The Severity of an Earthquake
The Richter Scale
The severity of an earthquake can be expressed in terms of both
intensity and magnitude. However, the two terms
are quite
different, and they are often confused.
Intensity is based on the observed effects of ground shaking
on people, buildings, and natural features. It varies from place
to place within the disturbed region depending on the location of
the observer with respect to the earthquake epicenter.
Magnitude is related to the amount of seismic energy
released at the hypocenter of the earthquake. It is based on the
amplitude of the earthquake waves recorded on instruments which
have a common calibration. The magnitude of an earthquake is
thus represented by a single, instrumentally determined value.
Earthquakes are the result of forces deep within the Earth's
interior that continuously affect the surface of the Earth. The
energy from these forces is stored in a variety of ways within
the rocks. When this energy is released suddenly, for example by
shearing movements along faults in the crust of the Earth, an
earthquake results. The area of the fault where the sudden
rupture takes place is called the focus or
hypocenter of the
earthquake. The point on the Earth's surface directly above the
focus is called the epicenter of the earthquake.
The Richter Magnitude Scale
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, location, 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 in the magnitude formula to compensate 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 of 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. Although the
Richter Scale has no upper limit, the largest known shocks have
had magnitudes in the 8.8 to 8.9 range.
Recently, another scale
called the moment magnitude scale has been devised for more
precise study of great earthquakes.
The Moment Magnitude Scale
The Richter Scale was developed to provide a measure of the local magnitude,
ML scale for moderate-size (3 < ML < 7) earthquakes in southern California.
The ML scale is often called the “Richter scale” by the press and
the public but Richter's original methodology is
no longer
used. It does not give reliable results when applied to M >=
7 earthquakes and it was not designed to use data from earthquakes recorded
at epicentral distances greater than about 600 km.
Instead the moment magnitude scale is now used. For practical lay purposes
the moment magnitude scale and the Richter scale are synonymous as most
most modern methods for measuring magnitude were designed to be consistent
with the Richter scale.
The moment magnitude 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.
The Modified Mercalli Intensity Scale
The effect of an earthquake on the Earth's surface is called
the intensity. The intensity scale consists of a series of
certain key responses such as people awakening, movement of
furniture, damage to chimneys, and finally--total destruction.
Although numerous intensity scales have been developed over the
last several hundred years to evaluate the effects of
earthquakes, the one currently used in the United States is the
Modified Mercalli (MM) Intensity Scale. It was developed in 1931
by the American seismologists Harry Wood and Frank Neumann. This
scale, composed of 12 increasing levels of intensity that range
from imperceptible shaking to catastrophic destruction, is
designated by Roman numerals. It does not have a mathematical
basis; instead it is an arbitrary ranking based on observed
effects.
The Modified Mercalli Intensity value assigned to a specific
site after an earthquake has a more meaningful measure of
severity to the nonscientist than the magnitude because intensity
refers to the effects actually experienced at that place.
There is a U.S. Geological
Survey project to collect information about ground shaking following significant
earthquakes. Did you feel an earthquake? If so, please
tell them what you felt by filling out the questionnaire for
the earthquake. The results of these questionnaires and information
furnished by other sources are used to assign an intensity value,
and to compile isoseismal maps that show the extent of various
levels of intensity within the felt area. The maximum observed
intensity generally occurs near the epicenter.
The lower numbers of the intensity scale generally deal with
the manner in which the earthquake is felt by people. The higher
numbers of the scale are based on observed structural damage.
Structural engineers usually contribute information for assigning
intensity values of Vlll or above.
The following is an abbreviated description of the 12 levels
of Modified Mercalli intensity.
- Not felt except by a very few under especially favorable conditions.
- Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
- Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
- Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
- Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
- Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
- Damage negligible in buildings of good design and construction; slight to
moderate in well-built ordinary structures; considerable damage in poorly
built or badly designed structures; some chimneys broken.
- Damage slight in specially designed structures; considerable damage in ordinary
substantial buildings with partial collapse. Damage great in poorly built
structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy
furniture overturned.
- Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
- Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent.
- Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
- Damage total. Lines of sight and level are distorted. Objects thrown into the air.
Another measure of the relative strength of an earthquake is
the size of the area over which the shaking is noticed. This
measure has been particularly useful in estimating the relative
severity of historic shocks that were not recorded by
seismographs or did not occur in populated areas. The extent of
the associated felt areas indicates that some comparatively large
earthquakes have occurred in the past in places not considered by
the general public to be regions of major earthquake activity.
For example, the three shocks in 1811 and 1812 near New Madrid,
Mo., were each felt over the entire eastern United States.
Because there were so few people in the area west of New Madrid,
it is not known how far it was felt in that direction. The 1886
Charleston, S.C., earthquake was also felt over a region of about
2 million square miles, which includes most of the eastern United
States.
This information is modified from the USGS publication *U.S.
GOVERNMENT PRINTING OFFICE: 1989-288-913. The original page may be found
at URL:http://pubs.usgs.gov/gip/earthq4/severitygip.html
last modified 04-17-01
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