The
Richter magnitude scale, also known as the
local magnitude (
)
scale, assigns a single number to quantify the amount of
seismic energy released by 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 Wood–Anderson torsion
seismometer output. So, for example, an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0. The effective limit of measurement for local magnitude
is about 6.8.
Though still widely used, the
Richter scale has been superseded by the
moment magnitude scale, which gives generally similar values.
The
energy release of an earthquake, which closely correlates to its destructive power, scales with the power of the shaking amplitude. Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 (
) in the energy released; a difference of magnitude of 2.0 is equivalent to a factor of 1000 (
) in the energy released.
Development
Developed in
1935 by
Charles Richter in partnership with
Beno Gutenberg, both of the
California Institute of Technology, the scale was firstly intended to be used only in a particular study area in
California, and on seismograms recorded on a particular instrument, the Wood-Anderson torsion
seismometer. Richter originally reported values to the nearest quarter of a unit, but decimal numbers were used later. His motivation for creating the local magnitude scale was to separate the vastly larger number of smaller earthquakes from the few larger earthquakes observed in California at the time.
His inspiration was the
apparent magnitude scale used in astronomy to describe the brightness of stars and other celestial objects. Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of one micrometre on a seismograph recorded using a Wood-Anderson torsion seismometer from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or lower limit, and sensitive modern seismographs now routinely record quakes with negative magnitudes.
Because M
L is derived from measurements taken from a single, band-limited seismograph, its values saturate when the earthquake is larger than 6.8.
To overcome this shortcoming, Gutenberg and Richter later developed a magnitude scales based on
surface waves,
surface wave magnitude M
S, and another based on
body waves,
body wave magnitude m
b.
M
S and m
b can still saturate when the earthquake is big enough.
These traditional magnitude scales have been superseded by the implementation of methods for estimating the
seismic moment and its associated
moment magnitude scale, although still widely used because they can be calculated quickly.
Richter magnitudes
The Richter magnitude of an earthquake is determined from the
logarithm of the
amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). The original formula is:
where A is the maximum excursion of the Wood-Anderson seismograph, the empirical function A
0 depends only on the
epicentral distance of the station, delta. In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the M
L value.
Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; in terms of energy, each whole number increase corresponds to an increase of about 31.6 times the amount of energy released.
Events with magnitudes of about 4.6 or greater are strong enough to be recorded by any of the seismographs in the world, given that the seismograph's sensors are not located in an earthquake's
shadow.
The following describes the typical effects of earthquakes of various magnitudes near the epicenter. This table should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, the depth of the earthquake's focus beneath the epicenter, and geological conditions (certain terrains can amplify seismic signals).
(
Based on U.S. Geological Survey documents.)
Great earthquakes occur once a year, on average. The largest recorded earthquake was the
Great Chilean Earthquake of
May 22,
1960 which had a magnitude (M
W) of 9.5.
The following table lists the approximate
energy equivalents in terms of
TNT explosive force
[, with mathematic equations] - though note that the energy here is that of the
underground energy release (ie a small atomic bomb blast will not simply cause light shaking of indoor items) rather than the overground energy release; the majority of energy transmission of an earthquake is not transmitted to and through the surface, but is instead dissipated into the crust and other subsurface structures.
See also
