Article: Earthquakes—The Pulse of the Planet
“Earthquakes and plate tectonics are a vibrant and critical element that keeps this Earth alive. They’re part of the pulse. They’re part of the breathing of the planet that makes it a great place to live.”
Tom Rockwell, professor of geological sciences, San Diego State University
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.
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.”
More About This Resource...
Our innovative Science Bulletins are an online and exhibition program that offers the public a window into the excitement of scientific discovery. This essay was published in April 2003 as part of the Quakes from Space Earth Feature.
- It begins by explaining that Earth's surface is like a giant jigsaw puzzle made of tectonic plates. But unlike the pieces of a jigsaw puzzle, the tectonic plates are moving in relation to one another.
- It then details how most earthquakes occur along plate boundaries, where plates are smashing together, pulling apart, or sliding by one another.
- The essay concludes with a look at how, in the big picture, earthquakes do more good than harm.
Supplement a study of earth science with a classroom activity drawn from this Science Bulletin essay.
- Ask students what they know about earthquakes. What causes them? Where do they occur?
- Have them read the essay (either online or a printed copy).
- Have them write a brief reaction to the article. Were they surprised to learn that, in the big picture, earthquakes do more good than harm? Why or why not?