An earthquake is a shaking or trembling of the crust of the earth caused by underground volcanic action or by the breaking and shifting of rock beneath the surface. The volcanic action and shifting rocks create strain which continues to build to a sudden release of pressure resulting in a shock wave. The vibrations produced in the crust can vary from barely noticeable to enormously destructive. Shock waves can be classified into two broad categories. Waves that send particles oscillating back and forth in the same direction as the waves are traveling are called primary. Primary waves, sometimes called compressional waves, travel through the earth beneath the crust. Secondary waves cause vibrations which move perpendicular to the wave. These waves travel on the surface of the earth and move much slower than primary waves. Thus, when an earthquake occurs, seismic centers throughout the world record primary waves before the secondary waves arrive.
Earthquakes have captured the imagination of people living in earthquake prone regions since ancient times. Ancient Greek philosophers thought quakes were caused by subterranean winds, while others blamed them on fires in the bowels of the earth. About AD 130, a Chinese scholar named Chang Heng reasoned that waves must ripple through the earth from the source of an earthquake. By 1859, an Irish engineer by the name of Robert Mallet proposed that earthquakes occurred by either a sudden movement of flexible materials which formed the earth's crust, or by their giving way and fracturing. In the 1870s, an English geologist called John Milne invented the forerunner of today's seismograph. The name comes from the Greek word "seismos," meaning earthquake. The modern seismograph was invented early this century by a Russian seismologist, Prince Boris Golitzyn. This device made possible the modern era of earthquake research.
Although earthquakes have occurred and affected mankind for as long as humans have been around, historical records are either lacking or considered unreliable before the middle of the 18th century. However, some records do exist and date major earthquake events back to 425 BC when one occurred off the coast of Greece resulting in an island named Euboea. Another occurred in Asia Minor about AD 17 and another leveled much of the Roman city of Pompeii in AD 63. During the Middle Ages, severe earthquakes struck England, in 1318; Naples, Italy, in 1456; and Lisbon, Portugal, in 1531. Casualties from major earthquakes have resulted in the greatest disasters in history. The worst occurred in 1556 in the Shannxi (Shensi) Province in China and killed an estimated 800,000 people. During the 18th century, the Japanese city of Edo (site of modern day Tokyo) was struck and killed 200,000. Earthquakes in Sicily in 1693 and Lisbon in 1755 each killed about 60,000 people. In 1797, approximately 40,000 died when Quito, Ecuador was struck.
Historically, the most powerful earthquakes to strike North America occurred in 1811 and 1812 in southeastern Missouri. However, the most famous earthquake to strike in North America was the one that hit San Francisco in 1906, killing 700. The Northridge earthquake which struck Southern California during the early morning hours of January 17,1994, was mild by comparison but resulted in 72 deaths and injured 11,846. The estimated 30 billion dollar loss makes it the most expensive disaster in the history of the United States.
All earthquakes may be classified as one of three types, tectonic, volcanic or artificial. Tectonic earthquakes are caused by movements of the plates which make up the crust of the earth. These earthquakes usually occur far below the surface of the earth (as much as 400 miles) at the boundaries of the plates, where they slide against oneanother causing a buildup of friction and pressure until a sudden release of energy results in an earthquake. Infrequently, these types of earthquakes can occur in the center of...
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