December
11, 2006 New results about
the
temperatures of rock deep below the New Madrid Seismic Zone in the
central Scientists from "Hot rocks are
weak,"
says Seth A. Stein, William Deering Professor of Geological Sciences in
the
Weinberg College of Arts and Sciences at Northwestern and a coauthor of
the
study. "So people suggested that the reason large earthquakes occur in
the
New Madrid area rather than in the many similar geologic settings in
other
parts of the eastern But the researchers
discovered
this is not the case. They looked at data used in the new edition of
the
Geothermal Map of North America (American Association of Petroleum
Geologists,
2004), which shows all the measurements of the heat coming to the
Earth's
surface (heat flow) taken from boreholes. They found that thermally New
Madrid
is surprisingly similar to other areas of the eastern "The New Madrid data
are
essentially no different from other sites in the eastern One of the most
difficult aspects
of assessing the earthquake hazard is deciding whether New Madrid is a
special
place or simply where central The new heat flow
results fit
into a growing idea that earthquakes can migrate among similar faults,
some of
which -- such as the Meers fault in Oklahoma -- appear to have been
active
about 10,000 years ago but show no activity today. Geological studies
find that
New Madrid earthquakes comparable to those of 1811-1812 occurred about
1450 and
900 AD. However, because this fault system has not generated
significant
topography, it is likely to have "turned on" relatively recently,
perhaps within the past few thousand years. With this view, say
the
researchers, prior earthquakes were concentrated on other faults, and
future
earthquakes will occur somewhere else when the New Madrid system "shuts
down." Once this happens, it may be a very long time -- thousands of
years
or longer -- before New Madrid becomes active again. "Although we don't
know when
the New Madrid fault system will shut down, it may be dying today,"
says
Stein. "The recent cluster of earthquakes may be coming to an end." Migrating
earthquakes also occur
in the interior of other continents, such as Precise measurements
taken by
Stein, coworkers and other investigators using the Global Positioning
System
(GPS) show that motion across the New Madrid Seismic Zone currently is
either
very slow or at zero. Because this motion has to accumulate for many
years to
cause a large earthquake, it will be at least hundreds of years and
perhaps
much longer, before another large earthquake happens. "Until recently
about all we
could say was that future earthquakes might occur in places where past
ones
had," says Stein. "Now we can actually test that idea by looking at
the motion accumulating for possible future earthquakes. Although we
can't be
sure yet, the longer the GPS data continue to show essentially no
motion, the
more likely it seems that the fault is shutting down and won't cause
large
earthquakes for a very long time. It's time to start thinking about
this
possibility and to use what we're learning to improve estimates of the
hazard
from future earthquakes." The possibility of
the fault
shutting down is important for assessing the earthquake hazard in the
central
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