Richter magnitude scale

The Richter magnitude test scale (or more correctly local magnitude ML scale) assigns a single number to quantify the size of an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismogram. So, for example, an earthquake of magnitude 5 is ten times greater than one of magnitude 4 and an earthquake of magnitude 8 is 10(8-4) or 10000 times greater than one of magnitude 4.

Problems with the Richter scale

The major problem with Richter magnitude is that it is not easily related to physical characteristics of the earthquake source. Furthermore, there is a saturation effect near 8.3-8.5, owing to the scaling law of earthquake spectra, that causes traditional magnitude methods (such as MS) to yield the same magnitude estimate for events that are clearly of different size. By the beginning of the 21st century, most seismologists considered the traditional magnitude scales to be largely obsolete, being replaced by a more physically meaningful measurement called the seismic moment which is more directly relatable to the physical parameters, such as the dimension of the earthquake rupture, and the energy released from the earthquake.

Related Topics:
Scaling law of earthquake spectra - 21st century - Seismic moment - Earthquake - Rupture

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In 1979 seismologists Tom Hanks and Hiroo Kanamori, also of the California Institute of Technology, proposed the moment magnitude scale (MW), which provides a way of expressing seismic moments in a form that can be approximately related to traditional seismic magnitude measurements.

Related Topics:
1979 - Tom Hanks - Hiroo Kanamori - California Institute of Technology - Moment magnitude scale

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Magnitude must not be confused with intensity. Intensity scales, such as the Modified Mercalli Intensity Scale and Japanese seismic intensity scale, are used to describe relative earthquake effects. Intensity is sensitive to a host of local site conditions and is not an absolute measurement of earthquake size.

Related Topics:
Intensity - Mercalli Intensity Scale - Japanese seismic intensity scale

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