Quakes from Space

At 5:47 A.M. on January 17, 1995, an earthquake struck Kobe, Japan. In the dark before dawn, office towers listed, walls collapsed, columns crumbled. Roadways twisted, and overpasses and elevated train tracks crashed to the ground. More than 100,000 buildings were destroyed. All of this happened in just 20 seconds. 

The Kobe earthquake left 5,500 people dead, another 35,000 injured, and 300,000 homeless. Had the earthquake struck a couple of hours later, the loss of life would have undoubtedly been much greater, as the freeways and elevated trains that collapsed would have been packed with people. Even so, the Kobe earthquake ranks as the most expensive natural disaster in history, causing approximately $150 billion in damage.

Earthquakes unleash awesome power. Rock folds, crumples, and slides, triggering shaking that can leave cities in ruins. What causes these catastrophic events and why are they clustered in certain parts of the globe? The answer lies in the inexorable motion of the tectonic plates that make up the Earth’s surface.

Fitting Together the Pieces 

The Earth’s surface is like a giant jigsaw puzzle. The enormous pieces of rock that make up this puzzle are called tectonic plates. But unlike the pieces of a jigsaw puzzle, the tectonic plates are moving in relation to one another.          

Most earthquakes occur along plate boundaries, where plates are smashing together, pulling apart, or sliding by one another. Kobe, for example, lies on the margin where the Philippine Plate is crashing into the Eurasian Plate. Turkey, which has experienced a number of devastating earthquakes in recent years, is being pushed westward as the African and Eurasian plates collide. 

Earthquake-prone California straddles the boundary between the Pacific and the North American plates. On its eastern side, the North American Plate stretches far into the Atlantic Ocean. This places New York in the relative safety of the center of the plate, so it experiences far fewer earthquakes than California. Although some earthquakes do occur in the interior of plates--two of the largest quakes in U.S. history shook New Madrid, Missouri, in 1811 and 1812--the vast majority of earthquakes strike the margins of the plates.

Up from the Mantle

What causes these mammoth plates of rock to move around? Think of the planet as a giant ball with five layers. It has a thin crust on the outside, which covers an upper solid layer of the mantle called the lithosphere. Next down comes the mantle, then a liquid outer core, and a solid inner core. The tectonic plates are composed of the lithosphere and crust. 

The movement of tectonic plates is caused by a circular loop of activity called convection. Below the plates, superheated solid mantle rock rises due to buoyancy. As this rock rises, a decrease in pressure causes the rock to partially melt, creating molten rock, or magma. Magma is of lower density than the surrounding mantle, and so it rises and collects beneath the lithospheric plates. At mid-ocean ridges, the plates are only a few kilometers thick--much thinner than elsewhere--allowing the rising magma to burst through the plate to the surface, where it cools to form new crust. The molten rock that does not reach the surface spreads out and adheres to the underside of the lithospheric plates. As the thickened plates cool, they begin to sink into the less dense mantle beneath. This constant rising of hot magma and sinking of cool lithosphere forms a convection cycle that is the driving force behind the movement of tectonic plates.

Stretching and Snapping

In the places where tectonic plates meet, the Earth’s crust fractures. These cracks in the Earth, where the rock on one side is moving with respect to the rock on the other side, are called faults. Movement along faults causes earthquakes.

There are three major types of faults. In California, the most common type is a strike-slip fault, where the two giant slabs of rock are sliding past each other horizontally. Thrust faults occur when one block of rock pushes up over another. Rarer in California are normal faults, where one side of the fault sinks down relative to the other side.

The slow, relentless movement of the tectonic plates deforms the rock along these faults. But much of the time, friction prevents the slabs of rock along a fault from moving smoothly. Instead, as the plates move, the rock deforms. Although rock seems firm and inflexible, it can actually be twisted, stretched, and bent. “Rock is squishy on very long time scales,” points out Andrea Donnellan, a geophysicist at NASA’s Jet Propulsion Laboratory. As the rock warps and bends, it stores up elastic energy, just as a rubber band stores up elastic energy when it is stretched. Eventually, like the rubber band, the rock will break to accommodate the plate motion, releasing the elastic energy in an earthquake.

Earthquakes occur when sufficient pressure has built up to overcome the friction between the plates along a fault. “A rock is slowly bent, slowly bent, and then occasionally it pops and readjusts itself by movement on one of these faults and we have an earthquake,” explains Tom Rockwell, a professor of geological sciences at San Diego State University. When the rock breaks, it sends seismic waves radiating out through the Earth. “It’s just like when I snap my fingers,” says Lucy Jones of the U.S. Geological Survey’s Earthquake Hazards Program. “I release energy that’s making the air vibrate. When a fault slips, it releases energy that makes the ground vibrate. It’s that secondary effect, that shaking, that we perceive as an earthquake.”

Shake, Rattle, and Roll

The sudden movement that triggers an earthquake reshapes the landscape. Mountain ranges are built by the earthquakes on thrust faults, where plates smash into each other. When an earthquake occurs, a mountain can abruptly leap up a meter (about three feet) or more, making all the world’s encyclopedias and almanacs instantly obsolete. The displacement caused by earthquakes on strike-slip faults, where plates slide past each other, is sometimes vividly evident. A fence that crosses a fault can break suddenly and then continue on in the same direction, but two meters (about six feet) to the right. It’s as if a photograph were torn in two and then taped back together so the image was not aligned.

The effects of earthquakes extend far from the actual fault. Most people don’t experience an abrupt leap of land under their feet during an earthquake. Instead, they feel the ground shaking and rolling, as if they were on a roller coaster. This shaking, which sometimes rattles dishes hundreds of kilometers from the earthquake’s epicenter, is responsible for much of an earthquake’s destructive power.

Rating Earthquakes

The energy released by an earthquake increases exponentially with magnitude. An increase of magnitude by one indicates a 30-fold increase in the energy released by the quake. Thus, a magnitude 8 earthquake releases 30 times the energy of a magnitude 7 and 900 times the energy of a magnitude 6.

Although magnitude is a good indicator of how much energy is released along a fault, it is just one variable used to determine how much damage an earthquake causes. Other critical factors include the total amount of human infrastructure in the area, the methods used to build that infrastructure, and the type of soil beneath it. The Kobe earthquake, the most expensive natural disaster in history, had a magnitude of 6.7. But the 1999 magnitude 7.1 Hector Mine earthquake, Southern California’s largest earthquake in the last decade, struck a sparsely populated area of the Mojave Desert. The Hector Mine quake cracked the concrete in one bridge, severed the rails in one section of train track, and knocked stucco from one building. That was the full extent of its damage.

More Good Than Harm

Because earthquakes can do so much damage so quickly, it is hard to think of them as anything other than disasters. But in the big picture, they do more good than harm. Though sometimes catastrophic, the damage caused by earthquakes is fairly localized, yet earthquakes are one of the essential geological processes that make Earth habitable. 

People tend to think of the Earth as static, as having an eternal unchanging solidity, but the planet’s apparent solidity is misleading. The Earth is constantly moving and changing. Molten rock rises through the upper mantle, volcanoes erupt, earthquakes shake the ground beneath us. The Earth is dynamic, vital, living. “Earthquakes are like the pulse of the Earth,” says Tom Rockwell. “They’re occurring all the time somewhere.” Thousands of earthquakes occur every day around the globe. Although most are too small to be felt, every year there are more than 1,000 magnitude 5 quakes, 100 magnitude 6s, 15 or 20 magnitude 7s, and about 1 magnitude 8. In less than 50 million years, earthquakes large and small pushed the Himalayas to their towering heights. This process hasn’t ended. The Indian Plate is still slamming into the Eurasian Plate, raising the mountains ever higher.

Earthquakes do more than create dramatic topography. “If we didn’t have active geological processes on this planet, we wouldn’t have people, we wouldn’t have plants,” says Tom Henyey, a professor of geological sciences at the University of Southern California. Volcanic eruptions, which also depend on plate tectonics, spew vital gases into the atmosphere, gases that help sustain life. And without earthquakes continually uplifting mountains, erosion would eventually wear them flat. If no mountains blocked the clouds to create rain shadows, the climate would be very different. Perhaps there would be no rainfall at all. “It would be a very sterile environment,” says Henyey. “Earthquakes are a manifestation of the active geological processes that give us life.”

Every day, people pack up their belongings and head to Southern California, their heads filled with visions of a sunny playground where hills and mountains overlook the golden shore. A few may hesitate because of California’s notorious history of devastating earthquakes. Ironically, it is exactly that long legacy of earthquakes that created the landscape that attracts people to Southern California. “Between the beaches and the snow on Mount Baldy, you can go skiing and surfing in the same day here in Southern California,” says geophysicist Ken Hudnut of the U.S. Geological Survey. “And the reason those mountains are so close to the ocean is because of the movement between the plates pushing up those mountains.” In other words, the mountains are there because of earthquakes.

All along the west coast of the United States, two enormous chunks of Earth, the Pacific and the North American tectonic plates, are slowly sliding past each other. Many places near this plate boundary are at high risk for earthquakes, including the San Francisco area, the Pacific Northwest, and Alaska, yet fully half the nation’s earthquake hazard is in Southern California.

Why is the potential for disaster so much greater in Southern California? Part of the answer is that the region is scarred with hundreds of active faults. But earthquake hazard is a measure not only of the expected number and magnitude of earthquakes but also of the potential damage–to people, to property, to infrastructure. A large network of faults plus a large number of people equals a large earthquake hazard. 

Each year, more than 15,000 earthquakes rumble underneath Southern California. Although most of these earthquakes are too small to be felt, the region averages two felt earthquakes per week. Fifteen million people living on top of such an active geologic area makes Southern California a natural focal point for earthquake research. 

Shattered Earth

To have an earthquake you need a fault, and Southern California has an exceptional abundance of them. The main reason for this is the Big Bend, a large curve in the boundary between the Pacific and North American plates. Up and down the entire boundary, the plates are slowly grinding past one another. But in the Los Angeles area, the Big Bend prevents them from sliding past each other as easily as they do elsewhere. Lucy Jones, the scientist in charge of Southern California for the U.S. Geological Survey’s Earthquake Hazards Program, compares it to trying to move a piece of plate glass around a corner. The glass gets smashed and “you have all these little pieces that need to be moved around.” This shattered glass effect has left Southern California with hundreds of faults capable of producing damaging earthquakes.      

The movement of tectonic plates does more than just shake the ground; it literally reshapes the landscape. As the Pacific Plate pushes northward, trying to force its way past the Big Bend, it is compressing the Los Angeles Basin and uplifting the San Gabriel Mountains to the north of Los Angeles. The 1971 San Fernando earthquake raised the San Gabriel Mountains by about 2.5 meters (eight feet). 

The Big One

The Big Bend is actually a 300-kilometer (200-mile) stretch of the San Andreas Fault, which runs most of the length of California--from Point Arena, about 175 kilometers (110 miles) north of San Francisco, to the Salton Sea near the Mexican border. Flying over parts of California, the San Andreas Fault looks like a giant scar cutting through the countryside. Though hundreds of other faults crisscross Southern California, in the public imagination the San Andreas is synonymous with catastrophic earthquakes. This isn’t completely irrational.

“The San Andreas Fault concentrates about two-thirds of the motion between the North American and the Pacific Plates,” Ken Hudnut points out, “so the San Andreas Fault has been the source of the greatest historical earthquakes.” Both the 1857 Fort Tejon earthquake in the Los Angeles area and the 1906 San Francisco earthquake were larger than magnitude 8. “The really big earthquakes, what we call ‘The Big Ones,’ tend to be on the San Andreas Fault,” says Hudnut.

The reasons for this are twofold. The size of an earthquake is directly related to the length of a fault, and the San Andreas is the longest fault in California by far. The San Andreas is also the fastest-moving fault in Southern California, which means that it has earthquakes more frequently than other major faults. While many other faults go tens of thousands of years between damaging earthquakes, each section of the San Andreas Fault has a big earthquake on average every 100 to 250 years.          

Although a large earthquake strikes the section of the San Andreas Fault nearest Los Angeles on average every 130 years, the last one was the Fort Tejon quake in 1857, when the area was relatively unpopulated. In that quake, there was about six meters (20 feet) of slip along the fault. The section of the fault farther south, near Palm Springs, hasn’t had a major earthquake since 1680. The fact of the matter is that Southern California is overdue for a big earthquake on the San Andreas Fault. “The next big one could happen any moment,” Hudnut says. “You just never know.”

But the San Andreas Fault is only a part of Southern California’s earthquake hazard. Southern California’s shattered ground has more than 300 active faults, most of which are part of the San Andreas Fault system. A few of these, including the San Jacinto and the Elsinore, are long, fast-moving faults likely to produce major earthquakes. But the most devastating quakes in recent years, such as the 1971 San Fernando earthquake and the 1994 Northridge earthquake, were on relatively minor faults that were not expected to move. These smaller faults actually produce the majority of damaging earthquakes in Southern California.

Shaking Like a Bowl of Jello 

Although Southern California as a whole has half the nation’s earthquake hazard, that hazard is not evenly distributed throughout the region. Los Angeles County alone can proudly claim a quarter of the nation’s earthquake hazard. Part of the reason is the sheer number of people living there. When the Fort Tejon quake hit in 1857, the area was relatively unpopulated. Today, about ten million people make Los Angeles County their home. Similarly, in 1857 few large structures had been constructed in Los Angeles. Today, office buildings, freeways, bridges, and ports blanket the area. 

But there is a second, geologic, reason that the Los Angeles Basin has a particularly high earthquake hazard. The mountains surrounding Los Angeles are granite, a very hard rock, but the Los Angeles Basin is made up of very soft, loose sediment. When the seismic waves from an earthquake that began in the hard mountains hit the soft basin, they slow down. To carry the same amount of energy, the waves must then get bigger in amplitude. These amplified waves cause greater shaking. In the L.A. Basin, the shaking is amplified by a factor of five to ten, with the worst shaking striking the center of the basin, where the sediment is the thickest.

Many earthquake experts compare the Los Angeles Basin to Jello inside a hard granite bowl. “If you jiggle that bowl of Jello, it jiggles for a long time,” says Andrea Donnellan, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena. “The L.A. Basin just sloshes back and forth as a result of earthquakes, so they can be very damaging here.” 

Living in Earthquake Country 

Southern California’s high earthquake hazard makes it a very desirable place to live if you happen to be an earthquake researcher. “We’ve got an earthquake-producing machine below us,” says Tom Henyey, a professor of geological sciences at the University of Southern California. “If I’m going to study earthquakes, I’ve got to live in earthquake country.”

Yet other people are also drawn to Southern California because of its earthquakes, though unwittingly, for it is the region’s active geology that gives it a dramatic landscape and pleasant climate. It is earthquakes that have built up Southern California’s hills and mountains, which in turn catch the clouds that drop rain on the region. “If we gave up our mountains, we’d have an empty desert that wouldn’t be a whole lot of fun to live in,” says Lucy Jones. 

“You have to take both,” explains Nancy King, a geophysicist with the U.S. Geological Survey. “You take an active geologic area and you get a lot of beauty. If you don’t have active plate tectonics, erosion is the major geologic force operating, and gradually over millions of years you get flat topography.”

If they are wealthy enough, people in California often take full advantage of the spectacular scenery earthquakes have provided them by building their homes on top of hills, often on stilts. Ironically, this puts them at greater risk from earthquakes. “One rule of thumb in Southern California is that if you live on a hill, you’re probably living near a fault, because the hills are uplifted by faults,” Tom Henyey says. Henyey thinks this is a reasonable risk since most faults go thousands of years between major quakes.

 Likewise, when most people weigh the pros and cons of living in Southern California, they find living in earthquake country a risk worth taking. No matter where you live in the world, you are in the shadow of some natural hazard. Although people in other parts of the country may face little danger from earthquakes, they have to put up with hurricanes and tornadoes, winter storms and withering heat. In Southern California, earthquakes are the big danger. Since earthquakes are inevitable, the question becomes, how do you minimize the damage they cause? In recent years, scientists have begun using space-based technology to paint a more detailed picture of Southern California’s earthquake hazard, information that engineers and planners can use to focus their attention and resources on the areas that need it most.

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 up – strain 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.”