previously known to have been shaken by
the quakes), as well as Charleston, S.C.,
and Louisiana. Still, he noted, “there are
huge dead zones in the ‘felt reporting,’ ”
most notably in areas to the west of the
quakes, which were nearly uninhabited
at the time.
Yet more reports from blank spots
on the map, even if they are accurate,
won’t solve some of the problems with
existing data, Hough suggests in
Seismological Research Letters. For instance,
the damage caused by a New Madrid
quake wasn’t always correlated with
distance from its epicenter, making
estimating magnitude quite tricky. In
St. Louis, about 300 kilometers north
of one quake’s presumed epicenter,
ground motions split open a few stone
homes and toppled a few chimneys. But
in Ste. Geneviève, Mo., about 75 kilometers closer to the source of shaking,
the quake was felt but reportedly caused
no damage, Hough notes.
The key difference in damage suf-
fered, she suggests, was the type of
terrain underlying the cities: While
St. Louis was built on floodplain sedi-
ments, the town of Ste. Geneviève had
been moved back from the Mississippi
River —presumably to higher, more
stable ground — after a flood inundated
the town’s former home in the late 1700s.
Similarly, homes and structures along
the Ohio River, as far as 800 kilometers
from the quakes’ epicenters, suffered
damage one or two classes higher on
the Modified Mercalli scale than those
located away from the river.
Thar she blows
Others are gaining clues to the New
Madrid quakes’ magnitudes by looking
at the geological scars left behind. One of
the most dangerous effects of a major
Filled ;ssure Silt and clay layers Lique;ed sand Earthquake waves
Sandy explosion Pressure from seismic shaking can cause sand buried beneath layers of silt and clay to liquefy and exude out at the surface
(diagram at left). Many such sandblows are still visible in the New Madrid region (aerial shot of a blow near Blytheville, Ark., at right).
