Earthquakes are vibrations produced in the earth's outer layer, or crust, when forces pushing on a mass of rock overcome the friction holding the rock in place and blocks of rock slip against each other. The vibrations can range from barely noticeable to very destructive. There are six types of shock waves. Two are classified as body waves which means they travel through the earth's interior and the other four are surface waves. The waves are changed by the rock types or formations they hit. Primary or compressional waves (P waves) send particles moving back and forth in the same direction as the waves are traveling, secondary or transverse shear waves (S waves) send vibrations perpendicular to their direction of travel.

P waves always travel at higher velocities than S waves. Three general classes of earthquakes are now recognized as tectonic, volcanic, and artificially produced. The tectonic variety is by far the most destructive. The most common cause of tectonic quakes is stresses by movements of the dozen of major and minor plates that make up the earth's crust. Most tectonic quakes occur at the boundaries of these plates, in zones where one plate slides past another Subduction-zone quakes account for nearly half of the world's destructive seismic events and 75 percent of the earth's seismic energy. They are along the so-called Ring of Fire, a narrow band about 38, 600 km long, that coincides with the sides of the Pacific Ocean.

The points at which crustal rupture occurs in such quakes tend to be far below the earth's surface, at depths of up to 645 km. Not all subduction zones are subject to frequent earthquakes. The frequency and magnitude of earthquakes around subduction zones are related to the direction in which the plates are moving. If two plates moving in the same general direction come close together, generally the edge of one plate will slide below the other at a sharp angle. This reduces the amount of area in which the plates touch, so the subduction zone does not produce many earthquakes and any earthquakes it does produce are not as strong. If two plates are sliding beside each other, one plate will often be forced under the other at a shallow angle, making a large area of friction.

This produces more frequent, stronger earthquakes. Tectonic earthquakes beyond the Ring of Fire occur in a variety of geological settings. Mid ocean ridges the seafloor spreading centers of plate tectonics are the sites of numerous such events of moderate intensity that take place at relatively shallow depths. One other category of tectonic earthquake includes the infrequent but large and destructive quakes that occur in areas far removed from other forms of tectonic activity. Of the two classes of non tectonic earthquake, those of volcanic origin are not very large or destructive. They are of interest chiefly because they often herald impending volcanic eruptions.

Such quakes originate as magma, molten rock from a layer in the earth's mantle called the asthenosphere, works its way upward, filling the chambers beneath a volcano. As the flanks and summit of the volcano swell and are tilted, rupture of the strained rocks may be signaled by many small earthquakes. Earthquakes produce various effects for types of land and depending where the tectonic place. They can cause great loss of life by destroying structures such as buildings, bridges, and dams, and they can also trigger landslides. Another effect of earthquakes is the generation, usually by sub sea tremors, of so-called tidal waves. Because such waves are not related to the tides, they are more properly called seismic sea waves or their Japanese name tsunamis.

These big waves of water have struck coastlines with such violent strength that entire towns have been destroyed. Seismologists have devised two scales of measurement to describe earthquakes quantitatively. One is the Richter scale-named after the American seismologist Charles Francis Richter-which measures the motion of the land surface 100 km from the epicenter, or focus, of the earthquake. It is a logarithmic scale; a magnitude 7 quake moves the land 10 times farther than a magnitude 6 quake, 100 times farther than a magnitude 5 quake, 1000 times farther than a magnitude 4 quake, and so on. Yearly, an estimated 800 quakes of magnitudes 5 to 6 occur worldwide, in comparison with about 50, 000 quakes of magnitudes 3 to 4, and only about one earthquake of magnitude 8 to 9. The other scale, by the Italian seismologist Giuseppe Mercalli, measures the intensity of shaking with gradations from I to XII.

Because seismic surface effects diminish with distance from the focus of the quake, the Mercalli rating assigned to the quake depends on the site of the measurement. Intensity I on this scale is defined as an event felt by very few people, whereas intensity XII is assigned to a catastrophic event that causes total destruction. Events of intensities II to III are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale, and XI to XII on the Mercalli scale can be correlated with magnitudes 8 to 9 on the Richter scale.