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"We interrupt this news flash to warn that a seismograph has picked up something, scientists think it's an earthquake. They say that just to be on the safe side, run as fast as your legs can carry you to an earthquake shelter, because the damage may be fatal. They are predicting that the earthquake should occur in the range of half an hour to one hour!" Everybody is fleeing from their homes to earthquake shelter, then, a few minutes later I felt shaking under my feet. It was the earthquake! I trembled like a mouse about to be pounced on. I could hear buildings crumbling and people screaming.
Causes
Earthquakes are caused by sudden slips along geological faults. They slip because of the movement of the tectonic plates. This concept is called the elastic rebound theory. These rocky tectonic plates move slowly, floating on top of a weak rocky layer. As the plates collide with each other or slide past each other, pressure builds within the rocky crust. When the pressure gets too intense, an earthquake occurs.
Ways to measure earthquakes
The Richter scale, named for the American seismologist Charles Richter (1900-1985) is one of several closely related methods of quantifying the magnitude (Strength) of an earthquake. It uses the amplitude of the seismic waves emitted by an earthquake as a measure of overall earthquake strength. An earthquake that causes 0.001 millimeter of ground motion at 100 km. distance is assigned at magnitude 3. The strength of another earthquake is gauged relative to this earthquake, with every factor of ten difference in ground motion corresponding to one magnitude unit. A barely felt earthquake typically has a magnitude of about one, and the largest about nine.
Location
One example is the 1906 San Francisco earthquake (Magnitude-7.7), which erupted the San Andreas Fault at the boundary between the North American and the Pacific plates. Wide zones of deformation are sometimes associated with these plate boundaries, especially where the plate boundary is curved sharply enough to inhibit easy sliding. The 1994 North Ridge (California) earthquake (M-6.7) was a thrust associated with a bend in the San Andreas Fault in southern California.
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