After the earthquake in northwestern Turkey, you may wonder how vulnerable U.S. cities might be to such destructive natural forces. Are our structures capable of resisting so powerful an earthquake? And are people living in California and Alaska the only Americans who need to worry about this?
Land near the eastern rim of the Pacific Ocean, where two of Earth's tectonic plates meet and push against each other, appears to be the most seismically active and geologically threatened in North America. But two of the strongest earthquakes ever recorded in the United States occurred nowhere near the Pacific Ocean.
In 1811 and 1812, hundreds of intense tremors and aftershocks hit New Madrid, Mo., in the state's southeastern corner. They were felt from the Atlantic Coast to the Rocky Mountains. And Charleston, S.C., experienced an earthquake in 1886 that collapsed or damaged most of the city's buildings and killed scores of people. The Charleston earthquake reportedly was felt in places as distant as Milwaukee, Chicago, Boston and Cuba.
In fact, during the last three centuries, more than 3,500 earthquakes have been recorded east of the Mississippi River, some of them strong enough to inflict measurable damage.
The vulnerability of buildings to earthquake is a function of two interrelated probability calculations, or bets. First, based on history and geology, we try to predict the likelihood that a region will be hit by severe earthquakes. Then, depending on that likelihood, we design structures to be safe under the conditions of stress considered probable for that region.
In Washington, we are betting that the earth won't move much. If probability calculations prove wrong, we lose.
For a strong earthquake, comparable to the one in Turkey, to hit Washington, Boston, Philadelphia, Richmond or Atlanta would be catastrophic. Although many buildings in those metropolitan areas would survive, thousands of others would be damaged or destroyed by forces that were never anticipated.
Under normal circumstances, buildings must resist two sets of forces: vertical forces of gravity--the weight of the building itself plus all superimposed loads, such as snow on the roof, furnishings and people on the floors; and abrupt lateral forces of wind or earthquake, making the building sway, bend, twist or even overturn.
Vertical forces are transmitted down to the building's foundations and the earth through columns, piers and walls. These vertical, load-bearing structural elements are tied to and support horizontal structural elements--trusses, girders and beams, rafters and joists, roof decks and floor slabs. Resistance to lateral forces depends on how well the combined vertical-horizontal system is woven together.
During a hurricane or earthquake, a building "feels" forces that are similar but generated differently. Violent hurricane winds tend to blow a building off its unmoving foundation, pushing the structure laterally and uplifting it vertically. Conversely, an earthquake violently shakes the ground and the foundation while the building naturally resists moving.
Thus, with both hurricanes and earthquakes, a building experiences whiplash as shock waves propagate rapidly through the frame. However, earthquakes can create extremely chaotic ground movements, jerking buildings vertically as well as horizontally, more like a bucking bronco than a bumper car.
A building can survive earthquakes if sited, designed and built properly:
* The ground under and around a building must be stable. In Turkey and elsewhere, earthquake-induced building collapses often resulted from poor soil conditions. In particular, soft or damp soils can become fluid when vibrated by strong tremors. Buildings that rest on bedrock or coherent soil strata are the most secure.
* The building must be connected to its foundations in one of two distinct ways. One strategy connects the superstructure and foundations firmly so they act as a rigid unit. The opposite strategy allows the superstructure to move laterally on the foundations, as if on roller skates. The latter approach ensures that the building actually stands still while the earth gyrates below it, transmitting much less force to the structure above.
* The building frame as a whole must be strong enough to hold together. Either the structure can be sufficiently massive and rigid, with no weak spots, to absorb extreme forces with minimal deformation, or the structural frame can be just flexible enough to deform slightly when shaken, safely absorbing and then dissipating the energy of an earthquake. This depends on proper bracing and detailing of connections to permit movement between abutting components.
* The building's cladding and other accouterments--elements attached to the structural skeleton--must be engineered to remain in place and unbroken when subjected to sudden shocks. Many earthquake-related injuries and deaths are caused not by collapsed buildings, but by falling cornices, pieces of masonry, concrete panels, windows and other elements.
* Finally there's the shape and proportions of a building. Long, thin buildings are structurally challenged, being much weaker in the short dimension than in the long. A tall, thin building is at risk since low-frequency ground vibrations can cause it to resonate at its own natural frequency, sharply intensifying deformations as the frame oscillates. Squat buildings are the least problematic, although engineers can stiffen any building shape with enough steel and concrete.
In this region, it would be difficult to predict which buildings would remain standing during an earthquake with a magnitude of more than 7. Most wood-frame houses probably would not collapse, given their geometric proportions and the elasticity of wood skeletal systems. However, many masonry-veneer facades, chimneys, parapets and cornices probably would come tumbling down.
The fate of larger structures--apartment houses, office buildings, schools--would depend on the standards to which they were engineered and constructed, and on how well they have been maintained. Maintenance can be a factor because only one weak point in a building need fail to initiate a catastrophic collapse.
Should you be worried after reading this? Please don't rush to the phone to call your nearest structural engineer. Probability is on your side. Occupying a building in Washington is a pretty safe bet. But you may want to take a good look at any building you spend time in near a geological fault line.
Roger Lewis is a practicing architect and a professor of architecture at the University of Maryland.