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دوشنبه ششم شهریور 1391 :: نویسنده : پانته آ

What is an earthquake?

An earthquake is a sudden motion or shaking of the Earth caused by the sudden release of energy that is stored in rocks. Most earthquakes are associated with rock movements along faults below the surface of the earth. Because of friction and high confining pressure, the fault blocks don't move until the tectonic stress becomes great enough to overcome the frictional force. Earthquakes may also result according to the elastic rebound theory . It suggests that in some cases energy is stored in rock that is being bent (deformed) by tectonic forces.

The rock breaks until the energy in the rock exceeds the bonding strengths between minerals. When the rock breaks, it suddenly returns to its predeformed shape. Then, the crust moves violently as a result of the quickly released force. This results in the formation of a new fault. Earthquakes can be some of the worst natural disasters, taking thousands of lives and creating billions of dollars of damage. For instance, in December of 2003, the ancient Iranian city of Bam was destroyed in an earthquake.

It was clear in reports that failed buildings were the main cause of the large number of deaths. Bam earthquake started at four in the morning when most of the residents were in bed. One-third of its population of 200,000 was either killed or seriously injured. Most of the buildings, including two of the citys hospitals, were destroyed. Seismologists hope to eventually predict earthquakes before they strike.

Earthquake waves

When rock masses suddenly move deep within the earth in response to tectonic stress, energy in the form of seismic waves moves outward through the rock from the point of origin, called the focus. The initia movement occurs at the focus. The epicenter is the point on the surface directly above the focus. There are three types of seismic waves: P and S body waves and surface waves.

a) Body waves: Body waves travel through the Earths interior. Body waves radiate outward from the focus in all directions and travel through solid rock.

A P body wave (primary body wave) is a compressional (longitudinal) wave that causes the particles in the rock to vibrate back and forth in the same direction the wave moves. An S body wave (secondary body wave) is only about half as fast as a P wave and causes the rock to vibrate at right angles (90 degree) to the direction of wave travel.

b) Surface waves: Surface waves are the slowest seismic waves and travel outward on the earth's surface from the epicenter much like ripples do from a stone thrown into water. They create most of the damage at the surface.

Measurement of seismic waves

Seismic waves are detected with a seismometer. The seismometer is connected to a seismograph. A seismograph is a device that records seismic waves as a series of wavy lines called a seismogram.

Measurement of earthquake strength

1. Magnitude: In 1935 Charles Richter devised the Richter scale to express earthquake magnitude. Richter magnitude is calculated from the height of the largest earthquake body wave recorded on a specific type of seismograph. The Richter scale is a numerical scale that lists earthquake magnitudes in logarithmic increments from about 2 to 8.6 – the highest value ever recorded on the scale. A magnitude 6 earthquake will move the ground 10 times that of a magnitude 5 event and release 32 times the energy. The Richter scale is more quantitative than earlier intensity scales, but it is not a precise measure of earthquake energy.

2. Intensity: The strength of an earthquake can be measured as a function of intensity. The modified Mercalli scale classifies intensity from 1 to 12 according to the amount of resulting damage. This system is not totally accurate because the amount of damage is often proportional to the following items:

a) population in an area,

b) the type of design and construction of buildings,

c) and the base on which the buildings sit (that is, bedrock or sediment).

Earthquakes and plate tectonic

Earthquakes are common at all three types of plate boundaries. The San Andreas fault zone is an example of a transform plate boundary, where two plates slide past one another. Subduction zone earthquakes occur when the subducting plate slips suddenly. Earthquakes occur at divergent plate boundaries as blocks of lithosphere along the fault drop downward. Earthquakes occur in plate interiors along old faults or where sediments press down the lithosphere.





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