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Tabletop Fault Model Shows Why Some Earthquakes Shake Faster

The more time it takes for an earthquake fault to heal, the faster the shake it will produce when it finally ruptures, according to a new study by engineers at the University of California, Berkeley, who conducted their work using a tabletop model of a quake fault. While the study does nothing to bring scientists closer to predicting when the next big one will hit, the findings could help engineers better assess the vulnerabilities of buildings, bridges, and other structures when a fault does rupture.

“The experiment in our lab allows us to consider how long a fault has healed and more accurately predict the type of shaking that would occur when it ruptures,” said Steven Glaser, UC Berkeley professor of civil and environmental engineering and principal investigator of the study. “That’s important in improving building designs and developing plans to mitigate for possible damage.”

To create a fault model, the researchers placed a Plexiglas slider block against a larger base plate and equipped the system with sensors. The design allowed the researchers to isolate the physical and mechanical factors, such as friction, that influence how the ground will shake when a fault ruptures.

It would be impossible to do such a detailed study on faults that lie several miles below the surface of the ground, the authors said. And current instruments are generally unable to accurately measure waves at frequencies higher than approximately 100 Hertz because they get absorbed by the earth.

Noting that fault surfaces are not smooth, the researchers roughened the surface of the Plexiglas used in the lab’s model. As the sides “heal” and press together, the researchers found that individual contact points slip and transfer the resulting energy to other contact points.

“As the pressing continues and more contacts slip, the stress is transferred to other contact points in a chain reaction until even the strongest contacts fail, releasing the stored energy as an earthquake,” said Steven Glaser, UC Berkeley professor of civil and environmental engineering and principal investigator of the study. “The longer the fault healed before rupture, the more rapidly the surface vibrated.”

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