Why some quakes lead to faster shaking
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,” said 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,” said Glaser. “The longer the fault healed before rupture, the more rapidly the surface vibrated.”
“It is elegant work,” said 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.”
The release notes that Glaser and McLaskey teamed up with Amanda Thomas, a UC Berkeley graduate student in earth and planetary sciences, and Robert Nadeau, a research scientist at the Berkeley Seismological Laboratory, to confirm that their lab scenarios played out in the field. The researchers used records of repeating earthquakes along the San Andreas fault that Nadeau developed and maintained.
The data were from Parkfield, California, an area which has experienced a series of magnitude 6.0 earthquakes two to three decades apart over the past 150 years.
Thomas and McLaskey explored the records of very small, otherwise identically repeating earthquakes at Parkfield to show that the quakes produced shaking patterns that changed depending on the time span since the last event, just as predicted by the lab experiments.
In the years after a magnitude 6.0 earthquake hit Parkfield in 2004, the small repeating earthquakes recurred more frequently on the same fault patches.
“Immediately after the 2004 Parkfield earthquake, many nearby earthquakes that normally recurred months or years apart instead repeated once every few days before decaying back to their normal rates,” said Thomas. “Measurements of the ground motion generated from each of the small earthquakes confirmed that the shaking is faster when the time from the last rupture increases.
This provided an excellent opportunity to verify that ground motions observed on natural faults are similar to those observed in the laboratory, suggesting that a common underlying mechanism —
fault healing — may be responsible for both.”
Understanding how forcefully the ground will move when an earthquake hits has been one of the biggest challenges in earthquake science.
“What makes this study special is the combination of lab work and observations in the field,” added Roland Burgmann, a UC Berkeley professor of earth and planetary sciences who reviewed the study but did not participate in the research. “This study tells us something fundamental about how earthquake faults evolve. And the study suggests that, in fact, the lab setting is able to capture some of those processes correctly.”
Glaser said the next steps in his lab involve measuring 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.
