Quake model aids in fault studies
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 Berkeley engineers, who conducted their work using a tabletop model of a quake fault.
“The high frequency waves of an earthquake—the kind that produces the rapid jolts—are not well understood because they are more difficult to measure and more difficult to model,” says study lead author Gregory McLaskey, a former Berkeley Ph.D. student in civil and environmental engineering. “But those high-frequency waves are what matter most when it comes to bringing down buildings, roads and bridges, so it’s important for us to understand them.”
The study, published in the November 1 issue of Nature and funded by the National Science Foundation, 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,” says Steven Glaser, 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 say. And current instruments are generally unable to accurately measure waves at frequencies higher than approximately 100 Hertz because they get absorbed by the earth.
“There are many people studying the properties of friction in the lab, and there are many others studying the ground motion of earthquakes in the field by measuring the waves generated when a fault ruptures," says McLaskey. “What this study does for the first time is link those two phenomena. It’s the first clear comparison between real earthquakes and lab quakes.”
Noting that fault surfaces are not smooth, the researchers roughened the surface of the Plexiglas used in the lab’s model.
“It’s like putting two mountain ranges together, and only the tallest peaks are touching,” says McLaskey, who is now a postdoctoral researcher with the U.S. Geological Survey in Menlo Park.
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,” says Glaser. “The longer the fault healed before rupture, the more rapidly the surface vibrated.”
“It is elegant work,” says seismologist John Vidale, a professor at the University of Washington who was not associated with the study. “The point that more healed faults can be more destructive is dismaying. It may not be enough to locate faults to assess danger, but rather knowing their history, which is often unknowable, that is key to fully assessing their threat.”
Glaser’s next step is to measure the seismic energy that comes from the movement of the individual contact points in the model fault to more precisely map the distribution of stress and how it changes in the run-up to a laboratory earthquake event.
Watch a video of the tabletop fault model in action.