Earthquake Magnitude:
The most familiar measure of earthquake magnitude is the Richter Scale, in which each increment represents an earthquake ten times larger then the previous one. This is an open ended scale, but most commonly recorded earthquakes are magnitudes 2 and higher, and the largest instrumentally recorded earthquake was about 9.5.   This scale was developed and calibrated to a particular type of seismograph.  Modern seismologists prefer a magnitude scale that reflects the actual area of rupture along a fault caused by an earthquake; this is the moment magnitude.  The moment magnitude scale is also logarithmic, and has been adjusted to look quite like the familiar Richter Scale.  While earthquakes of magnitude less than 2 (including negative magnitudes) do occur, these are difficult to detect without a sensitive local network of seismographs.  Thus maps of global seismicity tend to show earthquakes of magnitude 3 and larger.  Earthquakes of magnitude 5 and above are those that cause damage to structures and loss of human life, and these are the quakes reported in the news.  The students' maps of earthquake magnitude will illustrate that very large earthquakes occur most often along convergent plate boundaries.  Areas particularly prone to large earthquakes include the Pacific Rim and the mountainous region that stretches from the Middle East to China.

 

Click on the image mosaic below to see larger versions and descriptions of the photos. Photos courtesy of the USGS.

 

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