The National Institute for Computational Sciences

A New Consideration in Earthquake Simulations

Research Shows a Way Forward in Making Earthquake Scenarios More Accurate

By Scott Gibson

Some things in life are not a matter of if but when. Case in point—a major earthquake striking Southern California. The question is how to prepare and lessen the impact.

Nobody can prevent or stop earthquakes; however, buildings can be better designed and constructed, land selectively used, and prediction capability improved, the U.S. Geological Survey (USGS) states.

A project of the USGS called the ShakeOut Earthquake Scenario "applied the best current scientific understanding to identify what can be done now to avoid an earthquake catastrophe." The ShakeOut presents a 'what if' earthquake with a magnitude of 7.8 (severe) on the southern portion of the San Andreas Fault. The entire fault extends down the heavily populated state of California from San Francisco to Southern California.

The 2008 project report makes clear that the ShakeOut Earthquake Scenario is not a prediction but a product to help communities improve planning, mitigation, and response and serve as a basis for public drills and emergency response exercises.

Southern California has very good reason to be doing earthquake drills and exercises. An earthquake similar to the hypothetical one in the ShakeOut would result in about 1,800 deaths, 53,000 injuries requiring emergency room care, the destruction of the equivalent of 133,000 single-family homes, and $191 billion in economic loss, according to a 2011 article in the journal Earthquake Spectra that provides an update on the ShakeOut Earthquake Scenario project.

The USGS states in the ShakeOut report that because each earthquake has its own signature patterns of shaking and destruction, the next earthquake to strike Southern California will differ in details from the ShakeOut Earthquake Scenario, including total damages and losses. But it adds that the overall effects, both socially and economically, will be similar.

Research being done with the support of high-performance computing informs and enhances models such as the ShakeOut Earthquake Scenario.

A paper to be published in 2014 by Geophysical Research Letters titled "Expected seismic shaking in Los Angeles reduced by San Andreas fault zone plasticity" suggests that computer simulation models may need to take into account an aspect pertaining to ground motion recently explored by a team of scientists using the Kraken supercomputer at the National Institute for Computational Sciences.

The project involved the simulation of a magnitude-7.8 earthquake rupturing the southern San Andreas Fault.

Research team member Daniel Roten of the Swiss Seismological Service, ETH Zurich, Switzerland, and the San Diego Supercomputer Center (SDSC), explains that simulations have typically assumed a linear relationship between stress and strain in crustal rocks.

"Our recent work accounts for the limited strength of crustal rocks; that is, we simulate the absorption of rupture energy by permanent rock deformation," he says. "Our results suggest that this nonlinear behavior in rocks could reduce the previous simulation-based estimates of expected ground motion velocity [the speed at which the ground would shift] in the Los Angeles basin during a magnitude-7.8 event on the southern San Andreas Fault by 30 to 70 percent."

Roten explains that nonlinear material response occurs in soft soils near the surface, lessening high-frequency (>1 Hz) shaking that controls damage to low- and mid-rise buildings. The existence of such behavior is well established in earthquake research, he says, adding that this type of nonlinearity is routinely treated in engineering seismology equations, and was provided for in the original ShakeOut Earthquake Scenario—but until the recent research performed with Kraken, it was not factored into computer simulations.

"Our simulations show that nonlinear response in crustal rocks may also reduce the amplitudes of long-period surface waves that pose a hazard to high-rise buildings, meaning the degree of destruction would be less than anticipated," Roten says. These reductions, he explains, could be important because the collapse of high-rise structures represents a substantial aspect in the damage and casualty estimates of the ShakeOut Earthquake Scenario.

Roten points out that more research will be needed to quantify the impact of these findings on damage and casualty estimates for future magnitude-7.8 earthquakes on the San Andreas Fault.

"A challenge lies in the characterization of the strength of crustal rocks, which controls the reduction in shaking compared to previous [linear] estimates," he says. "However, the study shows a way forward for more-accurate earthquake scenarios that will be based on detailed representations of the nonlinear parameters in the earth's crust."

[Besides Roten, the other members of the research team who used Kraken for the new simulations discussed here were Kim Olsen and Steven Day, Department of Geological Sciences, San Diego State University, and Yifeng Cui, SDSC.]

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Article posting date: 13 May 2014

About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and state-of-the-art engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS runs the National Institute for Computational Sciences (NICS), which had the distinction of deploying and managing the Kraken supercomputer. NICS is a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment, known as XSEDE. In November 2012, JICS sited the Beacon system, which set a record for power efficiency and captured the number one position on the Green500 list of the most energy-efficient computers.