Researchers used the ocean's vibrations to create "virtual earthquakes" that could help them determine how much damage a real earthquake would impose on building and other structures.
The researchers determined the city of Los Angeles would experience "stronger-than-expected ground movement" if an earthquake were to occur, a Stanford University news release reported.
"We used our virtual earthquake approach to reconstruct large earthquakes on the southern San Andreas Fault and studied the responses of the urban environment of Los Angeles to such earthquakes," lead author Marine Denolleof the Scripps Institution of Oceanography, said in the news release.
The new research highlighted that earthquakes are not the only thing that causes seismic waves.
"If you put a seismometer in the ground and there's no earthquake, what do you record? It turns out that you record something," study leader Greg Beroza, a geophysics professor at Stanford University, said.
The equipment picked up an ambient seismic field, which is a weak but continuous signal that is created when ocean waves interact with dry land. The waves generate pressure that sweeps across the ocean's floor and moves into the Earth's crust.
"These waves are billions of times weaker than the seismic waves generated by earthquakes," Beroza said.
The ambient seismic field is not a new discovery, but researchers have always thought of it as a "nuisance" that interfered with other seismic readings. Over the past decade researchers have developed signal-processing techniques that allow them to better-isolate waves.
The research team was able to use the ambient seismic waves as "proxies" that for those created by real earthquakes.
The team installed seismometers along the San Andreas Fault to measure the ambient seismic waves. They then took the gathered data and employed a mathematical technique to make it seem as if the waves had come from within the Earth.
The researchers' findings helped them confirm the accuracy of computer simulations that were made back in 2006.
The initial simulation found that "if the southern San Andreas Fault section of California were to rupture and spawn an earthquake, some of the seismic waves traveling northward would be funneled toward Los Angeles along a 60-mile-long (100-kilometer-long) natural conduit that connects the city with the San Bernardino Valley. This passageway is composed mostly of sediments, and acts to amplify and direct waves toward the Los Angeles region," the news release reported.
This was the first time researchers were able to confirm this tunneling effect called the "waveguide-to-basin effect."
The predictions also suggested the waves would get stronger as they reached Los Angeles (which sits atop a sedimentary basin). The researchers believe Los Angeles could be at risk of significant ground movement if an earthquake with a magnitude of 7.0 or greater occurs along the San Andreas Fault.
"The seismic waves are essentially guided into the sedimentary basin that underlies Los Angeles," Beroza said. "Once there, the waves reverberate and are amplified, causing stronger shaking than would otherwise occur."
The researcher plan to use their technique to test the ground movement risk of other cities that sit atop basins such as "Tokyo, Mexico City, Seattle and parts of the San Francisco Bay area," the news release reported.
The technique could also be used to test earthquake risk in developing countries since it is relatively inexpensive, as well as to look at earthquakes that have occurred in the past.
"For an earthquake that occurred 200 years ago, if you know where the fault was, you could deploy instruments, go through this procedure, and generate seismograms for earthquakes that occurred before seismographs were invented," Beroza said.
WATCH:
