While exploring a pair of cave in Illinois in 2008, Samuel Panno noticed a peculiar thing: some of the stalagmites—rocky, cone-shaped formations that rise up from cave floors—had smaller, neighboring growths that were lighter in color, suggesting they had formed more recently.

The offset, white stalagmites were about two inches tall and were growing on taller, brown stalagmites throughout both caves. "It looked like somebody had dropped the ice cream from their cone on top of [the bigger stalagmites]," said Panno, who is a researcher at the University of Illinois and the Illinois State Geological Survey.

When he and his colleagues spotted similar stalagmites pairings in caves in two other Midwestern states, they decided to investigate. Using rock dating techniques, the scientists calculated the ages of the new stalagmite growths, and were surprised to find that they were all roughly 200 years old. "That correlated really well with a series of earthquakes in the region that happened in 1811 and 1812," Panno said.

An idea began to form in the scientists' minds: perhaps the stalagmite pairs they had uncovered were due to earthquakes disrupting the normal stalagmite-formation process. Stalagmites are formed by mineralized water droplets falling from cave ceilings, and earthquakes could leave their mark on stalagmite growth by shifting the ground and altering the flow of a water drip.

If the team’s hunch was correct, then stalagmites could serve as valuable "paleoearthquake indicators" that could provide clues about the timing, magnitude and origin of past earthquake activity.

In a new study, published in the September 13 issue of the Bulletin of the Seismological Society of America, Panno and his team tested this hypothesis. The scientists used a variety of dating techniques to determine the ages of new stalagmite growths in several caves in the Wabash Valley fault system in the Midwestern United States, and compared them to the timing of known earthquakes in the region.

"Sometimes we'll take an entire stalagmite and slice it in half to open it like a book so we can date the different growth bands," Panno said. "Other times, we'll use a drill to take a small core and date that."

Among four stalagmites in Donnehue cave in Indiana, the scientists found a twin pair stalagmite pair that had stopped growing around 100,000 years ago and then resumed growth again around 6,000 years ago, overlapping with a roughly magnitude 7 earthquake in the region. Another younger stalagmite began growing around 1,800 years ago—coinciding with a magnitude 6.2 quake—and showed later shifts in its growth axis that overlapped with other seismic events in the nearby New Madrid Seismic Zone.

Scientists already knew about both of these ancient quakes from other studies of shaken soil in ancient sediments. But whereas dates from so-called paleoliquifaction studies are only reliable up to about 35,000 years, due to limitations of the radiocarbon dating technique used to calculate sediment age, stalagmite ages are calculated using a different technique, called uranium-thorium dating, which could potentially allow scientists to uncover older earthquake signatures from as far back as half a million years ago.

"This has the potential for allowing us to look much farther back in time," said John Tinsley, a scientist emeritus at the United States Geological Survey who was not involved in the study.

Tinsley added that because stalagmite growth can also be affected by other factors, such as climate change that causes a drip source to dry up, building a strong case for ancient quakes that aren't already known about will require finding multiple stalagmite pairs of similar ages in multiple caves.

Indeed, Panno says that most of the stalagmite shifts his team observed were related to flooding or climate change, but the new study demonstrates that it's possible to disentangle these effects from those triggered by earthquakes.

Having a clearer picture of past earthquakes could provide valuable insight about future quakes, Panno said. "It's important to understand the periodicity of major quakes along these seismic zones because the more we know about them," he added, "the better that state, federal and local agencies can be prepared to make decisions about what to do if it looks like we're overdue for another one."