Samuel Panno isn't exactly Indiana Jones, but he did have an aha! moment during one of his caving adventures in southwest Illinois. The insight has led to a new way to study past earthquakes--and maybe even help predict the next major event.
While exploring limestone caves, Panno, a geochemist at the University of Illinois, Urbana-Champaign, and his colleagues noticed small white stalagmites on the cave floors. They seemed young enough to have formed after two big earthquakes that struck the region in the past 2 centuries, including the gigantic New Madrid event of the early 1800s.
Because all the stalagmites were about the same size and shape, the researchers suspected that a quake might have started them growing. So they chipped off a few of the deposits, brought them back to the lab, and cut them vertically down the center. Stalagmites form when water containing calcium carbonate drips from the surface of a cave and dries on the floor. Scientists can date them by counting the layers of the mineral deposited by annual drip cycles, much like naturalists use tree rings. The researchers also measured the relationship between two isotopes of thorium contained in the stalagmites, because that relationship offers a precise decay pattern that helps calculate a mineral formation's age.
The result, geochemist and team member Keith Hackley will report Sunday at a joint meeting of several geological and agricultural societies in Houston, Texas, is two populations of stalagmites, one that started growing about 200 years ago and the other about 90 years ago. The older formations coincide closely with the New Madrid quakes, and the younger stalagmites match "a smaller but significant earthquake that occurred quite close to the caves in 1917," says Hackley.
Hackley says the stalagmites probably started growing because earthquakes disturbed the structure of the cave ceilings enough to create cracks, through which water started seeping. By dating the birth of stalagmites, scientists can track past seismic events, he notes, and if scientists see a pattern in how often the stalagmites form, they may be able to predict the likelihood of a future quake.
It's a promising new technique, says independent paleoseismologist Martitia Tuttle in Georgetown, Maine. The method might help to narrow the timing of past events, but "we paleoseismologists would also like to know where the events were located and how big they were"--information stalagmites can't provide. For now, she says, "it's another tool in the toolbox."