Lawrence Livermore National Laboratory

Modeling an Earth-Shaking Event

(Page 3 of 3)

Earthquake in a Box

Rodgers describes the SWP modeling effort from a seismologist’s perspective.  “Think of it as though we built a box that represents the Bay area in three dimensions,” he says, “and we put an earthquake in the box, and the earthquake sets the box in motion.  By doing so we were able to put a simulated seismic station that measures the ground motions as a function of time at any place, which allowed us to compute the ground motion anywhere.”

Modeling the San Francisco earthquake has significance beyond commemorating the event.  The ability to create a computer simulation of such a complicated occurrence enables scientists to model other earthquakes before they happen.  Architects and civil engineers can use data gathered from those models to design structures that withstand tremors.

What DOE’s supercomputers and scientists can’t do is predict exactly where or when earthquakes will strike.

“We know that earthquakes are going to happen,” Rodgers says.  “The problem is that we have only been looking at earthquakes in detail for about 100 years.  The return times of large earthquakes are hundreds, if not thousands of years, so we haven’t got a statistical sample to allow us to do meaningful statistics.”

Nonetheless, “In the Bay area the most likely next earthquake will be along the Hayward fault,” he says.  This supposition is based on geological studies indicating there have been 11 earthquakes along the fault at intervals averaging 140 years.  The last such quake occurred in 1868, making 2008 the 140th year.

Using modeling, the researchers “can put in a hypothetical Hayward fault magnitude 7 earthquake, and see what happens,” Rodgers says.  Although models cannot tell experts precisely where along the fault the earthquake will start or in what direction it will run, “We can do lots of simulations to look at how the ground motion might vary depending on those types of factors.”

Such scenarios are valuable because “Our data set of actual large earthquake shaking is limited,” Rodgers say.  “So this modeling effort is very important because it allows us to, in the safety of our computer, compute the shaking that would occur if an earthquake were to happen on a specific fault of a certain size within a certain geology.”

From the Soil Up

Such projections are especially needed to avoid accidents at the many nuclear power plants being considered to meet increasing worldwide energy demands.  Seismic safety of nuclear power plants is guided by observed, as well as computed, ground motions.  The same computer modeling can also be used to simulate potential damage should an earthquake impact nuclear storage facilities, such as the controversial Yucca Mountain site.  “Then you would know how to design containers to withstand the possible motions of the Earth,” says Petersson.

Now that the SWP team has created a program that models how earthquake waves propagate from the source through rocks and soil to the foundations of buildings, the next logical step will be to follow those waves up from the soil through complicated structures, such as nuclear power plants, airports, and bridges, to learn how they will respond to the shaking of an earthquake.

“The foundations of buildings are embedded within soil so they need to be modeled together,” said Rodgers.  “We would like to be able to model what’s called the soil/structure interactions.”

Meanwhile, the SWP team recently made some waves of its own by receiving an internal award from the DOE’s Energy and Environment Directorate for its work on the 1906 earthquake.

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