A new underground map of the Los Angeles region tracks the possible impacts of seismic activity, providing a tool to assess building safety.

The structure and composition of the ground greatly influences the strength of seismic shaking, and whether it is felt as a quick jolt or a long reverberation. This affects the impact of earthquakes on the built environment.

A new Caltech study installed hundreds of temporary seismic stations throughout Los Angeles County to measure seismic activity and build a map of the shape of the basin below.

The map shows that soft sediments are up to 10 kilometers deep beneath downtown Los Angeles, while other regions such as the San Gabriel and San Bernardino valleys sit atop sediments that are much shallower.

Deeper sediments lead to longer-period shaking—long wavelengths of vibration analogous to a low musical note—which affects larger buildings more than shorter-period shaking that is more like a high-pitched note.

“When L.A. is hit by a large earthquake, there are going to be some city blocks that are damaged and some that are relatively unscathed," said seismologist Robert Clayton, Caltech professor of geophysics, emeritus.