What are the different magnitude scales, and why are there so many?
Earthquake size, as measured by the Richter Scale is a well known, but not well understood, concept. The idea of a logarithmic earthquake magnitude scale was first developed by Charles Richter in the 1930’s for measuring the size of earthquakes occurring in southern California using relatively high-frequency data from nearby seismograph stations. This magnitude scale was referred to as ML, with the L standing for local. This is what was to eventually become known as the Richter magnitude.
As more seismograph stations were installed around the world, it became apparent that the method developed by Richter was strictly valid only for certain frequency and distance ranges. In order to take advantage of the growing number of globally distributed seismograph stations, new magnitude scales that are an extension of Richter’s original idea were developed. These include body wave magnitude, Mb and surface wave magnitude, Ms. Each is valid for a particular frequency range and type of seismic signal. In its range of validity each is equivalent to the Richter magnitude.
Because of the limitations of all three magnitude scales, ML, mb, and Ms, a new, more uniformly applicable extension of the magnitude scale, known as moment magnitude, orMw, was developed. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. New techniques that take advantage of modern telecommunications have recently been implemented, allowing reporting agencies to obtain rapid estimates of moment magnitude for significant earthquakes.
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