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<p class="body"><font face="Georgia, Times New Roman, Times, serif" size="5">The
Severity of an Earthquake</font> </p>
<p class="body"><font size="3" face="Georgia, Times New Roman, Times, serif">Source:
"The Severity of an Earthquake". U.S. Geological Survey, U.S.
Government Printing Office 1997-421-530.</font></p>
<p class="body"><img src="../../images/coloredSquare.gif" width="10" height="10">
<font face="Georgia, Times New Roman, Times, serif" size="3" color="#663399">THE
SEVERITY OF AN EARTHQUAKE</font><font face="Georgia, Times New Roman, Times, serif" size="3">
can be expressed in terms of both intensity and magnitude. However, the
two terms are quite different, and they are often confused.</font></p>
<table cellspacing="0" cellpadding="8" width="236" align="left">
<tr>
<td class="caption" height="317"><a href="../../sample_pages/1980-01-08_full.jpg"><img src="00001-01.gif" width="211" height="265"><br>
</a><span class="capNum"><font size="2" face="Arial, Helvetica, sans-serif">Figure
1: San Francisco, 1906. Collapse of City Hall after the 8.3 magnitude
earthquake. Most of the property destruction was caused by the fire
that raged after the earthquake.</font></span></td>
</tr>
</table>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3"><b>The Richter
Magnitude Scale</b></font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<table cellspacing="0" cellpadding="8" align="right" width="233">
<tr>
<td class="caption"><img src="00001-02.gif" width="212" height="263"><span class="capNum"><br>
<font face="Arial, Helvetica, sans-serif" size="2">Figure 2: Mindanao,
Philippines, 1976. Apartment building destroyed by a magnitude 7.9
earthquake.</font></span></td>
</tr>
</table>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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 </font><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3"><b>The Modified
Mercalli Intensity Scale</b></font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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
t used in the United States is the Modified Mercalli (MM) Intensity Scale.
It was developed in 1931 bytheAmerican 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 mathe matical basis; instead it
is an arbitrary ranking based on observed effects. </font></p>
<table cellspacing="0" cellpadding="8" width="238" align="left">
<tr>
<td class="caption"><a href="../../sample_pages/1980-01-08_full.jpg"><img src="00001-03.gif" width="212" height="264"><br>
</a><span class="capNum"><font size="2" face="Arial, Helvetica, sans-serif">Figure3:
</font></span><font size="2" face="Arial, Helvetica, sans-serif">Long
Beach, California, 1933. Exterior walls collapsed onto parked cars
after this magnitude 6.3 earthquake (photo by Southern California
Earthquake Pictures).</font></td>
</tr>
</table>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">The Modified
Mercalli Intensity value assigned to a specific site after an earth quake
has a more meaningful measure of severity to the nonscientist than the
magni tude because intensity refers to the effects actually experienced
at that place. After the occurrence of widely-felt earthquakes, the Geological
Survey mails questionnaires to postmasters in the disturbed area request
ing the information so that intensity values can be assigned. The results
of this postal canvass and information furnished by other sources are
used toassign 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 VIII or above. </font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">The following
is an abbreviated description of the 12 levels of Modified Mercalli intensity.
</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">I. Not felt
except by a very few under especially favorable conditions.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">II. Felt
only by a few persons at rest, especially on upper floors of buildings.
Delicately suspended objects may swing.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">III. 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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">IV. 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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">V. Felt
by nearly everyone; many awakened. Some dishes, windows broken. Unstable
objects overturned. Pendulum clocks may stop.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">VI. Felt
by all, many frightened. Some heavy furniture moved; a few instances of
fallen plaster. Damage slight.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">VII. 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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">VIII. 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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">IX. 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.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">X. Some
well-built wooden structures destroyed; most masonry and frame structures
destroyed with foundations. Rails bent.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">XI. Few,
if any (masonry) structures remain standing. Bridges destroyed. Rails
bent greatly. </font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">XII. Damage
total. Lines of sight and level are distorted. Objects thrown into the
air.</font></p>
<p><font face="Georgia, Times New Roman, Times, serif" size="3">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 3
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.</font> <img src="../../images/coloredSquare.gif" width="10" height="10" alt="">
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