Geology And Geophysics

Difference between Moment Magnitude Scale and the Richter Scale



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The earth's continents and seas are carried on underground oceans of extremely hot and semi-solid materials by tectonic plates. These plates are actually huge slabs of rock 50-650 feet (15-200 km) thick. Cracks in these plates are called fault lines. When there is a shift in the tectonic plates an earthquake occurs.

Each year worldwide thousands of earthquakes occur. Some so slight they can't be felt, while others, which occur much less frequently, can cause catastrophic damage. For many years the magnitude from the seismic waves from these quakes have been measured with the use of the Richter Scale.

The Richter Scale was developed in 1035 by American seismologist Charles F. Richter. It was designed to measure medium-sized quakes (3.0-7.0) in Southern California. It is also known as the local magnitude scale.

Information of seismic activity is gathered using a seismograph. A seismograph functions by a weight that is freely suspended from a support that is attached to bedrock and each wave is recorded as the weight moves. Adjustments are made to compensate for the differences in distance between seismographs and the epicenter of the earthquake.

The numbers on the scale range from 0-10. The amplitude of the waves increases by powers of 1- in relation to the Richter magnitude numbers. One of the downfalls of this scale is it has an upper limit on highest measurable magnitude. It has also proved to be unreliable if measurements are taken at a distance of more than about 350 miles (600 km) from the epicenter.

Due to the short comings of the Richter Scale, two Harvard University seismologists, Thomas C. Hanks and Hiroo Kanamori introduced the Moment Magnitude Scale (MMS). This is now used by seismologists to measure the magnitude of earthquakes. The magnitude is based on the moment of the earthquake equal to the rigidity of the earth multiplied by the average amount of slip on the fault and the size of the area that slipped.

MMS is now the scale used to estimate the magnitude of all modern earthquakes by the U.S. Geological Society. It is based on a logorithmic scale which is a scale of measurement that uses the logarithm (power to which the base number must be raised in order to produce the number of a physical quantity or can be measured and/or calculated and expressed in numbers) instead of the quantity itself. On this scale, earthquakes one number higher is approximately 30 times more powerful.

Although both scales are s till used for medium earthquakes, MMS supersedes the Richter Scale. Since Moment Magnitude Scale is not based on instrument recordings but on the area of the fault that is ruptured by the quake, it is much more accurate with those of higher magnitudes.

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