The threat
of large earthquakes striking the New Madrid seismic zone remains
all too real for people in St. Louis, Memphis and other parts
of the central United States - despite recent reports to the contrary.
That is the conclusion of a new study by geophysicists Shelley
J. Kenner and Paul Segall published in the journal Science.
According to the authors, devastating earthquakes could rip through
the New Madrid seismic zone along the Mississippi River sometime
this century - potentially causing widespread destruction from
Arkansas to Iowa.
``We can`t say for certain there aren`t going to be any earthquakes
in the next few decades,`` says Kenner, a postdoc in geophysics
at Caltech and lead author of the Sept. 29 Science study. ``There
was a sequence of large earthquakes in the past, and it could
happen again,`` adds Segall, a professor of geophysics at Stanford.
According to the U.S. Geological Survey (USGS), the central Mississippi
Valley is the most earthquake-prone region of the United States
east of the Rocky Mountains. Most quakes in the area are relatively
small and occur within the 100-mile-long New Madrid seismic zone
that extends along the Mississippi from Missouri to Arkansas.
During the winter of 1811 to 1812, the sparsely populated New
Madrid, Mo., area was jolted by a series of three powerful earthquakes
now estimated to have been of magnitude 7.5 to 8.3. The quakes
caused church bells to ring a thousand miles away in Boston and
even changed the course of the Mississippi River. Paleoseismic
evidence collected in recent decades indicates that strong ``earthquake
triplets`` similar in magnitude to the 1811-12 temblors have occurred
approximately every 500 years along the New Madrid fault and are
likely to happen again.
Some geologists say there is a 90 percent chance that earthquakes
of magnitude 6 to 7 will strike the region in the next 50 years,
causing extensive damage and injury - especially in now heavily
populated urban areas such as St. Louis and Memphis, which are
built along the Mississippi on unstable mud banks. But in April
1999, Science magazine published a controversial study, which
concluded that ``the hazard posed by great earthquakes in the
New Madrid seismic zone appears to be overestimated.``
 The
authors of the 1999 study, led by geologist Seth Stein of Northwestern
University, wrote that a large earthquake would not strike the
region for at least 1,000 years. ``It is also possible that 1811-12-style
earthquakes may never recur,`` according to the Stein research
team. The
authors used Global Positioning System (GPS) satellite measurements
to track ground movement in the New Madrid region between 1991
and 1997, but observed ``little, if any motion`` within the fault
zone.
According to Stein and his co-authors, this lack of movement suggests
that it would take at least 1,000 years to accumulate enough stress
along the fault to generate a magnitude 7 earthquake - and at
least 2,500 years for a magnitude 8 temblor to occur.
But Kenner and Segall disagree with the conclusions reached by
Stein and his colleagues. ``We`re not disputing their GPS data,``
says Segall. ``We`re saying that, if you use good data in an inappropriate
model, your conclusions will be inappropriate.``
Segall points out that the 1999 Stein study used earthquake-prediction
models for California`s San Andreas fault and applied them to
the New Madrid seismic zone - a completely different type of fault
system. The San Andreas is a ``transform fault`` that marks the
boundary between two tectonic plates - the North American and
the Pacific. These two massive land formations constantly grind
past one another. As the Pacific plate moves northward, stress
and strain accumulate along the fault, triggering large and small
earthquakes. The relatively rapid movement of the Pacific plate
is easily observed by GPS and ground-based detectors.
But the New Madrid is an ``intraplate fault`` located in the middle
of the North American plate, not at the boundary. And while large
segments of the San Andreas are clearly visible, the New Madrid
fault lies hidden under deep layers of sediment. ``Intraplate
zones are something of a mystery,`` notes Segall. ``We can`t even
map the fault, so we don`t really know how big it is.`` ``As scientists,
we don`t understand what`s happening in the intraplate setting
all that well,`` adds Kenner. ``What we tried to do in our current
Science study is to come up with a model that`s a better approximation
of the New Madrid seismic zone.``
The Kenner-Segall model suggests that there is a preexisting weak
zone beneath the New Madrid fault where stress accumulates over
time. ``You may not be able to see very much stress accumulation,``
observes Segall, ``but it`s there.``After many years, the stress
shifts from the weak zone toward the surface, where it builds
up and eventually causes the fault to rupture, triggering a large
earthquake as the ground slips some 15 to 30 feet.
How often do these major quakes occur? Every 250 to 4,000 years,
say Kenner and Segall, which means that another sequence of large
temblors could strike by the middle of the century - perhaps sooner.
``The New Madrid fault is like a crack in the wall,`` comments
Segall. ``You try to cover it with plaster, but the crack reappears
over and over again. It`s always going to be there, because it`s
a preexisting weakness. ``I can`t emphasize enough that there
is a huge uncertainty when it comes to intraplate
events,`` he adds. ``For example, we don`t know why the earthquakes
come in triplets, but it has happened on a regular basis in the
past.``
Earlier this
month, the Federal mergencyManagement
Agency (FEMA) released a study listing the U.S. metropolitan areas
at greatest economic risk from earthquakes. St. Louis ranked 16th.
``If you live in the New Madrid area, you may still have an earthquake
problem,`` says Segall. ``The most prudent thing for people to
do is to prepare.``
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