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January 28, 2014

Virtual Earthquakes Predict Real Risk

Tiffany Trader
earthquake waveguide to basin effect

Scientists at Stanford devised a “virtual earthquake” technique capable of predicting the effects of a major quake occurring along the southern San Andreas Fault.

The remarkable thing about the new technique is that it relies on weak vibrations generated by the Earth’s oceans to create these ‘virtual earthquakes’ in order to forecast resultant ground movement and shaking hazard in the event of a real quake.

The innovative research appears in the Jan. 24 issue of the journal Science. The results show that if a major quake occurs south of the city, Los Angeles will experience stronger-than-expected ground movement.

Lead author Marine Denolle recently received a PhD in geophysics from Stanford and now works at the Scripps Institution of Oceanography in San Diego. “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,” she explains.

The research is based on the fact that even in the absence of a quake, there is still background seismic activity. “If you put a seismometer in the ground and there’s no earthquake, what do you record? It turns out that you record something,” notes study leader Greg Beroza, a geophysics professor at Stanford.

The instruments are perceiving a continuous signal called the ambient seismic field. The field is generated by ocean waves interacting with solid Earth. The crashing of the waves creates a pressure pulse that undulates through the ocean to the sea floor and into the Earth’s crust. Beroza explains that the waves are billions of times weaker than the seismic waves caused by earthquakes.

Although the scientific community has known about the ambient seismic field for a century or so, it was mainly viewed as a nuisance to conducting earthquake research. Because the field is weak and appears randomly in the earth’s crust, it was also difficult to isolate, but techniques have improved over the last decade. New signal-processing techniques are better able to track the waves as they travel through one seismometer to another.

The research group further refined these techniques and set up seismometers along the San Andreas Fault to measure ambient seismic waves. Using the data from the seismometers, the group employed mathematical techniques to make the waves appear as if they came from deep within the Earth, where real earthquakes occur.

The team confirmed the accuracy of their virtual earthquake approach by comparing the new predictions with supercomputer simulations from 2006. The promising aspect of the new technique is that it does not require the same level of computational power, so it is much more affordable.

The primary finding from the study was that Los Angeles is at a risk for increased ground movement if a large earthquake, magnitude 7.0 or greater, were to take place along the southern San Andreas Fault, near the Salton Sea.

“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 next step for the group is to test their virtual earthquake technique in other cities around the world that are located on top of sedimentary basins. Examples of such locales include Tokyo, Mexico City, Seattle and parts of the San Francisco Bay area.