You can get the full story on earthquakes in How Earthquakes Work, but a review of the basics will help here. Earthquakes occur when masses of rock in Earth's crust slip and slide against one another. This kind of movement is most common along a fault, a break in a body of rock that can extend for miles or even hundreds of miles. When pieces of crustal rock suddenly slip and move, they release enormous amounts of energy, which then propagates through the crust as seismic waves. At the Earth's surface, these waves cause the ground to shake and vibrate, sometimes violently.
Geologists classify seismic waves into two broad categories: body and surface waves. Body waves, which include P and S waves, travel through the Earth's interior. P waves resemble sound waves, which means they compress and expand material as they pass. S waves resemble water waves, which means they move material up and down. P waves travel through both solids and liquids, while S waves only travel through solids.
After an earthquake strikes, P waves ripple through the planet first, followed by S waves. Then come the slower surface waves -- what geologists refer to as Love and Rayleigh waves. Both kinds move the ground horizontally, but only Rayleigh waves move the ground vertically, too. Surface waves form long wave trains that travel great distances and cause most of the shaking -- and much of the damage -- associated with an earthquake.
If earthquakes only moved the ground vertically, buildings might suffer little damage because all structures are designed to withstand vertical forces -- those associated with gravity -- to some extent. But the rolling waves of an earthquake, especially Love waves, exert extreme horizontal forces on standing structures. These forces cause lateral accelerations, which scientists measure as G-forces. A magnitude-6.7-quake, for example, can produce an acceleration of 1 G and a peak velocity of 40 inches (102 centimeters) per second. Such a sudden movement to the side (almost as if someone violently shoved you) creates enormous stresses for a building's structural elements, including beams, columns, walls and floors, as well as the connectors that hold these elements together. If those stresses are large enough, the building can collapse or suffer crippling damage.
Another critical factor is the substrate of a house or skyscraper. Buildings constructed on bedrock often perform well because the ground is firm. Structures that sit atop soft or filled-in soil often fail completely. The greatest risk in this situation is a phenomenon known as liquefaction, which occurs when loosely packed, waterlogged soils temporarily behave like liquids, causing the ground to sink or slide and the buildings along with it.
Clearly, engineers must choose their sites carefully. Up next, we'll discover how engineers plan for and design earthquake-resistant buildings.