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Earthquake history recorded in stalagmites

Caves could yield insight into how often major Midwestern quakes have happened

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10:47am, October 6, 2008
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HOUSTON — The dates at which some Midwestern cave formations began to grow could help researchers chronicle the earthquake history of Missouri and surrounding states, according to work reported October 5 in Houston during the annual meeting of the Geological Society of America.

During late 1811 and early 1812, a series of major quakes rocked the New Madrid Seismic Zone, a fault system named for a small town in southeastern Missouri near the center of those temblors. Scientists often can estimate the age of older, prehistoric quakes along those faults by analyzing wood or other organic debris trapped in pockets of sand and forced to Earth’s surface during a quake, says Keith C. Hackley, a geochemist with the Illinois State Geological Survey in Champaign. But many such features have long been plowed or otherwise disturbed by farming, rendering results of analyses ambiguous.

Hackley and his colleagues have now used geochemical techniques, including uranium-thorium dating, to analyze material at the base of stalagmites found in caves between 180 and 230 kilometers north of the epicenters of the 1811–1812 quakes. Many of those stalagmites started growing about 195 years ago, when the massive temblors — estimated to range around magnitude 8 — may have cracked rocks overlying the caves. When these rocks cracked, mineral-rich groundwater seeped into the caverns from new locations and started generating new stalagmite formations. Other stalagmites that the team analyzed began growing about 90 years ago, about the time that a magnitude-5 quake shook a region just east of the caves, says Hackley.

These results hint that stalagmites could provide useful information about ancient quakes in the area, Hackley says. Preliminary analyses of about 60 formations found in caverns throughout southern Illinois, Indiana and Missouri suggest that major quakes occur in the region about once every 500 years or so. If correct, that frequency would confirm similar results obtained by less-accurate analyses of material gathered from sand blows or trenches dug during previous field studies.

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