Los Angeles is especially vulnerable to any major earthquake that may take place south of the city of almost 4 million, a new method of constructing virtual earthquakes determined.

Published in the journal Science, the technique takes advantage of the ambient seismic field generated by ocean waves.

Whenever waves bump up against one another, a pulse is created that then makes its way down all the way to the Earth's crust, creating seismic waves billion of times less intense than those produced by earthquakes, the researchers, from Stanford University, explained.

For years, these ongoing waves were thought to get in the way of earthquake research; however, this is starting to change as scientists develop methods capable of isolating specific waves.

Marine Denolle, the lead author of the new study, expanded on these techniques to produce a system that takes the information provided by the seismic waves to create "virtual earthquakes" capable of predicting the potential repercussions of a much stronger quake.

According to the results, the citizens of Los Angeles have reason to fear anything that might originate south of them.

"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," Denolle, who recently received her PhD in geophysics from Stanford and is now at the Scripps Institution of Oceanography, said in a statement.

Their results confirmed a 2006 prediction, which said that if the southern San Andreas Fault were to produce an earthquake, a portion of the waves would be siphoned off along a natural conduit connecting Los Angeles to the San Bernardino Valley. What's more, this pathway is made up largely of sediments, which would both amplify and direct waves toward the city, the researchers report in the study.

"The seismic waves are essentially guided into the sedimentary basin that underlies Los Angeles," said study leader Greg Beroza, a geophysics professor at Stanford. "Once there, the waves reverberate and are amplified, causing stronger shaking than would otherwise occur."

Anything of a magnitude of 7.0 or more, the scientists said, could put Los Angeles at risk for stronger and more variable ground motion.