Earthquake
An earthquake is an unpredictable event in which masses of rock shift below the Earth's surface, releasing enormous amounts of energy and sending out shock waves which sometimes cause the ground to shake dramatically. Not all earthquakes cause calamity, but they are known to be one of the Earth's most destructive forces. Entire structures, including houses and dams, have been known to collapse in an earthquake.
Earthquakes occur along fault lines, which are boundaries of different layers, or plates, of rock masses on the crust and within the Earth. The movement of one immense plate can shift great masses of weight and pressure onto other weaker layers. When this pressure gives way, an earthquake can take place.
An earthquake's power can be measured in two ways: by intensity and magnitude. While intensity of a quake is usually described through people's perceptions and the severity of building destruction, magnitude is measured by using seismographs or devices which detect ground movement. These magnitude measurements also allow scientists to precisely compare earthquakes around the world.
People have been experiencing and describing the effects of earthquakes well before scientific measuring devices were available. But intensities were more formally rated in the late 1800s and later refined around 1930 when the United States adopted and revised an early European version called the Mercalli scale.
The Modified Mercalli scale has twelve different ratings, ranging from Level I, which is described as " barely felt except by a very few," to Level V, which is described as "more widely felt, causing some windows to break." By Level VIII, damage to buildings is evident and by Level X, many structures completely collapse. Despite the usefulness of its precise ratings, the Modified Mercalli scale could not accurately measure earthquakes in uninhabited areas. This is why the shock waves, or the measurements of ground vibrations, are recorded to help scientists understand the true power of each earthquake. When earthquakes occur, three different types of shock waves vibrate from the source: the P, or primary wave, the S, or secondary wave, and the L, or Love wave that moves along the surface. These waves are detected by seismometers.
Today seismologists use the Richter scale to measure earthquake magnitude, which is named after its inventor, Charles F. Richter (1900-1985), a professor of seismology at the California Institute of Technology. The Richter scale takes the largest amplitude of a seismometer reading and uses a logarithmic formula to further calculate it. Because of the logarithmic nature of the Richter scale, each rating of an earthquake actually represents a ten-fold increase in the power of the previous rating.
Readings from seismographic stations also provide scientists with information about the location of the earthquake's epicenter. From the epicenter, shock waves travel outward in the same way that rings of water ripple out in a pond when a pebble is dropped.
Although earthquakes cannot be predicted, keeping accurate records of their intensity and magnitude helps scientists discover where Earth tremors happen repeatedly. In the United States, many populated areas are located along fault lines, requiring structures to be built with added support to help them withstand intense ground shaking. Earth tremors are almost always occurring within the Earth. It is estimated that as many as one million earthquakes take place every year. Fortunately, most are of low intensity.
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