Earthquake intensity and magnitude is currently
measured scientifically using several methods: the Richter Scale, the Modified Mercalli
Intensity Scale, and the Moment Magnitude Scale.
Richter Scale
The Richter
Magnitude Scale was invented in 1935 by Charles F. Richter at the California
Institute of Technology. The scale is based on a mathematical logarithm
intended to compare the intensity of earthquakes. The logarithm uses the
amplitude of seismic waves recorded by seismographs to determine the magnitude
of each earthquake in whole numbers and decimal fractions. Distance
between seismographs measuring the same earthquake and the distances between
seismographs and the epicenter are taken into account in the Richter
logarithm.
In
considering the numbers outputted by the Richter Magnitude Scale, one must
remember that because the scale is based on logarithmic computations, the
difference between a 5.3 earthquake and a 6.3 earthquake is tenfold in
amplitude. Or, a 6.3 Richter magnitude earthquake produces approximately
31 times more energy than does a 5.3 Richter magnitude earthquake. Unlike
the Modified Mercalli Intensity Scale which has 12 levels, the Richter Magnitude
Scale has no top level. The largest earthquakes recorded or discovered
through paleoseismology have been between 8.8 - 9.0 on the Richter Scale.
For
more information on the Richter Scale and Charles F. Richter:
National
Earthquake Information Center "The
Richter Magnitude Scale"
Earth Quest Online "The
Story of the Richter Scale"
National Earthquake Information Center "Charles
F. Richter - An Interview"
Geological Survey of Canada "Earthquake
Magnitude - Richter Scale"
Nevada Seismological Laboratory "What
is Richter Magnitude?" - includes mathematical formulas
The Modified
Mercalli Intensity Scale
Originally created in 1902 by Italian seismologist and
volcanologist Guiseppe Mercalli and modified in 1931 by seismologists Harry Wood
and Frank Neumann, the Modified Mercalli Intensity Scale is based on the effect
the intensity of an earthquake has on the surface of the planet. This
magnitude scale has no mathematical basis. Mercalli intensities are assigned to
an earthquake after non-scientific information has been compiled, such as how
scared people were or how much destruction the earthquake caused. Although
the assignation of Mercalli intensities is arbitrary, the differences in levels
is much more comprehensible to the layperson. The 12 levels of the
Modified Mercalli Intensity Scale are as follows.
- Felt under only rare or very favorable circumstances by a
miniscule number of people.
- Felt by few people who are usually stationary or on upper
levels of buildings.
- People indoors will feel a motion similar to that produced
by a passing truck. Stationary vehicles may rock slightly.
- Felt indoors and out. Light objects such as dishes
and picture frames will be disturbed.
- Felt by nearly everyone and will awaken most people.
Can cause pendulum clocks to stop.
- Felt by all. May move furniture. Slight damage.
- Well-designed buildings will have minor damage if any, but
an earthquake of this intensity can demolish neighborhoods constructed of
poorly-built edifices.
- Considerable damage or partial collapse to most average
buildings.
- Buildings designed to withstand earthquakes or buildings
that have been retrofitted will sustain considerable damage. Buildings
may shift off their foundations.
- Almost all buildings save some well-build wooden structures
will be damaged. An earthquake of this intensity will bend railroad
tracks.
- Few buildings remain, bridges are destroyed, railroad
tracks greatly warped.
- Total damage. Horizon is no longer parallel to line
of sight.
For more information on the Modified
Mercalli Intensity Scale, Guiseppe Mercalli, Harry Wood, or Frank Neumann:
National
Earthquake Information Center "The
Modified Mercalli Intensity Scale"
California Institute of
Technology "Harry
Wood" (photograph)
The Moment Magnitude Scale
Used
to measure the total amount of energy released in a single seismic event, the
moment magnitude scale is often preferred over the Modified Mercalli Scale and
the Richter Scale because it is more scientifically precise. The Moment
Magnitude Scale uses physical properties of an earthquake instead of relying
upon human reactions to judge the size of a quake. What this scale focuses
on primarily, is the area of the fault that was affected by the
quake.
The Moment Magnitude Scale was
developed in 1977 by Japanese seismologist Hiroo Kanamori.
For more information on the Moment
Magnitude Scale and Hiroo Kanamori:
Arkansas Center for Earthquake Education and Technology
Transfer "The Moment Magnitude Scale"
American
Geophysical Union "Kanamori
Receives the Bucher Medal" (summary of speech given at ceremony
provides good background the biographical information about Kanamori)
California
Institute of Technology "Hiroo
Kanamori" (Dr. Kanamori's homepage)
For more information on magnitude and intensity scales, including mathematical formulas and comparisons
between methods:
National
Earthquake Information Center "Magnitude
/ Intensity Comparison"
National Earthquake Information Center "Magnitude
and Intensity"
National Earthquake Information Center "Measuring
the Size of an Earthquake"
National Earthquake Information Center "Why
Are There So Many Magnitude Scales?"
National Earthquake Information
Center "Magnitude
Definitions Used by the NEIC" (includes formulas for calculating magnitude)
National Geophyscial Data Center "NGDC
Seismological Algorithms"
National Geophysical Data Center "Earthquake
Intensity Database Search"
Cascades Volcano Observatory "DESCRIPTION:
Richter Magnitude Scale, Local Magnitude Scale, Moment Magnitude Scale"
United
States Geological Survey "The
Severity of an Earthquake"
St. Louis University Department of Earth
& Atmospheric Sciences "Earthquake
Size" (though this site is simply posted lecture notes for an
introductory course, the comparison and discussion of magnitude scales is worth
the trouble)
Global Seismological Services "Seismic
Scaling Relations" (great lengthy examination, with equations, of the
types of magnitude and intensity scales)
United States Geological Survey
"Parent's Guide to USGS: Magnitude"
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