vibrations produced in the earth's crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. The vibrations can range from barely noticeable to catastrophically destructive. Six kinds of shock waves are generated in the process. Two are classified as body waves—that is, they travel through the earth's interior—and the other four are surface waves. The waves are further differentiated by the kinds of motions they impart to rock particles. Primary or compressional waves (P waves) send particles oscillating back and forth in the same direction as the waves are traveling, whereas secondary or transverse shear waves (S waves) impart vibrations that are perpendicular to the direction of travel. P waves always travel at higher velocities than S waves, so whenever an earthquake occurs, P waves are the first to arrive and to be recorded at geophysical research stations worldwide. See also Geology: Geological Processes; Seismology.

History of Earthquake Study.

Questions regarding the nature of earthquakes have occupied the minds of people living in earthquake-prone areas since ancient times. Some of the ancient Greek philosophers attributed quakes to subterranean winds, whereas others blamed them on fires in the depths of the earth. Around ad 130 the Chinese scholar Chang Heng (78–139), reasoning that waves must ripple through the earth from the source of an earthquake, constructed an elaborate bronze vessel to record the passage of such waves. Eight balls were delicately balanced in the mouths of eight dragons placed around the circumference of the vessel; a passing earthquake wave would cause one or more of the balls to drop.

Earthquake waves were observed in this and other ways for centuries, but more scientific theories as to the causes of quakes were not proposed until modern times. One such concept was advanced in 1859 by the Irish engineer Robert Mallet (1810–81). Perhaps drawing on his knowledge of the strength and behavior of construction materials subjected to strain, Mallet proposed that earthquakes occurred “either by sudden flexure and constraint of the elastic materials forming a portion of the earth's crust or by their giving way and becoming fractured.” Later, in the 1870s, the English geologist John Milne (1850–1913) devised a forerunner of today's earthquake-recording device, or seismograph (Gr. seismos, “earthquake”). A simple pendulum and needle suspended above a smoked-glass plate, it was the first instrument to allow discrimination of primary and secondary earthquake waves. The modern seismograph was invented in the early 20th century by the Russian seismologist Prince Boris Golitzyn (1862–1916). His device, using a magnetic pendulum suspended between the poles of an electromagnet, ushered in the modern era of earthquake research.

Kinds and Locations of Earthquakes.

Three general classes of earthquakes are now recognized: tectonic, volcanic, and artificially produced. The tectonic variety is by far the most devastating, and such quakes pose particular difficulties for scientists trying to develop ways to predict them. The ultimate cause of tectonic quakes is stresses set up by movements of the 13 or so major and minor plates that make up the earth's crust (see Plate Tectonics). Most tectonic quakes occur at the boundaries of these plates, in zones where one plate slides past another—as at the San Andreas Fault in California, North America's most quake-prone area—or is subducted (slides beneath the other plate). Subduction-zone quakes account for nearly half of the world's destructive seismic events and 75 percent of the earth's seismic energy. They are concentrated along the so-called Ring of Fire, a narrow band about 38,600 km (about 24,000 mi) long, that coincides with the margins of the Pacific Ocean. The points at which crustal rupture occurs in such quakes tend to be far below the earth's surface, at depths of up to 645 km (400 mi). Alaska's powerful Good Friday earthquake of 1964 is an example of such an event.

Tectonic earthquakes beyond the Ring of Fire occur in a variety of geological settings. Mid-ocean ridges—the seafloor-spreading centers of plate tectonics—are the sites of numerous such events of moderate intensity that take place at relatively shallow depths. These quakes are seldom felt by anyone and account for only about 5 percent of the earth's seismic energy, but they are recorded daily on the instruments of the worldwide network of seismological stations. Another setting for tectonic earthquakes is a zone stretching across the Mediterranean and Caspian seas and the Himalayas, terminating in the Bay of Bengal. Within this zone, which releases about 15 percent of the earth's seismic energy, continental landmasses riding on the Eurasian, African, and Australian plates are being forced together to produce high, young mountain chains. The resulting quakes, which occur at shallow to intermediate depths, have often devastated areas of Portugal, Algeria, Morocco, Italy, Greece, the Balkan countries, Turkey, Iran, and India.

One other category of tectonic earthquake includes the infrequent but large and destructive quakes that occur in areas far removed from other forms of tectonic activity. Prime examples of these so-called midplate quakes are the three massive tremors that shook the region around New Madrid, Mo., in 1811 and 1812. Powerful enough to be felt more than 1600 km (1000 mi) away, these shocks produced movements that rerouted the Mississippi River. Another example is the quake that struck Charleston, S.C., in 1886. Geologists believe that the New Madrid quakes are a symptom of forces tearing apart the earth's crust, forces such as those that created Africa's Great Rift Valley.

Of the two classes of nontectonic earthquake, those of volcanic origin are seldom very large or destructive. They are of interest chiefly because they often herald impending volcanic eruptions, as they did in the weeks preceding the eruption of Mount Saint Helens, Wash., in May 1980. Such quakes originate as magma works its way upward, filling the chambers beneath a volcano. As the flanks and summit of the volcano swell and are tilted, rupture of the strained rocks may be signaled by swarms of small earthquakes. On the island of Hawaii, seismographs may register as many as 1000 small quakes a day before an eruption occurs.

Humans can induce earthquakes through a variety of activities, such as the filling of new reservoirs, the underground detonation of atomic explosives, or the pumping of fluids deep into the earth through wells. For example, in 1962 the city of Denver, Colo., began to experience earthquakes for the first time in its history. The discovery was made that the tremors correlated in time with the pumping of waste fluids into deep wells at an arsenal east of the city. When pumping was discontinued, the earthquakes persisted for a while and then ceased.

Earthquake Effects.

Earthquakes produce various effects of concern to the inhabitants of seismically active regions. They can cause great loss of life by destroying structures such as buildings, bridges, and dams, and they can also trigger devastating landslides. For example, a quake near Hebgen, Mont., in 1959 caused a slide that killed several people and temporarily blocked the Madison River, thereby creating a lake and threatening the town of Ennis with a catastrophic flood.

Another destructive effect of earthquakes is the generation, usually by subsea tremors, of so-called tidal waves. Because such waves are unrelated to the tides, they are more properly called seismic sea waves or—their Japanese name—tsunami. These towering walls of water have struck populated coastlines with such violent fury that entire towns have been destroyed. In 1896 Sanriku, Japan, with a population of 20,000, suffered such a devastating fate. On Dec. 26, 2004, a devastating tsunami triggered by a magnitude 9.0 earthquake on the Indian Ocean floor killed at least 178,000 people in Indonesia, Sri Lanka, S India, and other countries of S Asia and E Africa; nearly 50,000 other people were missing and presumed dead.

Where buildings have been constructed on filled ground, the liquefaction of soils is another seismic hazard. When subjected to the shock waves of a quake, soil used in landfill may lose virtually all its bearing strength and behave, in effect, like quicksand. Buildings resting on these materials have literally been swallowed up, as in the San Francisco earthquake of 1906.

Intensity Scales.

Seismologists have devised two scales of measurement to enable them to describe earthquakes quantitatively. One is the Richter scale—named after the American seismologist Charles Francis Richter (1900–85)—which measures the energy released at the focus of a quake. It is a logarithmic scale that runs from 1 to 9, though no upper limit exists; a magnitude 7 quake is 10 times more powerful than a magnitude 6 quake, 100 times more powerful than a magnitude 5 quake, 1000 times more powerful than a magnitude 4 quake, and so on. An estimated 800 quakes of magnitudes 5 to 6 occur annually worldwide, in comparison with about 50,000 quakes of magnitudes 3 to 4, and only about one earthquake of magnitudes 8 to 9. Until 1979 an earthquake of magnitude 8.5 was thought to be the most powerful possible; since then, however, improvements in seismic measuring techniques have enabled seismologists to refine the scale, and 9.5 is now considered to be the practical limit. On the basis of the newly refined scale, the magnitude of the 1906 San Francisco earthquake has been revised from 8.3 to 7.9, while the Alaskan earthquake of 1964 has been upgraded from 8.4 to 9.2.

The other scale, introduced in the late 1800s by the Italian seismologist Giuseppe Mercalli (1850–1914), measures the intensity of shaking with gradations from I to XII. Because seismic surface effects diminish with distance from the focus of the quake, the Mercalli rating assigned to the quake depends on the site of the measurement. Intensity I on this scale is defined as an event felt by very few people, whereas intensity XII is a catastrophic event that causes total destruction. Intensities II to III on the Mercalli scale are roughly equal to magnitudes 3 to 4 on the Richter scale, and XI to XII to 8 to 9.

Earthquake Prediction.

Attempts at predicting when and where earthquakes will occur have met with some success. China, Japan, Russia, and the U.S. are most actively supporting such research. In 1975 the Chinese predicted the magnitude 7.3 quake at Haicheng, evacuating 90,000 residents two days before the quake damaged 90 percent of the city's buildings. One of the clues that led to this prediction was a chain of low-magnitude tremors, called foreshocks, that had begun about five years earlier. Other potential clues being investigated are tilting or bulging of the land surface and changes in the earth's magnetic field, in the water levels of wells, and even in animal behavior. A new method under study in the U.S. involves measuring the buildup of stress in the crust of the earth. On the basis of this method the U.S. Geological Survey, in April 1985, predicted that an earthquake of magnitude 5.5 to 6 would occur on the San Andreas fault, near Parkfield, Calif., sometime before 1993. It never occurred, but four quakes shook California after the prediction: San Francisco (1989), 7.1; Cape Mendocino (1992), 7.0; Yucca Valley (1992), 7.5; and Los Angeles (1994), 6.6.

Devastating Earthquakes.

Historical records of earthquakes before the middle of the 18th century are generally lacking or unreliable. Among the ancient quakes for which reasonably trustworthy records exist include the one that occurred off the coast of Greece in 425 bc, making Euboea an island; the one that destroyed the city of Ephesus in Asia Minor in ad 17; the one that leveled much of Pompeii in 63; and those that partially destroyed Rome in 476 and Constantinople (now İstanbul) in 557 and again in 936. In the Middle Ages, severe quakes occurred in England in 1318, Naples in 1456, and Lisbon in 1531.

An earthquake in 1138 in Aleppo, Syria, killed 230,000. The earthquake in 1556 in Shaanxi (Shensi) Province of China, which killed over 800,000 people, was one of the greatest natural disasters in history. In 1693, an earthquake in Sicily took about 60,000 lives; and early in the 18th century the Japanese city of Edo (the site of modern Tokyo) was destroyed, with the loss of some 200,000 lives. In 1755 the city of Lisbon was devastated by a quake and about 60,000 people died. Quito, now the capital of Ecuador, was shaken by an earthquake in 1797, and more than 40,000 died.

In North America, the series of earthquakes that struck southeastern Missouri, near New Madrid, in December 1811–January 1812 were probably the most powerful experienced in the U.S. in historical time. The most famous U.S. earthquake, however, was the one that shook the San Francisco area in 1906, causing extensive damage and taking about 700 lives.

For further information on this topic, see the Bibliography, sections 421. Plate tectonics, 423. Earthquake, volcano, 431. Storm, 1224. San Francisco.