Article: Earthquake Research in the Satellite Age

“We’re trying to figure out why earthquakes occur and how they occur. We’re doing this partly because it’s an interesting scientific problem, but the main reason we’re doing it is because it has societal value. If we can understand earthquakes and figure out which areas are subject to seismic hazard, we hope that we can save lives and property.”

 

            Nancy King, U.S. Geological Survey

 

Ken Hudnut is standing on a rugged hilltop in the mountains outside of Los Angeles. Nearby is a small gray dome supported by four narrow legs planted securely in the ground. Hudnut is a geophysicist with the U.S. Geological Survey, and the contraption next to him is a highly sophisticated Global Positioning System receiver. “This station was installed a couple of years ago,” Hudnut says. “Within those two or so years since it was installed, this whole mountain that we’re standing on has moved towards the northwest by about four inches [ten cm] with respect to the interior of the North American continent.”        

Four inches (ten cm) in two years may not sound like much--it’s about how fast your fingernails grow--but for a mountain, that’s moving at a good clip. In 15 million years, Los Angeles will run into San Francisco.

A decade ago, earthquake researchers had no way to measure the motion of a mountain with such accuracy. But now, with GPS, they can monitor the millimeter-by-millimeter movement of the Earth’s crust. Such advances are revolutionizing the study of earthquakes.

Monitoring Plates from Space

In 1994, the magnitude 6.7 Northridge earthquake struck Southern California. The most expensive earthquake in U.S. history, it caused $40 billion in damage and served as a vivid reminder of the earthquake hazard in Southern California. In the quake’s aftermath, a state-of-the-art earthquake monitoring system was installed.

The centerpiece of this system is the Southern California Integrated GPS Network (SCIGN), an array of 250 Global Positioning System stations located throughout the region. SCIGN continually records the position of these GPS stations. “With a GPS network, we are essentially just measuring the location of points,” explains Andrea Donnellan, a geophysicist with NASA’s Jet Propulsion Laboratory. “So we can take two points on the Earth, and we see how these points get closer together or farther apart.” 

Tracking the movement of these stations lets scientists actually monitor plate tectonics. They can observe the pieces of Southern California’s shattered plates move around, stretching, squeezing, and folding the Earth’s surface. Measuring this motion helps earthquake researchers assess where faults are slipping, or moving, and where strain is building upstrain that might eventually be released as an earthquake. “We work backwards from the deformation of the surface of the Earth, which is what we can measure, to slip on faults, which is what we actually want to know,” says Nancy King, a geophysicist with the U.S. Geological Survey. “The more slip there is on a fault, the more hazard there is associated with it.”

Such information complements rather than replaces information gathered from seismographs. Unlike GPS, seismographs can measure shaking and locate an earthquake’s focus, but they do not reveal where strain is building up between earthquakes. 

How GPS Works

To collect data, SCIGN relies on the Global Positioning System, a group of 27 satellites in orbit around Earth. At any given time, 12 of these satellites might be in view of a receiving station on the ground. The GPS satellites continually broadcast radio signals with their time and position, which the receiver on the ground records. The length of time it took a satellite’s signal to reach the receiver is used to calculate the receiver’s exact distance from that satellite. By comparing information from multiple satellites, it is possible to precisely pinpoint the location of the receiver. GPS receivers used in boats and cars record only part of the GPS signal and can locate positions to within about six to nine meters (20 or 30 feet). The SCIGN receivers are much more sophisticated and can be positioned to within about three millimeters (an eighth of an inch). 

SCIGN is a cooperative effort of NASA’s Jet Propulsion Laboratory, the U.S. Geological Survey, and the Scripps Institution of Oceanography under the auspices of the Southern California Earthquake Center. The SCIGN data are put on the Internet, where anyone can access them for free. Besides researchers, engineers and surveyors are increasingly taking advantage of this valuable information.

Where’s the Hazard?

By using GPS to measure the buildup of strain on the fault system, scientists are attempting to more accurately map the seismic hazard in Southern California. “We’re hoping to be able to zoom in on where the hazard is the greatest,” says Hudnut. “Because there are so many people in Los Angeles, we really need to map the strain and the hazard in more detail.”

The SCIGN data have already given researchers a more detailed picture of the compression of the Los Angeles Basin, revealing that it is concentrated in the north of the basin. Scientists have also identified previously unknown blind faults, or faults that do not reach all the way to the surface, enabling them to better model the region’s fault system.

No Predictions, Please 

Earthquake researchers don’t like the word “prediction.” Most don’t think it will ever be possible to predict the next big earthquake, because earthquake processes are entirely too chaotic. Yet that is exactly what people living in earthquake zones want to know. Lucy Jones, a scientist with the U.S. Geological Survey, understands the problem. “We want, on human scales, to say there’s going to be an earthquake next week or next year or in this decade,” she says, “but the faults are moving on time scales of millennia. And so we cannot at this point, with any of the information we have, tell you whether it’ll happen this year.”

Instead of prediction, earthquake researchers prefer to talk about “forecasting.” “This is looking at a bigger picture, and saying, over the next 30 years, how frequently will the shaking exceed a certain value?” explains Hudnut. Though less dramatic than prediction, forecasting is actually much more useful. A prediction of an earthquake in Los Angeles tomorrow would likely result in freeways clogged with cars as people flee the city. Such hysteria would do nothing to protect lives or property. Forecasting, on the other hand, can give engineers and planners time to prepare. 

“Engineers know how to make buildings withstand earthquakes, even very strong earthquakes,” says Hudnut. New buildings tend to survive earthquakes with minimal damage. It is the older structures, those that predate current building codes, that crumble. After the 1971 San Fernando earthquake, the California Department of Transportation began reinforcing freeway overpasses in the Los Angeles area. By 1994, they had strengthened more than 100 of them. When the Northridge quake hit, not one of the reinforced overpasses collapsed, though seven others did. But it takes time and money to strengthen and replace older buildings and bridges. “If we have better information on seismic hazard, that can be used to help guide a retrofitting program,” says Hudnut. 

Hudnut also has hopes that GPS can be used to develop an earthquake early warning system in Southern California. “When a fault ruptures it’s just like a zipper,” Hudnut explains. “It breaks and then the crack propagates along the fault.” The crack moves down the fault at over three kilometers (about two miles) per second, but current technology can communicate faster than that. If instruments were able to detect motion along the fault instantly, they could signal Los Angeles that the quake was on its way. Although the shaking would begin before humans could respond, an automatic signal might give computers sufficient time to shut down electrical systems, secure hard drives, or open firehouse doors so the trucks are ready to go. Early warning systems are already in place in Japan and Taiwan, but in Los Angeles the faults are closer to the city, so creating an effective system is a greater challenge.

Earthquake Insights

Scientists are making great leaps in their understanding of earthquake processes as a result of the SCIGN data. In 1999, during the magnitude 7.1 Hector Mine earthquake in the desert east of Los Angeles, scientists were able to monitor the deformation of the Earth moment by moment as a nearby GPS station rolled with the quake. Information like this should provide researchers with a better understanding of how cracks propagate during an earthquake. In the aftermath of the Hector Mine quake, scientists measured how the Earth continued to relax and readjust for several years. Researchers also observed as the fault that produced the Northridge quake continued to slip quietly, without shaking, for two years, raising the mountains another 12 centimeters (five inches).         

Researchers are combining the GPS data with other sophisticated tools to improve their grasp of earthquake mechanics. By putting the SCIGN data into computer models, they are gaining insight into how faults interact. “In the computer we can speed up time and run models that go for thousands of years,” says Donnellan. “One of the very interesting things we’ve learned is that one earthquake can speed up an earthquake on another fault and make it happen, or actually turn off an earthquake on another fault.” 

The Next Generation

The future of earthquake research seems wedded to satellite technology. Plans are already in the works for a Plate Boundary Observatory, a network of about 1,000 GPS stations from Mexico to Alaska that should give researchers a clearer picture of crustal deformation all along the Pacific-North American Plate boundary.

GPS is just one example of how satellite technology is revolutionizing the study of earthquakes. The space-based technology that currently holds the greatest promise for earthquake research is interferometric synthetic aperture radar, or InSAR. By combining radar images taken from space, InSAR gives scientists a better three-dimensional view of the deformation of the Earth’s crust before, during, and after earthquakes than even GPS provides. InSAR has the added advantage of being able to precisely measure changes practically anywhere on the globe, not just where GPS stations have been installed. “You would need a GPS receiver every 20 meters [about 66 feet] to get the same information,” says Gilles Peltzer, a geologist at the Jet Propulsion Laboratory.

In the coming years, earthquake researchers will continue to look to space to help unlock the processes going on underground here on Earth. “Space is really revolutionizing our understanding of earthquake processes and plate tectonics,” says Donnellan. “We’re just now getting hints and evidence of things that occur that we never even knew happened before. So it’s very, very exciting, and I think the next hundred years will take us way beyond what we know now.”