How Are Earthquakes Measured Earthquake Magnitude Scale

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					                         How Are Earthquakes Measured?
              The Richter Scale
                                            The magnitude of most earthquakes is measured on the Richter
                                            scale, invented by Charles F. Richter in 1934. The Richter
                                            magnitude is calculated from the amplitude of the largest seismic
                                            wave recorded for the earthquake, no matter what type of wave
                                            was the strongest.

                                           The Richter magnitudes are based on a logarithmic scale (base
                                           10). What this means is that for each whole number you go up on
                                           the Richter scale, the amplitude of the ground motion recorded by
                                           a seismograph goes up ten times. Using this scale, a magnitude 5
                                            earthquake would result in ten times the level of ground shaking
         CHARLES RICHTER STUDYING           as a magnitude 4 earthquake (and 32 times as much energy would
         A SEISMOGRAM.                      be released). To give you an idea how these numbers can add up,
    think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much
    energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as
    detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that
    occur each year are magnitude 2.5 or less, too small to be felt by most people.

    The Richter magnitude scale can be used to describe earthquakes so small            Class          Magnitude
    that they are expressed in negative numbers. The scale also has no upper
    limit, so it can describe earthquakes of unimaginable and (so far)                  Great          8 or more
    unexperienced intensity, such as magnitude 10.0 and beyond.                         Major          7 - 7.9

    Although Richter originally proposed this way of measuring an earthquake's          Strong         6 - 6.9
    "size," he only used a certain type of seismograph and measured shallow             Moderate       5 - 5.9
    earthquakes in Southern California. Scientists have now made other
                                                                                        Light          4 - 4.9
    "magnitude" scales, all calibrated to Richter's original method, to use a
    variety of seismographs and measure the depths of earthquakes of all                Minor          3 -3.9
    sizes.
                                                                                       Earthquakes are also classified
                                                                                       in categories ranging from
                                                                                       minor to great, depending on
      Earthquake Magnitude Scale                                                       their magnitude.



Magnitude       Earthquake Effects                                           Estimated Number
                                                                             Each Year

2.5 or less     Usually not felt, but can be recorded by seismograph.        900,000

2.5 to 5.4      Often felt, but only causes minor damage.                    30,000

5.5 to 6.0      Slight damage to buildings and other structures.             500

6.1 to 6.9      May cause a lot of damage in very populated areas.           100

7.0 to 7.9      Major earthquake. Serious damage.                            20

8.0 or          Great earthquake. Can totally destroy communities near the
                                                                             One every 5 to 10 years
greater         epicenter.
           The Mercalli Scale
                                               Another way to measure the strength of an earthquake is to use the
                                               Mercalli scale. Invented by Giuseppe Mercalli in 1902, this scale uses
                                               the observations of the people who experienced the earthquake to
                                               estimate its intensity.

                                    The Mercalli scale isn't considered as scientific as the Richter scale,
                                    though. Some witnesses of the earthquake might exaggerate just how
                                    bad things were during the earthquake and you may not find two
                                    witnesses who agree on what happened; everybody will say something
                                    different. The amount of damage caused by the earthquake may not
                                    accurately record how strong it was either. Some things that affect the
                                    amount of damage that occurs are: the building designs, the distance
       GIUSEPPE MERCALLI
                                 from the epicenter, and the type of surface material (rock or dirt) the
                                 buildings rest on.
        Different building designs hold up differently in an earthquake and the further you are from the
        earthquake, the less damage you'll usually see. Whether a building is built on solid rock or sand
        makes a big difference in how much damage it takes. Solid rock usually shakes less than sand,
        so a building built on top of solid rock shouldn't be as damaged as it might if it was sitting on a
        sandy lot.

Modified Mercalli Intensity Scale
       Mercalli Eq. Richter Witness Observations
       Intensity Magnitude
       I                1.0 to 2.0             Felt by very few people; barely noticeable.

       II               2.0 to 3.0             Felt by a few people, especially on upper floors.

       III              3.0 to 4.0             Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake.

       IV               4.0                    Felt by many indoors, few outdoors. May feel like heavy truck passing by.

                                               Felt by almost everyone, some people awakened. Small objects moved. trees and poles may
       V                4.0 to 5.0
                                               shake.

                                               Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls.
       VI               5.0 to 6.0
                                               Chimneys may be slightly damaged.

                                               Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly
       VII              6.0
                                               built structures. Some walls may fall.

                                               Little damage in specially built structures. Considerable damage to ordinary buildings, severe
       VIII             6.0 to 7.0
                                               damage to poorly built structures. Some walls collapse.

                                               Considerable damage to specially built structures, buildings shifted off foundations. Ground
       IX               7.0
                                               cracked noticeably. Wholesale destruction. Landslides.

                                               Most masonry and frame structures and their foundations destroyed. Ground badly cracked.
       X                7.0 to 8.0
                                               Landslides. Wholesale destruction.

                                               Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground.
       XI               8.0
                                               Waves seen on ground.

       XII              8.0 or greater         Total damage. Waves seen on ground. Objects thrown up into air.


http://www.geo.mtu.edu/UPSeis/intensity.html                                         http://earthquake.usgs.gov/eqcenter/dyfi/ (Did you feel it?)

				
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posted:11/29/2011
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