California ,

Forecast: Clear with a chance of tremors


Seismologists have developed a technique called probabilistic seismic hazard analysis (PSHA), which creates maps of unstable zones based on the nature of the fault and the different types of soil and rock in the region. More than 220 PSHA hazard curves were assimilated to create the map above. New analyses tend to raise hazard estimates in the Los Angeles and Ventura Basins and reduce hazard estimates for mountainous regions in southern California. (Courtesy: U. TEXAS)

USC/U. TEXAS (US)—Imagine if alongside your local weather outlook there also was an earthquake forecast. You’d know if you needed to bring an umbrella and secure loose items before leaving the house in the morning.

The CyberShake project, based at the Southern California Earthquake Center (SCEC), is advancing science toward that goal. The project is producing seismic hazard maps that predict how much ground motion can be expected throughout the L.A. basin over the next 50 years.

Emergency response managers count on these predictions to determine what areas will be hardest hit in a quake. Building engineers rely on them to construct structurally sound buildings. All told, the maps have the potential to save billions of dollars in damages and preserve thousands of lives in the case of a catastrophic earthquake.

“We want buildings to last at least 50 years,” says Philip Maechling, information technology architect for SCEC, which is associated with the University of Southern California. “We ask, ‘what are the peak ground motions that this building, or this site, will experience over that timeframe?'”

To answer this question, seismologists have developed a technique called probabilistic seismic hazard analysis (PSHA), which creates maps of unstable zones based on the nature of the fault and the different types of soil and rock in the region.

For nearly a decade, seismologists have been using numerical algorithms and computer simulations to predict these future ground motions based on their knowledge of earthquake physics, but until recently they were not considered as reliable as historically-based PSHA. With the latest Cybershake maps, this perception is changing.

To create these maps, SCEC teamed with the Texas Advanced Computing Center (TACC), whose massive supercomputer, Ranger, enabled the production of hazard predictions more comprehensive than anything that has been created before.

PSHA requires two inputs: information about the geology of the area being studied, and knowledge of how fast seismic waves travel, and how much shaking they would cause.

“CyberShake is a very integrative project,” Maechling says. “It combines a lot of other activities at SCEC to produce these probabilistic maps, which are an interface between seismologists and the public.”

The difficulty of creating accurate PSHA maps lies in simulating all the likely earthquakes at a given site. Since every earthquake has a number of potential outcomes, SCEC had to simulate 415,000 earthquakes to properly characterize southern California. These impacts were then summed to determine the potential ground motion for each location on the hazard map.

With 200 data points, the researchers were able to present a map that matched, and even exceeded, the detail and accuracy of traditional attenuation maps.

“CyberShake essentially enables us to obtain a more customized hazard estimates at a site,” says Ned Field, USGS seismologist and leader for developing official earthquake forecast models for California. “During the 1994 Northridge earthquake, neighboring areas of Santa Monica experienced very different levels of shaking. Attenuation-relationship-based hazard studies would never predict these details, but CyberShake gives us hope.”

The new simulations mainly agreed with the attenuation maps. However, they predicted significantly more ground motion in the most densely populated areas of the L.A. basin, which could cause greater damage than previously anticipated.

The U.S. Geological Survey are combining the results of the two models to create the newest hazard maps, but over time, researchers believe computational simulations will become the dominant predictor of seismic hazard.

“Once we gain confidence in the methodology, CyberShake will enable both more reliable loss estimates for insurance purposes and better building codes for avoiding catastrophic collapse,” Field says.

Static PSHA maps are not the final objective, according to Maechling. Soon, CyberShake will be joined to a system that dynamically incorporates seismic changes as they happen to produce daily ground motion forecasts for volatile regions.

“In the next five years, just like weather forecasts, we’re going to have ground motion forecasts,” Maechling says, “and the technique should be applicable anywhere in the world.”

With insights into the seismic future of the L.A. basin and improved methods for using computers to predict the impact of earthquakes, the CyberShake project is a textbook example of how computational science impacts society.

“I see the USGS and emergency management agencies looking at our simulations and saying, ‘we need to take into consideration these new results coming out of SCEC,'” Maechling says. “That’s exciting.”

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