On Earthquakes

On Earthquakes

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.

Historical Overview

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...

... middle of paper ...

...nia prepares for the

"big one." Despite the tremendous impact of the Northridge earthquake,

no one believes that it is the last nor even the largest which will

strike the greater Los Angeles area. A new 36,000 square foot

Emergency Operations Center has been constructed in East Los Angeles.

A series of laws and procedures entitled the "Standardized Emergency

Management System" has continued to make the Mutual Aid System even

more effective. The Los Angeles Sheriff's Department has taken a

leading role in establishing courses in crisis action planning, crisis

decision making, command and control, operations, logistics and

intelligence. Liaisons and working groups with other agencies and

disciplines has provided additional insight and contributed to a high

state of readiness as they prepare for the next major earthquake.