The moment magnitude provides an estimate of earthquake size that is valid over the complete range of magnitudes, a characteristic that was lacking in other magnitude scales. The result is called the moment magnitude. The moment is then converted into a number similar to other earthquake magnitudes by a standard formula. The moment can be estimated from seismogram s (and also from geodetic measurements). Moment is a physical quantity proportional to the slip on the fault multiplied by the area of the fault surface that slips it is related to the total energy released in the earthquake. In particular, for very large earthquakes, moment magnitude gives the most reliable estimate of earthquake size. The original formula for a Richter Magnitude Scale is ML log10A minus log10A0 (delta), where ‘A’ equals maximum excursion of the Wood-Anderson Seismograph and the empirical A0 depicts the epicentre distance denoted as delta and measured in degrees. In its range of validity, each is equivalent to the Richter magnitude.īecause 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, or Mw, was developed. Each is valid for a particular frequency range and type of seismic signal.

These include body wave magnitude ( Mb) and surface wave magnitude ( Ms). 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. This is what was to eventually become known as the Richter magnitude.Īs 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. This magnitude scale was referred to as ML, with the L standing for local. 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.

Small earthquakes have about the same value on the Richter scale and the moment magnitude scale, so Richter is fine for those. Earthquake size, as measured by the Richter Scale is a well known, but not well understood, concept. The moment-magnitude scale is logarithmic, so an increase of one unit means an earthquake is 10 times bigger, with about 30 times the energy produced.

Moment magnitude scale formula