The
New Madrid Seismic Zone
The magnitude of an earthquake
is determined from
the logarithm of the amplitude of waves recorded by seismographs. Each
whole
number increase in magnitude represents a tenfold increase in measured
amplitude
This map of the New Madrid
Seismic Zone shows where
a hypothetical magnitude 8 (VIII) earthquake would be felt and at what
magnitude in that area.
In contrast to
California, an earthquake in the New Madrid seismic zone could have
consequences more far-reaching because faults in the Mississippi Valley
are
buried under sedimentary deposits up to a mile deep. These conditions
allow
seismic waves to travel as much as 20 times farther than they do in
California.
As a result, a moderate New Madrid quake would shake a seven-state
region --
Missouri, Illinois, Arkansas, Tennessee, Kentucky, Mississippi and
Indiana --
like a bowl of jelly. "Good firm soil will not amplify the ground
motion
as much as a weak soil," says John C. Theiss, a structural engineer for
EQE-Theiss in St. Louis and who is a member of the Building Seismic
Safety
Council. "If you're near a river and you build on soil that was
previously
on a floodplain, that's the worst type of soil, because it magnifies
the ground
motion."
Memphis,
Shelby County Seismic Hazard Maps