This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In computer simulations of the Big One—a large-magnitude earthquake that may occur along the San Andreas Fault in California—seismic waves ripple from the epicenter of a magnitude 7.8 earthquake near the Salton Sea into the Los Angeles Basin. For minutes, in this simulation, the ground beneath downtown Los Angeles shakes like Jell-O because the region is built on a deep basin of soft sediments, which can amplify seismic waves.
The structure and composition of the ground have a large impact on the strength of seismic shaking and whether it is felt as a quick jolt or a long reverberation. To understand and prepare for how a major earthquake will affect different areas of Southern California, researchers in a new Caltech study installed hundreds of temporary seismic stations throughout Los Angeles County to measure seismic activity and build a map of the basin's shape below. The map shows that soft sediments are up to 10 kilometers (6 miles) deep beneath downtown Los Angeles, whereas other regions, such as the San Gabriel and San Bernardino valleys, sit atop much shallower sediments.
This kind of detailed information about underground structure has a direct effect on understanding building safety. The study was led by Caltech graduate student Valeria Villa and conducted with seismologist Robert Clayton, professor emeritus of geophysics. A paper describing the research appears in the journal Journal of Geophysical Research: Solid Earth on Aug. 4.
"When LA is hit by a large earthquake, there are going to be some city blocks that are damaged and some that are relatively unscathed," Clayton says. "The underground basin variations control how the damage will be distributed." 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, which is more like a high-pitched note. To create the map of the basin, Clayton teamed with volunteers, including Villa and other Caltech graduate students and postdocs, to install 273 small seismic sensors throughout the LA region at intervals of 1–2 kilometers (0.6–1.2 miles).
The sensors, measuring about 5 inches (13 centimeters) high and protruding from the ground, were spread from Pasadena to Long Beach and from Santa Monica to Orange County. In many cases, residential neighborhoods proved to be optimal locations for the sensors, and the researchers engaged with local homeowners and renters to gain community participation. The sensors were deployed for a month.
Villa developed a detailed method that incorporated measurements of gravity and seismic waves from the sensors, allowing researchers to peer into the ground's structure far below. These measurements were then transformed into a 3D map of the region. "Such a study could not have been accomplished before," she says.
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