Peter Steffen/AFP/Getty Images
The Sendai event, which struck Japan in March with a magnitude of 9.0, resulted from a far more dangerous "megathrust," or subduction fault, in which one plate slides underneath another. The contiguous U.S. hasn't experienced such an unleashing of geological violence since Jan. 26, 1700, when a subduction fault off the coast of the Pacific Northwest ruptured with a magnitude of 9.0, sending a 30-foot wall of water roaring across the ocean. Twelve hours later, it smashed buildings and killed people in coastal Japan, 4500 miles away.
In 1700, the Pacific Northwest was sparsely inhabited. Today, the combined population of Portland and Seattle is more than 1.1 million people, and based on geological evidence, there's a 10 to 15 percent chance another quake of that magnitude will strike those cities in the next 50 years. Other cities in quake-prone regions are also vulnerable, including such unlikely places as Memphis, Tenn., Charleston, S.C., and New York City. And California's San Andreas Fault, while not of the megathrust type, remains formidable. Running close to most of the state's big population centers, it has a history of producing a large quake roughly every 150 years. (The last one was 154 years ago.) In all, 75 million Americans live in areas where quakes could cause substantial damage.
Despite decades of research, seismologists are still unable to predict these catastrophic events. Some experts fear they may be chaotic phenomena and hence fundamentally unpredictable. Instead, scientists focus on more attainable goals, deploying sensor arrays, satellite instrumentation and computer simulations to develop a clearer picture of how earthquakes happen, where they are likely to strike and how much damage they can do. "We're working on the basic physics," says Bill Ellsworth, a seismologist with the U.S. Geological Survey. "We need to answer these questions: How strong will the shaking be? Is there an upper limit? Those are critical in terms of designing structures that remain standing."
Halfway between Los Angeles and San Francisco, a leveled patch of dirt on the shoulder of a grassy hill is ringed with a chain-link fence and dotted with a few small buildings. Beneath one of them is something quite extraordinary: a peephole deep into one of the most active parts of the Earth's crust. A team led by Ellsworth spent four years drilling a 2-mile-deep, 8-inch-diameter well for a project called the San Andreas Fault Observatory at Depth, or SAFOD.
The borehole descends straight down, then curves eastward toward the shear zone between the Pacific and North America plates. The original borehole cut across the fault, but the movement of the platesabout an inch and a half per year, relative to each othersoon began severing it. Today, an instrument package that contains a seismometer, an accelerometer, a tiltmeter and other sensors rests in the shaft a short distance from the slippage. Ellsworth hopes SAFOD will operate for at least another decade, collecting fault-line data.
In a sister program, Ellsworth's colleagues are trying to deduce the behavior of the Earth's crust through precise measurements on the ground. The Plate Boundary Observatory consists of a network of ground sensors spread over thousands of miles along the Pacific Coast. Thanks to super-sensitive GPS technology, these sensors can collectively measure deformation of the landscape with millimeter accuracy. Some scientists believe that sudden deformations might offer an early quake warning.
Farther south stands another outpost of earthquake science: America's largest shake table, the Large High Performance Outdoor Shake Table (LHPOST) at the University of California, San Diego. On a recent sunny spring morning, a three-story masonry building atop the facility's 40-foot-long platform begins lurching violently from side to side. Within seconds, it starts to crumble. Bricks above a window spill down in sheets. Cracks spider-web out through the cinder block wall. It shatters and tumbles. By the time the movement stops 22 seconds later, the ground is heaped with rubble.
It's been a bad day for this particular building but a jackpot for science. By testing the effects of shaking on full-scale structures, LHPOST provides engineers the real-world data they need to hone their computer models and figure out how to make buildings safer in the future.
In this case, the ruined structure was made of a reinforced concrete frame with walls of unreinforced masonrya style that was popular in the 1920s and that can still be found throughout the California danger zone. "Since many of these buildings have historic value, they can't just be torn down and replaced," says UCSD professor of structural engineering Benson Shing. In the wake of the destructive test, Shing's team rebuilds the structure, then retrofits it with antiearthquake measures. On the ground floor, they spray the walls with a 1-inch layer of cement mixed with sand and glass fibers. On the second floor, they glue sheets of glass fiberreinforced polymer onto the wall much as one would apply wallpaper. Once the shake table gets rocking, a few cracks appear, but the walls remain intact.
Japan is the most earthquake-savvy nation on earth. Its population is psychologically prepared, its building codes are tough, and it lavishes millions of dollars on research. Its E-Defense shake table, located at the Hyogo Earthquake Engineering Research Center, north of Kobe, is the world's largest, capable of holding full-scale buildings up to seven stories high and weighing up to 2.5 million pounds.
Japan has also spent decades implementing an Earthquake Early Warning (EEW) system that transmits an alarm signal to population centers. Because a quake's motion travels at the speed of sound and a fiberoptic signal can travel near the speed of light, such a system could provide anywhere from a few seconds to more than a minute of warning.
The EEW helped save the lives of more than 150 passengers aboard a bullet train in 2004, when a magnitude 6.8 quake struck the island of Honshu. The train was traveling at 125 mph about 11 miles from the epicenter. Within 3.6 seconds of the quake, an observatory detected the tremors. Simultaneously, an alarm issued and the train's power supply was cut. The operator applied the brakes, significantly reducing speed. Two and a half seconds later, violent tremors hit. The train derailed, but no one aboard was injured. A similar system gave Tokyo 80 seconds' warning of the Sendai quake.
The U.S. is just starting to explore these kinds of technologies. The USGS has begun to install 120 seismic monitors in the most vulnerable areas of California, mainly in the Los Angeles basin and the northern part of the state. Still, building the system will take time. "We're talking at least three years, and probably five" before it becomes operational, project chief David Oppenheimer says.
With public support, similar systems could be placed in every quake-prone part of the U.S.even in places whose residents may not realize that slowly, invisibly, inexorably, the earth is moving beneath their feet.