Why Earthquakes Result in Shaking?

Till date one of the biggest challenge in earthquake science is to understand how forcefully the ground will move when hit by an earthquake.

A new study by the University of California helps engineers better assess the vulnerabilities of buildings, bridges and other structures when a fault does rupture.

Based on a tabletop model of quake fault, they state that the more time it takes for an earthquake fault to heal, the faster the shake it will produce when it finally ruptures.

"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," said study lead author Gregory McLaskey, a former UC 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 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."

In order to create a fault mode, the researchers placed a Plexiglas slider block against a larger base plate and equipped the system with sensors. With the help of this design the researchers were to isolate the physical and mechanical factors, such as friction, that influence how the ground will shake when a fault ruptures.

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

"The longer the fault healed before rupture, the more rapidly the surface vibrated," said Glaser.

"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."

In order to proceed to confirm that their lab scenarios played out in the field, 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.

They based their finding on records of repeating earthquakes along the San Andreas fault that Nadeau developed and maintained. The data were from Parkfield, Calif., 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.

"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."

According to Glacier 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.