January 24, 2014
‘Virtual Earthquake’ Technique Used To Predict Los Angeles Earthquake Risk
[ Watch the Video: Forecasting Los Angeles Earthquakes ]
redOrbit Staff & Wire Reports - Your Universe Online
A new technique developed by scientists at California’s Stanford University has confirmed that the Los Angeles would experience stronger-than-anticipated ground movement should major seismic activity occur to the city’s south, according to research published in the latest edition of Science.
The method used weak vibrations generated by the Earth’s oceans to produce “virtual earthquakes” that can be used to predict ground movement and shaking hazards to buildings posed by real tremors, Stanford School of Earth Sciences associate director of communications Ker Than said in a statement Thursday.
“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,” explained lead author Marine Denolle, a former Stanford PhD student who is now with the Scripps Institution of Oceanography in San Diego.
According to Stanford geophysics professor Greg Beroza, who headed up the research, the new technique takes advantage of the fact that earthquakes are not the sole source of seismic waves. Placing a seismometer in the ground when there is no earthquake present will allow the instrument to record a faint, continuous signal known as the ambient seismic field.
The ambient seismic field is produced when ocean waves interact with solid Earth, the researchers explained. When waves collide with one another, they generate a “pressure pulse” which travels through the ocean to the sea floor and then into the Earth’s crust. Beroza said that these waves are “billions of times weaker” than those of earthquakes.
The existence of the ambient seismic field is not a new discovery. Scientists have reportedly known about it for around a century, but it has primarily been viewed as a hindrance to earthquake research because it interferes with traditional work in the field, since the waves propagate through the crust in every direction.
However, in the past 10 years, seismologists have come up with new signal-processing techniques which allow them to isolate specific waves – namely, those traveling through one seismometer and then a second one downstream. Building upon these methods, Denolle and her colleagues came up with a way to make these ambient seismic waves serve as proxies for the seismic waves generated by actual earthquakes.
“By studying how the ambient waves moved underground, the researchers were able to predict the actions of much stronger waves from powerful earthquakes,” Than said. They started by installing several seismometers along the San Andreas Fault in order to specifically measure this type of wave activity.
Once that was finished, they used the data they collected along with newly-developed mathematical techniques to make it seem as though the waves originated deep below the Earth’s surface. By doing so, they were able to correct for the fact that the seismometers were installed at the surface, not at the depths where real earthquakes occur.
“In the study, the team used their virtual earthquake approach to confirm the accuracy of a prediction, made in 2006 by supercomputer simulations, 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,” Than said.
“This passageway is composed mostly of sediments, and acts to amplify and direct waves toward the Los Angeles region,” he added. “Until now, there was no way to test whether this funneling action, known as the waveguide-to-basin effect, actually takes place because a major quake has not occurred along that particular section of the San Andreas Fault in more than 150 years.”
Furthermore, their virtual earthquake technique predicts that the seismic waves will become even more amplified once they reach Los Angeles, due to the fact that the city rests upon a large sedimentary basin. As a result, the study authors suggest that the city could face the risk of stronger, more variable ground motion if an earthquake of at least magnitude 7.0 were to take place on the southern San Andreas Fault.
The Stanford team is now planning to use the relatively inexpensive technique in other international cities that are also built on top of sedimentary basins, such as Seattle, Mexico City and Tokyo. They believe that the method could also be used to forecast ground motion in developing countries, as well as to recreate the seismic signatures of massive earthquakes that occurred in the distant past.