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
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....