The crust Earth is constantly moving. Massive forces are continually shaping and reshaping its rocks - twisting and cracking them, thrusting then into enormous folds. These changes are generally slow, and most humans are rarely aware of them - except when they culminate in a major earthquake.

Each year, the world experiences about 200 quakes large enough  to cause major damage. But earthquakes are much more common that this figure indicates. When all shocks are considered, from the greatest to the smallest year. Sizable shocks average about 10,000 each year. Worldwide, about one person in 8,000 will be killed and roughly 10 times as many will be injured by earthquakes.

Throughout humans history, the unpredictability and ruinous effects of earthquakes have provoked some fanciful explanations. In japan, Earth tremors were thought to be caused by the subterranean stirrings of of a giant catfish, normally restrained with a big mallet by a watchful deity. 

This belief was paralleled by similar ideas in China and India-except that a big tortoise and a giant mole were the respective culprits. Around 300 B.C., the Greek philosopher Aristotle theorized that winds from above were drawn into hollow passageways deep inside Earth. Agitated by fire and seeking to escape, these winds caused quakes and sometimes erupted as volcanoes.

Modern geologic research has to led to a much clearer understanding of Earth's structure and the reasons for earthquakes. Scientists now know that deep-seated geologic forces in Earth lead to movements within its uppermost layer.

Stresses within Earth's rock structure are the driving forces that cause displacements, or strains, int that structure. They take a long time to build, but they finally may reach a point at which even a slight addition of stress, or a small weakening of rock, causes a rupture.

Most earthquakes seem to be cause by sudden movements along preexisting faults located near plate boundaries. Because of friction and the tremendous pressure between the plates, the rocks on either side of the fault do not slide freely against each other. they are virtually locked together and do not move at first, despite building forces. Great strains build up, and the rocks bends. Eventually the plates reach their elastic limit, or breaking point. They suddenly shift to the fault surface, and "snap" into their new positions - a phenomenon called ELASTIC REBOUND. The resultant sudden jar is felt as an EARTHQUAKE.

As with any tearing or breaking action, an earthquakes begins at a specific point and spreads outward. The point of initial fracture is called the "focus"(or hypocenter) of an earthquake. The point on Earth's surface directly above the focus is called the EPICENTER. The distance from the surface to the focus varies from 3 to 25miles in shallow focus earthquakes, to more than 300miles in the deep-focus quakes characteristics of subduction zones.

The rocks on opposite sides of a fault and offset in proportion to the size of the earthquakes. The displacement varies from a fraction of an inch to 20 feet. In a small quake, the fracturing stops within a few seconds. In the largest quakes, it may continue for more than a minute.

Energy is released from the rocks at the leading edge of the fracture. Most of the released strains is dissipated by the breaking and crushing of rocks, by the moving of adjoining blocks, and by the creation of heat. A small portion of energy radiated outward in the form of seismic waves. The ground motion felt during an earthquake is caused by the arrival of these waves at the surface.

The motion of Earth's surface caused by earthquakes waves can be so slight that no one feels it. Such minor quakes are detected only by sensitive instruments called SEISMOGRAPHS.

Many larger quakes cause a noticeable swaying of the ground and rattling, but they cause neither serious damage nor visible changes in the surface of Earth. The vast majority of quakes fall into these categories. Truly catastrophic quakes are few in number. When earthquakes of a cataclysmic nature do occur, nearly all human-made structures are destroyed.

The first effective seismographs were built around 1890. Although today's instruments are much more sophisticated, the basic principle remains unchanged. A pendulum mass mounted on a spring is freely suspended from a frame attached to the ground. This construction allows the mass to be reasonably independent of the frame's motion. When the supporting frame is shaken by earthquakes waves. the inertia of the mass causes it to lag behind the motion is measured electronically by the seismometer, and recorded in a computer or on paper as a seismogram.

Since the motion of the ground at any point is three-dimensional, three directions of movement, perpendicular to one another, are recorded. One vertical and two horizontal seismometers generate three seismograms from which seismologists can calculate the movement in space.

Skilled technicians can interpret the initial arrival times of the P and S waves at the recording station. The distance from the earthquake focus to the station os calculated from the known speed of wave travel between different regions on the globe. The data from three or more recording stations, at different locations, are necessary to pinpoint the source of the earthquake. Today seismographic stations are set up all over the world. More than 20,000 earthquakes are documented every year by this worldwide network. 


Post a Comment