Seismic waves are the vibrations from
earthquakes that travel through the Earth;
they are recorded on instruments called
seismographs. Seismographs record a zig-zag
trace that shows the varying amplitude of
ground oscillations beneath the instrument.
Sensitive seismographs, which greatly
magnify these ground motions, can detect
strong earthquakes from sources anywhere
in the world. The time, locations, and magnitude
of an earthquake can be determined
from the data recorded by seismograph stations.
The Richter magnitude scale was developed
in 1935 by Charles F. Richter of the
California Institute of Technology as a
mathematical device to compare the size of
earthquakes. The magnitude of an earthquake
is determined from the logarithm of
the amplitude of waves recorded by seismographs.
Adjustments are included for the
variation in the distance between the various
seismographs and the epicenter of the earthquakes.
On the Richter Scale, magnitude is
expressed in whole numbers and decimal fractions.
For example, a magnitude 5.3 might be computed for a
moderate earthquake, and a strong earthquake might be
rated as magnitude 6.3. Because of the
logarithmic basis of the scale, each whole
number increase in magnitude represents a
tenfold increase in measured amplitude;
as an estimate of energy, each whole number
step in the magnitude scale corresponds to
the release of about 31 times more energy
than the amount associated with the preceding
whole number value.
At first, the Richter Scale could be applied
only to the records from instruments of
identical manufacture. Now, instruments are
carefully calibrated with respect to each other.
Thus, magnitude can be computed
from the record of any calibrated seismograph.
Earthquakes with magnitude of about 2.0 or less are
usually called microearthquakes; they
are not commonly felt by people and are
generally recorded only on local
seismographs. Events with magnitudes of about 4.5 or
greater - there are several thousand such shocks annually -
are strong enough to be recorded by sensitive
seismographs all over the world. Great earthquakes,
such as the 1964 Good Friday earthquake
in Alaska, have magnitudes of 8.0 or higher.
On the average, one earthquake of such size occurs somewhere
in the world each year.
The Richter Scale has no upper limit.
Recently, another scale called the moment magnitude
scale has been devised for more precise study
of great earthquakes.
The Richter Scale is not used to express damage. An
earthquake in a densely
populated area which results in many deaths
and considerable damage may have the same
magnitude as a shock in a remote area that
does nothing more than frighten the wildlife.
Large-magnitude earthquakes that occur
beneath the oceans may not even be felt by humans.
See also: Charles Richter
See also: Magnitude / Intensity Information and Links
Abridged from
The Severity of an Earthquake,
a U. S. Geological Survey General Interest
Publication.
U.S. GOVERNMENT PRINTING OFFICE: 1989-288-913
This publication is one of a series of general interest
publications prepared by the U.S. Geological Survey to provide
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Survey," write:
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Information Services
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