Geology And Geophysics

Understanding the Richter Scale and Moment Magnitude Scale



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The Moment Magnitude and the Richter Scales are modern seismic scales, used to measure and compare the severity of earthquakes. Both these scales are classified as Magnitude scales, that measure the magnitude or original force of an earthquake. The 'magnitude' of an earthquake is expressed in Arabic numerals indicating the seismic energy released.

The RICHTER SCALE is the popular name for the Richter Magnitude or Local Magnitude Scale, in use since 1935, when Charles Richter attempted to study earthquakes in parts of California. The Richter magnitude is based on a scale of 10, with the magnitudes starting from a less than 2.0 reading to a 10+ reading, though the latter has never been recorded. The readings help to arrive at the level of magnitude of an earthquake, from micro and light to strong and great, though after taking into account the earthquake intensity as well. It is derived from the logarithm of the amplitude of waves, recorded by a seismograph.

The MOMENT MAGNITUDE SCALE is newly devised scale for measuring the size of an earthquake vis-a-vis the energy released. This was developed in 1979 to overcome the shortcomings of the historic Richter Scale. The Moment Magnitude Scale is also a logarithmic scale, with each number denoting a 30 or more powerful magnitude than the previous number.

# While the Richter Scale was developed as a Local Magnitude Scale for of medium-sized earthquakes in Southern California, the Moment Magnitude Scale was developed to address the shortcomings of the Richter Scale while measuring earthquakes of larger intensity.

# The Richter Scale records responses of seismographs and the distance from epicentre, the Moment Magnitude Scale records the physical property, i.e. the seismic moment of the earthquake.

# The symbol used for Richter Scale is 'ML' where 'L' denotes the logarithmic power; while that for Moment Magnitude Scale is 'Mw' with the subscript 'w' denoting mechanical work. # 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. The Moment Magnitude is however represented by

Mw = 2/3 log10(M0) - 10.7, where 'M0' equals the seismic moment measured in dyne centimetres (10 to the power - 7 Nm)

# While the Richter Scale fails in consistency and accuracy for earthquakes of larger magnitudes, like say exceeding 7; the Moment magnitude Scale is capable of measuring earthquakes of larger magnitudes with accuracy. There is no upper limit to possible measures of earthquake magnitudes.

# When earthquakes occur at distances of more than miles from the earth's epicentre, the Richter Scale is unable to give accurate readings. However, the Moment Magnitude Scale does away with such possibility.

# Whereas the Richter Scale can render a detailed picture for low magnitude earthquakes, the Moment Magnitude Scale often gives a blurred reading for low magnitude earthquakes.

# Thus, Richer Scale is used for accurately measuring earthquakes of smaller intensities, say those less than a 3.5 reading; while the Moment Magnitude Scale is used for earthquakes of medium and large magnitudes.

Both of these seismic scales have their own properties for measuring the magnitude of earthquakes. While Richter Scale continues to be in use for smaller earthquakes, it may sometimes seem that the Richter Scale is more popular, just because smaller earthquakes are more common. However, the Moment magnitude Scale is being increasingly favoured for its advantages of measuring earthquakes of larger magnitudes and distances from epicentre.

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