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"virtual quakes" predict quake risk

Study uses ambient seismic waves to forecast ground motion

January 27, 2014

Los Angeles would experience stronger-than- expected shaking in a major earthquake, U.S. researchers producing “virtual earthquakes” say.

Stanford scientists are using weak vibrations generated by the Earth’s oceans to produce “virtual earthquakes” that can be used to predict the ground movement and shaking hazard to buildings from real quakes.

The new technique has produced a prediction Los Angeles will experience stronger-than-expected ground movement if a major quake occurs south of the city, the university reported recently.

“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,” said lead author Marine Denolle.

The researchers took advantage of the fact earthquakes aren’t the only phenomenon that can produce 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,” Stanford geophysics professor and study leader Greg Beroza said.

What is recorded is a weak, ongoing signal known as the ambient seismic field, generated by ocean waves interacting with solid Earth.

“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 stronder shaking than would otherwise occur.

As a result, the scientists say, Los Angeles could be at risk for stronger and more variable, ground motion if a large earthquake-magnitude 7.0 or greater-were to occur along the southern San Andreas Fault, near the Salton Sea.

In addition to studying earthquakes that have yet to occur, the technique could also be used as a kind of “seismological time machine” to recreate the seismic signatures of temblors that shook the Earth long ago, according to Beroza.

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