How are Earthquakes Measured?

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Richter Magnitude Scale

Charles Richter developed the Richter Scale, in collaboration with colleague Beno Gutenberg, in 1935.  The scale uses seismograph readings to express the magnitude of an earthquake, taking into account the different readings from a number of distances.  The scale is logarithmic, so an earthquake of magnitude 6 is ten times stronger than a magnitude 5 and a hundred times stronger than a magnitude 4.   The scale was developed while Richter was working in California and was quickly adopted by seismologists there, spreading to the rest of the world as soon as instruments could be calibrated.  The traditional Richter Scale Magnitude is useful for tremors up to 600 KM from the recording instrument.

 

Surface Wave Magnitude

In 1936 Richter and Gutenberg developed the Surface Wave Magnitude Scale, to describe high energy earthquakes happening at greater distances more accurately.  The scale depends on the amplitude, that is the highest peak on the seismogram, for surface waves with a 20 second period.   This is a useful scale as the seismometer can be far away from the epicenter of the earthquake and still allow for an accurate depiction of the magnitude.

Moment Magnitude Scale

The question of accurate magnitude statements for all earthquakes has led to the Moment Magnitude Scale being adopted by the USGS.  This scale uses the strength of the rock base, the size of the fault and the average movement, or displacement of the fault.  Visualized as energy produced within a rectangular area, the Moment Magnitude Scale gives a more accurate depiction of the energy released by an earthquake but takes a little time to figure out so there will be an estimate given, usually based on instruments, followed by the Moment Magnitude, given as MM. The press and public always want to know how big an event is, and the work of seismologists and mathematicians is ongoing.

It is likely that, in the future, new methods will be adopted as ways of detecting and recording tremors are invented, but the current system of seismometers and reporting is quite impressive, on a global scale.   The USGS Earthquakes Program presents maps of earthquakes as they happen and gives information on past earthquakes, predictions and Probabilities.

Sources

University of California Berkeley Seismology Laboratory, Why are there so many magnitudes? Accessed May 17, 2011. USGS, The Richter Magnitude Scale, 2009.  Accessed May 17, 2011 Michigan Tech UPSeis,  How are Earthquake Magnitudes Measured, 2007.  Accessed May 18 2011. Thomas, D.S.G., and Goudie, A., (2000), The Dictionary of Physical Geography, Blackwell Publishers Ltd.

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