Prehistoric Indiana earthquakes

The point on the Earth’s surface directly above the center of an earthquake is called the quake’s epicenter. During the last two centuries earthquakes with epicenters in Indiana have been relatively minor events. This has not always been the case. Indiana University archaeologists Pat Munson and Cheryl Munson and U.S. Geological Survey geologist Steve Obermeier have found hundreds of ancient sandblows (see figures 2 and 3) that suggest the occurrence of at least six major earthquakes with epicenters in Indiana during the last 12,000 years. The largest of these quakes appears to have had an epicenter near Vincennes and has been estimated to have been many times more powerful than the quake that struck the Los Angeles area in January 1994.

Figure 2. When strong earthquakes release their energy, the violent shaking may cause underground layers of saturated sandy soil to behave like a fluid under pressure. This process is called liquefaction, and sometimes the pressure forces the liquefied sand to move up through cracks in the overlying soil and flow out over the surface, creating a feature called a sandblow (A). The photo above (B) shows a cross-sectional view of an ancient sandblow exposed in the bank of the Wabash River near Vincennes. After the sandblow formed, it was covered by layers of silt deposited during floods.
A. Image by Jeff Kirby.
B. Photo by Ned Bleuer.

Figure 3. Map of southern two-thirds of Indiana showing sites where ancient sandblows have been found, and showing areas of liquefaction for six major prehistoric earthquakes.
Modified from Munson, Obermeier, Munson, and Hajic, 1997.

The ages of the sandblows were determined using radiocarbon dating on organic materials found in soil layers below, above, or at the same level as the tops of the sandblows. Included in the organic materials is charcoal from campfires made by people living in Indiana at that time. Other artifacts, including projectile points, were found at many sites and helped to date the earthquakes.