Earthquakes are one of the most dangerous natural disasters on Earth. This is because they strike with little or no warning, and can cause catastrophic damage.Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake.
Earthquakes are believed to result from a sudden dislocation in the earth’s crust. What causes the shift? Opinions vary. But one view gaining wider acceptance is that the earth’s crust is not rigid, but is made up of a series of “plates” that move over the earth’s “mantle.”
According to this view, when these “plates” (which are several miles thick) collide, one slides under the other, bending rocks in the process. In other cases, the horizontal action of these “plates” passing one another exerts tremendous pressure on crystal rock. Such movements occur along a “fault” or fracture in the earth’s surface. Then, it is believed, when the crust can no longer tolerate the strain, it “snaps” into a new position.
If the region above the rupture is heavily populated, there may be destruction, injury and death. The extent of the damage depends on the quake’s “magnitude,” that is, the power it releases, its location and other factors. The “snapping” sets off jarring energy vibrations called shock waves that travel through the earth at the speed of several miles per second. These waves are detected by sensitive instruments called seismographs.
On the basis of seismograph readings the earthquake’s magnitude is expressed on the Richter Scale, a logarithmic scale. A reading of 7 signifies a disturbance ten times as great as a reading of 6, which is, itself, ten times as great as 5. Any reading over 6 is considered to be an earthquake of major magnitude. Only one or two earthquakes each year will reach 8.