Hazard zones Many people think just of the West Coast when discussing major quakes in
the United States, but the New Madrid region and coastal South Carolina face similar threats.
quake is liquefaction, a process in which
the pressures created by seismic waves
temporarily turn moist, poorly drained
sediments — especially those bearing
great weights, such as from buildings — into something akin to quicksand.
Liquefaction played a large role in devastating San Francisco’s Marina District,
largely built on landfill used to reclaim
former wetlands, during the 1989 Loma
Prieta quake. More recently, during the
megaquake that slammed Japan in
March, ground motions that lasted
several minutes triggered liquefaction
that damaged homes, buildings and
infrastructure such as roads, port facilities and buried gas and water lines.
During the New Madrid quakes,
of course, there was little large-scale
infrastructure to be damaged. But soils
in the area are largely made of thick layers of dense, river-deposited silt and
mud interleaved with layers of water-saturated sand, conditions that primed
the ground to liquefy. When the seismic
shaking commenced, the weight of overlying mud pressurized aquifers, causing
massive geysers of sandy water to spew
onto the surface.
Deposits left by these “sandblows” or
“sand boils” were often huge, says
Thomas Holzer of the USGS in Menlo
Park, Calif. While some measured a few
meters across, one sand boil covered at
least 136 acres, he noted at the April
meeting. And because the sand is typi-
cally so different from the silty soil in
color and texture, the features can easily
be discerned at ground level and in
satellite images, despite decades of
plowing and other agricultural activity
on the fertile floodplains.
In 1815, the U.S. Congress passed its
first disaster relief act, to aid victims of
the New Madrid quakes, appropriating $50,000 — which, in today’s dollars,
amounts to a little less than $600,000.
Damages from a modern-day New
Madrid quake would dwarf that figure.
A magnitude 6. 4 to 6. 9 quake at
the southern end of the New Madrid
Seismic Zone, near Memphis, could
cause damage and economic losses
to private property and businesses of
between $80 billion and $130 billion,
Mary Lou Zoback, a geologist with Risk
Management Solutions in Newark,
Calif., reported at the April meeting.
A magnitude 7. 7 located on the worst
possible spot in the New Madrid Seis-
mic Zone could trigger losses exceeding
$250 billion, she said.
s For more accounts of the New Madrid
Sid Perkins is a freelance writer based in
December 3, 2011 | SCIENCE NEWS | 25