PRELIMINARY EVIDENCE FOR NEW MADRID SEISMIC ZONE EARTHQUAKES RECORDED IN SOUTHWESTERN ILLINOIS CAVES

Predictions of seismic activity along the New Madrid Seismic Zone (NMSZ) will require an accurate reconstruction of paleo-seismic history. Ongoing research by the authors, beginning in 2001, has shown that cave deposits in southwestern Illinois may be unique and unexpected repositories for such records. Geological features in two southwestern Illinois caves, located within 250 km of the New Madrid Seismic Zone (NMSZ), and within 150 km of the epicenter of the 1811-1812 earthquakes, appear to be related to earthquake activity. The features consist of three types: 1) stalagmites with deviated growth axes; 2) hundreds of relatively small, actively growing, white speleothems; the white stalagmites are growing on older stalagmites (4000 to 5000 years BP), on older flow stone, and on breakdown in both caves; and 3) fallen soda straw stalactites from an isolated area in one of the caves. Each of these features, including associated collapse features, could provide temporal evidence of a seismic event.

The white stalagmites range from 2 to 6 cm in height, about 3 cm in diameter. Because these stalagmites are porous along their central axis, microsampling for U-series dating was restricted to their nonporous flanks. Resulting U-Th ages for calcite near the base of their flanks were 114, 121, and 123 years BP. Preliminary thin section work suggests the stalagmites may be at least 40 years older. Because of the similar morphology, appearance, and ages of the white speleothems, we propose that a single event triggered their growth. Possibilities include: 1) the rejuvenation of dormant speleothems resulting from the 1811-1812 NMSZ earthquake series (about 190 years BP), and 2) agricultural activities on the surface that began in the area in the early 1840s (about 160 years BP). Both scenarios could have reopened former flowpaths to the caves and initiated speleothem growth.