EARTHQUAKES

EARTHQUAKES occur when the lithospheric plates that compose the surface of the earth shift in relation to one another. Earthquakes are happening constantly all over the world, but major quakes seem to occur only once every two or three years. The size of an earthquake is generally described in terms of intensity and magnitude. The Modified Mercalli scale gauges earthquake intensity by assessing the effect of the quake on the inhabitants of an area. Intensity assessments do not depend on seismographic instruments, but are subjective appraisals of (1) human and animal reaction to shaking and, (2) damage to structures of human origin and to the ground surface. Seismologists use the scale to assign to each earthquake an intensity ranking from I (felt by only a few people under favorable conditions) to XII (total damage).

Magnitude of energy released by an earthquake at its point of origin is a strictly quantitative measure based upon data from seismographs that record maximum wave amplitude (the extreme range of vibrations—or shock waves—caused by the sudden movement of the earth's crust). Charles Richter developed the first magnitude scale in 1935, but a variety of magnitude scales are used today. The Richter magnitude scale has no upper or lower numerical limits; some very small earthquakes are actually given negative numbers. The scale is logarithmic, meaning that each increase of one Richter number represents

a tenfold increase in the magnitude of the earthquake. An earthquake of magnitude 5 releases energy equivalent to that released by 1,000 tons of TNT. Recently, seismologists and earthquake engineers have begun to use a measure called "seismic moment" to estimate the size of seismic sources. Moment magnitude measures the leverage of the forces (couples) across the whole area of the fault slip rather than just wave motion, which is affected by fracture and friction in the rocks.

Scientists have used intensity and magnitude data to prepare seismic risk maps of the United States. One map places locales in one of four zones: Zone 0, such as Florida, is an area where no damage is expected; Zone 3 is one in which a quake intensity of VIII and higher is expected, as in parts of CALIFORNIA. The western United States exhibits the greatest seismic activity in the country—especially Alaska, California, Nevada, Utah, and Montana—although the upper part of the Mississippi embayment, southwest Kentucky, southern Illinois, and southeastern Missouri are also seismically active.

The historical record of earthquakes in the United States goes back to 1638 in NEW ENGLAND and to about 1800 in California. One of the earliest major earthquakes to affect the colonies occurred in the Three Rivers area north of Quebec, along the lower SAINT LAWRENCE RIVER, on 5 February 1663. It caused chimneys to break as far away as Massachusetts Bay. In the early nineteenth century, the Midwest was hit with a series of earthquakes that began in New Madrid, Missouri. The largest of the shocks from these quakes, which occurred in 1811 and 1812, were felt over an area of about 950,250 square miles. Nor has the southern part of the United States been spared. An unpredicted earthquake occurred near Charleston, South Carolina, on 31 August 1886 that did considerable damage in Charleston (much of which was built on filled land) and killed, by some estimates, more than one hundred people. It was the largest seismic event in recorded history on the eastern seaboard. Tremors were felt as far away as New York, Boston, Cuba, and Bermuda. The most notorious earthquake in U.S. history was the one that hit SAN FRANCISCO on 18 April 1906. It was associated with a rupture of the San Andreas fault from the vicinity of Point Delgada to a point in San Benito County near San Juan, a distance of more than 250 miles. The shock hit at 5 A.M. and, almost instantly, building after building crumbled to the ground. Thousands of fires ignited and burned out of control for three days fed by severed electrical wires, overturned coal burners, ruptured gas mains, broken water lines that prevented fighting the fires, and bungled efforts of troops trying to create backfires with

dynamite. The earthquake and fire caused extensive damage throughout northern California, but in San Francisco it obliterated 500 city blocks, caused nearly $500 million in damages, and killed more than 3,000 people.

California was hit again by major earthquakes in 1925 and 1933, but it was almost sixty years before the United States experienced another quake of the magnitude of the 1906 San Francisco earthquake. That event occurred during the late afternoon of 27 March 1964, at 5:36 P.M. local time. An earthquake of magnitude 8.6 on the Richter scale occurred in the sparsely inhabited mountainous area of northern Prince William Sound in south central ALASKA. It caused serious damage within an area of approximately 7,500 square miles, creating large changes in land levels and vertical displacements of nearly thirty-six feet in places along the continental margin. Three hundred people were killed, some from the effects of the quake itself and others by drowning in the seismic sea-wave (tsunami, or tidal wave) caused by the quake.

During the last third of the twentieth century, California again rocked from seismic activity. On 9 February 1971, an earthquake of magnitude 6.5 on the Richter scale struck the San Fernando Valley. This earthquake demonstrated the extent of damage that can occur from a moderate shock centered in a large metropolitan area (the Los Angeles Basin, with a population of 5 million). It caused sixty-five deaths, and damage was estimated to exceed $500 million. Southern California experienced an earthquake measuring 6.4 on the Richter scale in 1979. Eight years later, another quake in the area measured 5.9. In October 1989, the Loma Prieta earthquake struck the San Francisco Bay area, killing at least sixty-three people and collapsing several elevated highways, including a section of the bridge between San Francisco and Oakland. Damages from this earthquake, that registered 7.1 on the Richter scale, reached $6–7 billion. In 1992, a quake measuring 7.4 on the Richter scale struck the desert east of LOS ANGELES, with one fatality. That same year, a quake of 6.9 struck northern California, with no fatalities. And in 1994, a major quake struck the Los Angeles area, with its epicenter in the city's Northridge section. This quake, measuring 6.6 on the Richter scale, damaged many structures in the city, including freeways, and killed at least fifty-one people. Property losses exceeded $4 billion. Scientists have not yet determined how to predict the precise onset of an earthquake; however, since the 1960s, engineers have developed earthquake-resistant building techniques that can reduce the impact of ground shaking. Regardless, public acceptance of earthquake probability estimates and mandated hazard abatement measures often has been slow.

EARTHQUAKES

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