Earthquake
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. Earthquakes can release energy thousands of times greater than the world's first atomic bomb.

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 travelling, whereas secondary or transverse shear waves (S waves) impart vibrations perpendicular to their 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 be recorded at geophysical research stations throughout the world.

II 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. In around ad 130 the Chinese scholar Chang Heng, 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. Perhaps drawing on his knowledge of the strength and behaviour 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 devised a forerunner of today's earthquake-recording device, or seismograph (Greek, seismos, “earthquake”). A simple pendulum and needle suspended above a smoked-glass plate, it was the first instrument used in seismology 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. His device, using a magnetic pendulum suspended between the poles of an electromagnet, ushered in the modern era of earthquake research.

III 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 plate tectonic quakes is stresses set up by movements of the dozen or so major and minor plates that make up the Earth's crust. Most tectonic quakes occur at the boundaries of these plates, in zones where one plate slides past another—as at the Pacific Rim and at the San Andreas Fault in California—or is subducted (slides beneath the other plate). Subduction-zone quakes account for nearly half of the world's destructive seismic events and 75 per cent of the Earth's seismic energy. They are concentrated along the so-called Ring of Fire, a narrow band about 38,600 km (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 680 km (422 mi).

Tectonic earthquakes beyond the Ring of Fire occur in a variety of geological settings. Mid-ocean ridges—the seafloor-spreading centres 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 per cent of the Earth's seismic energy, but they are recorded daily on the instruments of the worldwide network of seismological stations.

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