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A: a)
Severe earthquakes have occurred in the East:
In November of 1755, an
earthquake with an estimated magnitude of 6.0
and a maximum intensity of VIII occurred 200 miles
off the coast of Cape Ann, Massachusetts. Boston
was heavily damaged. The strongest earthquakes
recorded in the continental US were not in the
West; they were centered in eastern Missouri near
the border with Kentucky and Tennessee. In the
winter of 1811-1812, a series of three earthquakes
of magnitudes 8.4 to 8.7 and maximum intensities
of XI occurred near New Madrid, Missouri. These
shocks were so strong that observers reported
that the land distorted into visible rolling waves.
They changed the course of the Mississippi River;
they made church bells ring in Boston and Washington,
D.C. Because the surrounding area was mostly undeveloped
at the time, few deaths were reported and these
events stirred relatively little attention then.
In August of 1886, a strong earthquake occurred
in Charleston, South Carolina. Magnitude is estimated
at 6.6 and maximum intensity was X. Most of the
city of Charleston was damaged or destroyed. Earthquakes
in the East are not confined to these areas; they
have been recorded in every State east of the
Mississippi. Damaging earthquakes have occurred
historically in nearly every eastern State.
b) Earthquakes of the same
magnitude affect larger areas in the East than in
the West:
The size of the geographic
area affected by ground shaking depends on the
magnitude of the earthquake and the rate at which
the amplitudes of body and surface seismic waves
decrease as distance from the causative fault
increases. Comparison of the areas affected by
the same Modified Mercalli intensity of ground
shaking in the 1906 San Francisco, California,
the 1971 San Fernando, California, the 1811-12
New Madrid, Missouri, and the 1886 Charleston,
South Carolina, earthquakes shows that a given
intensity of ground shaking extends over a much
larger area in the Eastern United States. Ground
shaking affects a larger area because amplitudes
of seismic waves decrease more slowly in the east
than in the west as distance from the causative
fault increases.
Comparison of isoseismal
contours for a great earthquake, the 1906 San
Francisco; a moderate earthquake, the 1971 San
Fernando; and two large earthquakes, the 1811-12
New Madrid and 1886 Charleston. The contour lines
connect sites having the same value of Modified
Mercalli intensity, a numerical index of the effects
of an earthquake on man, the Earth's surface,
and on buildings. Each area shown in red corresponds
to an intensity of VII or greater and denotes
the zone of most severe ground shaking and damage.
The area shaded blue corresponds to an intensity
of VI; in this area, the ground shaking is felt
by all, many are frightened and run outdoors,
but damage is slight. The effects of ground shaking
extend over a much larger area in the Eastern
United States.
Taken from: Hays, W.W.,
ed., 1981, Facing Geologic and Hydrologic Hazards
-- Earth Science Considerations: U.S. Geological
Survey Professional Paper 1240B, 108 p.
c) Eastern state's building
codes:
Modern building codes
in Eastern states are not as strict as those in
California and much of the West: Not only that,
but older buildings, which predate modern building
codes completely, are more prevalent in the East
than in the West.
d) Causes of earthquakes
in the East are not well-understood:
Hundreds of millions of
years ago, the East coast was this continent's
active plate tectonic boundary, as the West coast
is today. If the East is not now in an active
plate margin, why do we have earthquakes here
and why do we have them in the center of the continent?
One possible explanation is that ancient faults
or rifts are stressed. If this is true, what is
the cause of the stress? In many areas of the
East where earthquakes have occurred historically,
specific faults causing the quakes have not been
mapped or even identified. Another problem we
encounter when evaluating earthquake risk is that
we only have earthquake records for the last couple
of hundred years. Establishing geologic patterns
over human time scales is difficult at best.
For further information:
Where
Are the Fault Lines in the U.S. East of the Rocky
Mountains?
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