The actual distance that buildings move during an earthquake is not usually that great compared to their size, and on its own does not cause much damage. It is the buildings' acceleration -- how quickly they speed up when the ground begins to shake -- that can cause them to collapse. That acceleration and the buildings' weight are the two main factors that determine how much strain an earthquake exerts on a structure.

A factor that can exacerbate the damage an earthquake causes is each building's natural tendency to sway back and forth at a particular rate. That rate tends to vary depending on a building's height -- with shorter buildings tending to sway more quickly than taller buildings. When the ground is shaking at the same rate as a building, its response to the earthquake is amplified and in some cases it can begin to accelerate more than twice as quickly as the ground beneath it.

In the 1985 Mexico City earthquake, over half the buildings that collapsed were about 20 stories tall, because those buildings' natural tendency to sway matched the ground's movement during the earthquake.

Another factor influencing a building's ability to withstand temblors is the ground under the building. Looser soil shakes more violently than firmer soil, which itself moves more than hard rock during a quake. Scientists blamed this phenomenon for the collapse of a stretch of a double-decker Oakland, Calif., highway during the 1989 earthquake in San Francisco. According to the U.S. Geological Survey, the ground under that stretch of the road was man-made fill over soft mud, and so it shook more than the ground under the rest of the highway.

Engineering buildings that can survive earthquakes
The conventional approach to building earthquake-resistant buildings was to make them strong enough to withstand the force of the ground's movement. The engineers working on such buildings configured combinations of floors, walls, beams and columns knowing each one's ability to keep the building upright during an earthquake.

Over the past 25 years, more advanced techniques have aimed to make structures safe during an earthquake by reducing the amount that ground shaking impacts them.

One method that scientists are using is placing something underneath the building that can absorb much of the energy from an earthquake. Placing layers of rubber and steel between a building and its foundation is one way to lessen the damage. The buffer is designed to keep up-down motion to a minimum, while being able to move side-to-side when the ground moves.

In an earthquake, a building that sits on top of such a device will still sway but is much less likely to be damaged. According to researchers at the University of Buffalo, experiments show that these devices can cut the forces that shake a building during an earthquake by 25 percent.

The device also slows down the side-to-side motion of an earthquake on buildings, bridges and other structures. During an earthquake, structures with one of these systems will rock back and forth like a boat on the ocean, but will not break apart, Michael Constantinou, co-director of the University of Buffalo's earthquake simulation laboratory, told the Online NewsHour.

Often there is a lead plug in the middle of the device that gives off heat as it bends back and forth during an earthquake. By converting some of the energy from the shaking into heat, it makes the overall device more effective by preventing some of the energy from being transferred into the building.

Another system that works on a similar principle uses a device that slides along a curved surface that is higher on the edges than it is in the middle. During an earthquake, the device moves freely along the curved surface and the upward slope of the sliding surface dampens the overall motion. Researchers at the Multidisciplinary Center for Earthquake Engineering Research tested these devices on shaking tables that mimic earthquake movements and found that they made a building able to withstand an earthquake six times stronger than it would have been able to without protection.

This technology was used in 1994 to make San Francisco's U.S. Court of Appeals building, which dates back to 1905, safe for possible future earthquakes. The system was made up of 256 individual devices that rolled back and forth, according to Constantinou.

Another strategy that engineers are using is outfitting buildings with devices that dissipate energy that would shake a building back and forth during an earthquake. These dampening devices are typically part of the bracing connecting floors to one another and assist in stabilizing the building. During an earthquake, the devices work like giant car shock absorbers to slow the floors' movement.

The dampening device used in the San Francisco civic center and many other structures involves a cylinder filled with oil. Engineers design these devices so that when one end of the cylinder is pushed, the oil moves. The oil's temperature increases when it moves, converting much of the energy that would have shaken the building into heat.

Scientists have been working on advanced versions of these systems that would respond to measurements taken during an earthquake and adjust almost instantaneously to the particular characteristics of those ground movements. Various technical difficulties have prevented these systems from being used to help protect structures, Constantinou said.

-- By Karyn Schwartz, Online NewsHour