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Shake Rattle Roll THE SCIENCE AND ENGINEERING OF EARTHQUAKES | |
Above and on the cover: Sand and cement collided as the sudden Loma Prieta (California) earthquake of 1989 caused buildings to collapse.
Cover photo by C.E.Meyer, U.S. Geological Survey |
To understand Jacobo Bielak's work, think of California's San Fernando Valley as a bowl of gelatin. The gelatin isn't very well mixed. Parts of it shake strongly with an earthquake, others much less.
Like a bowl of gelatin, the valley, a sedimentary basin, experiences earthquake ground motion that varies considerably from one location to another. Determining the severity of ground motion and understanding why certain regions shake more strongly than others is an important step towards better earthquake-resistant design. Bielak, a civil engineer at Carnegie Mellon University (CMU), is the principal investigator of NSF's Grand Challenge Quake Project, funded by NSF's Earthquake Hazard Mitigation Program (EHMP), part of the National Earthquake Hazards Reduction Program (NEHRP). The goal of this project is not to predict earthquakes, says Bielak, but rather to predict what will happen when an earthquake does occur. "What are the resulting ground motions?" he asks. "What is the overall severity? What will the duration be? And what are the other details?" Bielak's questions aren't simply academic. Like other projects in the NEHRP, the information gathered has practical applications for builders and city planners, as well as for people who live in earthquake-prone areas. |
Mitigating Hazards
For people in California and other shake-prone parts of the country, earthquakes are a fact of life. But while earthquakes themselves cannot be controlled, the damage from hazards can be minimized, or mitigated, through better planning and preparedness.NEHRP is a multi-agency effort headed by the Federal Emergency Management Agency. In addition to NSF's involvement, the program includes teams from the United States Geological Survey, whose focus is on hazard assessment, and the National Institute of Standards and Technology, whose emphasis is on design guidelines.
Within NSF, the emphasis is on basic earthquake research and earthquake engineering, explains Bill Anderson, Head of EHMP within NSF's Engineering Directorate. "A lot of the research that we support is intended to allow other agencies to take action."
When the program started in 1977, the emphasis was on reducing the loss of life, but now, says Anderson, buildings have to do more than simply remain standing. They have to remain functional, as do other parts of the infrastructure, so as to mitigate economic losses as well as casualties.
Partly because of infrastructure issues, the economic damage of earthquakes can be devastating. Explains Andersen's colleague Cliff Astill, Program Director within EHMP, "The Northridge [Los Angeles 1994] and Kobe [Japan 1995] quakes involved losses at the economic limits of what is insurable. If The Big One came today, would it be insurable? I just don't know."
Showing the Quakes
One thing that is known about The Big One is that it wouldn't affect the San Fernando Valley evenly. Different soils and geological structures make the seismic waves travel differently, explains Bielak. A case in point is the Mexico City earthquake of 1985. The damage was not distributed uniformly throughout the city; rather, it showed up in patches. "The lake bed, which contained soft soils, shook much more than the surrounding hills and the transition zones where the soil was stiff," says Bielak.The Quake Project uses differential equations to describe how the seismic waves move, or propagate, through all sorts of materials. The resulting computer models are tailored to the varying soil properties to balance accuracy and efficiency. They simulate the earthquake response of the San Fernando Valley in southern California and are run on the Pittsburgh Supercomputing Center's Cray T3E.
To get all of its information, the Quake Project uses a multidisciplinary team from several universities. In addition to Bielak, the CMU team includes computer scientists David O'Hallaron, Jonathan Shewchuk and Thomas Gross, and computational mechanicist Omar Ghattas. Seismologist Francisco Sanchez-Sesma works at the National University of Mexico, and seismologist Keiiti Aki is from the Southern California Earthquake Center (SCEC), an NSF-funded Science and Technology Center.
The team is currently testing the system by "forecasting" the effects of past quakes. They have also created animations that visually demonstrate how seismic energy is trapped in the basin, causing the waves to reverberate off of the valley walls and amplify.
While the Quake Project's tailored modeling approach is unique, other teams, including the SCEC's strong motion section, are creating comparable models. The models are valuable tools, but all of them have their limitations, says seismologist Ralph Archuleta, who directs the strong motion team. Most notably, the physics of wave propagation aren't completely understood yet. "When waves are propagating in the granite and other hard rock, the pattern is linear and fairly well known. But we get a non-linear response with the soft soils near the surface of the earth. We don't have a very good understanding of these non-linear responses yet."
Limitations aside, both Archuleta and Bielak say the information from the models is a boon not only to scientists, but also to city planners who want to get away from the one-size-fits-all building requirements that discount on-site geological and geotechnical factors. Says Bielak, "We may find that in certain regions the building requirements are too stringent, because we'll know that even if there is an earthquake, this area won't be damaged. Or, in another area, we may find the requirements need to be strengthened."
Closing in on When and Where
City planners are looking to SCEC's other projects as well. Since its formation in 1991, SCEC's mission has included the task of "estimating when and where future damaging earthquakes will occur, calculating the expected ground motions, and communicating that information to the public."Among the highest priorities are hazard maps that tell us where and how frequently earthquakes are likely to occur, and what the level of ground shaking will be, says SCEC Director Tom Henyey. This does not mean, however, that the Center predicts a specific date and time of the next quake. It is a matter of probabilities. "We just know that if there have been occurrences [earthquakes] along such and such a fault at intervals that average 200 years, and it's been 200 years since the last one, then the probability of another occurrence in the not-too-distant future is high."
To create the information of when and where the earthquakes are most likely to happen, Henyey says they work with three sets of data: historical records of California's earthquakes dating back to about 1850; paleoseismic information from the faults; and new data captured via the Global Positioning System (GPS).
The GPS, which uses the Global Positioning Satellites and permanent receiving stations affixed to the earth, allows scientists to determine how the ground in a seismically active region is deforming as the stresses build up toward the next big event. This may enable them to spot areas with an increased seismic potential before a full-blown quake occurs. "We can monitor the relative changes in positions of points on the earth's surface down to one millimeter a year," says Henyey.
Even if no one knows exactly when or where the earthquake will happen, the hazard maps help structural engineers, emergency preparedness officials, and city planners, explains Henyey. Looking at the hazard maps, engineers will know how likely their structures are to face a damaging earthquake during the next few decades, and they can design accordingly.
Building Flexibility
But what about buildings that are already standing? Structural retrofit research is one focus of another EHMP, the National Center for Earthquake Engineering Research (NCEER), headquartered at the State University of New York at Buffalo (UB).
San Francisco's U.S. Court of Appeals is the most ornate federal building west of the Mississippi. So that it will safely withstand earthquakes, the building has been retrofitted with FPS seismic isolation bearings.
Photo courtesy of Skidmore, Owings & Merrill
The FPS system has been installed in a six-story model before being placed on a shake table at the National Center of Earthquake Engineering Research.
Photo credit: Photo courtesy of NCEER |
One such building is San Francisco's historic U.S. Court of Appeals. Completed in about 1905, the structure is listed on the National Register of Historic Places. Its granite exterior is adorned with lions' heads; inside, there are mosaic tile floors, vaulted ceilings, marble columns and statues.
The question of how to make the building safe during future quakes, including The Big One, surfaced after it was damaged in the Loma Prieta temblor of 1989. Architects and engineers proposed the use of the Friction Pendulum System (FPS), a method that would enable the 60,000-ton building to ride out a quake by gently swaying back and forth, like a pendulum. "As far as I can see, this system should do very well in protecting certain structures when there is extremely strong seismic excitation," says Michalakis C. Constantinou, a civil engineer at UB who headed up five years of testing that helped qualify the FPS system for use. These tests helped the General Services Administration, the owner of the courthouse, to choose FPS over earthquake protection devices from the United States, Japan, New Zealand, and England. In addition to seeing the building made safe, NCEER was pleased to see the technology they had tested being transferred from UB's laboratory to real-life application. At Carnegie Mellon, Bielak is also pleased with the amount of use the Quake Project's modeling systems is getting. The software tool that creates the model has turned out to be remarkably versatile. It has been adopted for such diverse projects as describing electrical currents in the heart, transport processes in estuaries and real-time simulation of terrain by the U.S. military. But, of course, the real technology transfer is focused on earthquakes. Within the United States, researchers share what they have learned with groups from various agencies The research centers frequently host groups from other quake-prone countries. Researchers in Japan and New Zealand are using the Quake system to simulate earthquake ground motion in the populated basins in their own countries. "What we have developed are the tools for performing more realistic seismic hazard analysis," says Bielak. "Considering the economic severity of recent quakes, it is not surprising that many people are interested." |
