Geography Reference
In-Depth Information
numbers of the Mercalli scale mean, as well as (in parentheses) their approximate Richter Scale equi-
valents (Note: because the Mercalli is a 12-point scale and the Richter is a 9-point scale, not every
Mercalli numeral will have a corresponding Richter equivalent).
The New Madrid Earthquake(s)
On December 16, 1811, perhaps the most powerful earthquake in the recorded history of
the United States occurred near New Madrid, Missouri, located on the Mississippi River
in the extreme southeastern part of that state. I say “perhaps” for two reasons. Scientif-
ic instruments that accurately measure earthquakes were not then available. And in the
days and weeks that followed, two other major quakes (and literally thousands of much
lesser ones) rocked the region. Either or both of those may have been stronger than the
first.
How powerful were they? Apparently, each may have registered an 8.0 or higher on the
Richter Scale. The Mississippi River changed course. An island in the river disappeared.
New land rose. Forests were knocked down for miles around. The ground rolled in visible
waves, wiping away houses, gardens, and fields. Fortunately, remarkably few people per-
ished, mainly because the region was then only lightly populated.
The quakes were not flukes, but instead the product of a fault zone that is minor in terms
of its length, but major in respect to seismic potential. Geographically, what is most inter-
esting is that New Madrid is more than a thousand miles from the nearest plate boundary.
Thus, while the vast majority of earthquakes occur on the fringe of tectonic plates, the
New Madrid episodes demonstrate that earthquakes (even very serious ones) can po-
tentially happen anywhere.
Table 6-1 The Modified Mercalli Intensity Scale
Intensity levelExtent of Impact (approximate Richter Scale equivalent)
I
Not felt. Usually detected only by instruments. (2 or less)
II
Felt by a few people at rest, especially on upper floors
III
Hanging objects swing. Felt quite noticeably outdoors. (3)
IV
Felt indoors by many, outdoors by few. Sensation is like a heavy
truck striking a building. (4)
V
Felt by nearly everyone; sleepers awakened; trees and tele-
phone poles shaken, some dishes and windows broken.
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