGR Solid Earth, emphasized that previous surveys were limited to linear cross-sections. “Previous surveys were just linear arrays that went through just one cross section on the LA Basin, but this was the first full three-dimensional map,” Villa said.

To construct the map, scientists deployed 273 temporary seismic stations around the city. These instruments recorded faint tremors from distant earthquakes that were too weak for humans to feel. By analyzing the paths and timing of these waves as they moved through the earth, researchers were able to chart the rock layers beneath the surface.

The data shows that the deepest sediments are concentrated in the central part of the basin, directly under downtown, while the edges contain thinner layers ranging from 0.6 to 2.5 miles (1 to 4 km) before hitting the hard crystalline bedrock. Although the map does not predict exactly where shaking will be most severe, it provides a critical tool for updating simulations and testing various earthquake scenarios.

Experts warn that the severity of damage will depend on factors such as which fault system triggers the quake. Los Angeles is surrounded by numerous faults, including the San Andreas, which has the potential to generate a magnitude 7.8 event.

A key concern is “resonance,” a phenomenon where a building’s natural vibration frequency matches the shaking of the underlying sediments, leading to increased destruction. Cochran explained that the new map will help engineers design structures that avoid this dangerous synchronization.

“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.”