Terremotos

EARTHQUAKE

What is an earthquake?

An earthquake is caused by sudden, violent shifting of massive rocks called plates under the earth's surface. This movement of the plates releases stress that accumulates along geologic faults. A fault is a deep crack that marks the boundary between two of these plates. Earthquake belts occur along faults around the world. Many run along coastal areas.

The epicenter is the point on the surface where the earthquake is the strongest. Small tremors occur constantly, but every few months a major earthquake occurs somewhere in the world. Scientists are researching ways to predict earthquakes, but their predictions are not always accurate.

EARTHQUAKES MEASURE

Seismologists have developed various ways to measure the strength of earthquakes. The first and most well-known is the Richter magnitude scale, developed earlier this century by California seismologist Charles Richter. The calculation of Richter magnitude is based on the maximum strength of the vibrations (measured by a seismograph) and the distance of the instrument from the epicenter of the earthquake. The Richter scale is logarithmic, which means that each increase in magnitude indicates a tenfold increase in the strength of the quake. A magnitude-6.0 earthquake, for instance, is ten times stronger than a magnitude-5.0. In terms of the energy released, the differences are even greater. A magnitude-6.0 earthquake releases 32 times the seismic energy as a magnitude-5.0.

But Richter magnitude is only accurate for earthquakes up to about 310 miles (500 kilometers) from the instrument that detects it. Seismologists have developed other magnitude scales based on measurements of body waves or surface waves. But these, too, are not accurate for all earthquakes. Body-wave magnitudes, for example, aren't accurate for very strong earthquakes. Another system has come into wide use, called "moment magnitude,"or Mercalli Scale used to measure "intensity" or the effects of earthquakes on buildings, structures and people which takes into account the actual area of fault ruptured and gives a more consistent measure of earthquake size across the spectrum -- from minor jiggles to devastating jolts.

Seismic waves weaken with increasing distance. All things being equal, the strongest groundshaking occurs at the epicenter of the quake -- the point on the surface directly above the focus of the earthquake. (The focus can be a few miles below the ground or, more rarely, as deep as 435 miles. Beyond that depth, rocks are too hot and malleable to store strain, and they simply deform, like a block of soft clay.) In the most powerful earthquakes, groundshaking can actually exceed the acceleration of gravity and toss boulders into the air, as happened during the great Assam (India) earthquake of 1897.

Faults

At the locations

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