Its moment magnitude number, often mistakenly called the Richter scale that is now considered obsolete, reports the magnitude (M) of earthquakes. The moment magnitude scale is logarithmic, which means that a magnitude 5.0 earthquake (M 5.0) is about 31 times weaker than an M 6.0 quake, and is 1,000 times weaker than an M 7.0 temblor. The primary danger to people posed by earthquakes is from the structural failure of buildings due to ground shaking. A building can collapse partially or totally when the building loses structural integrity. This is more likely to happen when buildings are built on unconsolidated soils, such as sand or clay, which tends to amplify the ground motion. People are killed or injured when buildings or other structures collapse; the major cause of death in the Loma Prieta (San Francisco) earthquake of 1989 was the collapse of an elevated freeway in Oakland.
A great deal of experience has been amassed on how to build or locate structures to reduce the risk of building collapse in an earthquake. Engineers know how to design buildings that may be damaged in an earthquake, but retain sufficient structural integrity to allow occupants to escape. Modern steel-frame skyscrapers tend to withstand earthquakes remarkably well. The most dangerous types of buildings are unreenforced masonry buildings, and concrete and steel structures that are not built with seismic safety in mind. The 1906 San Francisco and 1925 Santa Barbara earthquakes in California revealed the problems with unreenforced masonry buildings, but the 1933 Long Beach earthquake damaged or destroyed down many masonry buildings, particularly schools. As a result, the California legislature passed the Field Act, which required that public buildings such as schools be built to withstand earthquake forces. In the years that followed, as experience accumulated with other great quakes (1964 Alaska, 1971 San Fernando), improved building codes and practices were adopted that greatly reduced the risk to human life from moderately large earthquakes.
In the United States, various efforts, notably including the National Earthquake Hazard Reduction Program (NERHP), have promoted research, improved building practices, and better public information about the earthquake hazard. Thus, in the United States, the long-term trend has been fewer casualties, but more property damage in earthquake stricken areas. However, seismologists, engineers, and emergency managers have warned that cities like San Francisco, Los Angeles, and Anchorage have not recently experienced “the Big One,” an earthquake that would catastrophically damage the region. Continued efforts to improve buildings and to manage risk will make that earthquake, when it occurs, much less likely to kill as many people as it would without these measures. Other nations in the world are not so advanced. For example, in the 1988 earthquake in Armenia, then part of the Soviet Union, poor building practices—in particular, in high-rise concrete and steel buildings that would not have been built under American building codes—catastrophically failed, killing 25,000 people and injuring at least 15,000. Recent earthquakes in Indonesia and along the India-Pakistan border have further illustrated the importance of improved building techniques.
BIBLIOGRAPHY
P. R. Berke; Timothy Beatley, Planning for Earthquakes: Risk, Politics and Policy (Johns Hopkins University Press, 1992).
B. A. Bolt, Earthquakes and Volcanoes: Readings from Scientific American (W.H. Freeman, 1980).
B. A. Bolt, Earthquakes (W.H. Freeman, 2003).
Ian Burton; R. W. Kates; G. F. White, The Environment as Hazard (Guilford Press, 1993).
Carl-Henry Geschwind, California Earthquakes: Science, Risk, and the Politics of Hazard Mitigation (Johns Hopkins University Press, 2001).
Roy D. Hyndman, “Giant Earthquakes of the Pacific Northwest,” Scientific American (v.273/6, 1995).
Thomas A. Birkland
State University of New York, Albany
Copyright © 2007 by SAGE Publications, inc.
