Seismicity of the Lower Wabash Valley: Fact Sheet John R. Hill Associate Director and Geologist Indiana Geological Survey Bloomington, Indiana 47405

*Continued from previous page -

Modern east-west compressive stresses affects accumulates in the bedrock and along preexisting faults until sudden movement along a fault plane causes an earthquake. Faults responsible for causing most known earthquake activity within the Lower Wabash Valley are in Precambrian igneous and metamorphic units at depths of between 12 and 20 km. In fact, not a single earthquake in Indiana has been linked with shallow faults that cut Paleozoic age sedimentary formations. The deep structures are not well understood, but they appear to be responsible for the elaborate splay (flower structures) of faults within the Paleozoic rocks.

Recent historic earthquake activity linked with the Lower Wabash Valley Fault System has been relatively moderate; the strongest event occurring on June 10, 1987 near Lawrenceville, Illinois. The magnitude 5.1 quake resulted in minor damage to a number of structures in Illinois and Indiana. On June 18, 2002, a temblor shook the Greater Evansville area and had its epicenter scarcely 10 miles due west of the city. The magnitude 4.8 earthquake damaged a number of older masonry structures in town and was felt throughout the region. A more significant magnitude 5.3 temblor took place in 1968, but its epicenter was west of the Lower Wabash Valley Fault System in south-central Illinois.

Even though earthquakes of the last two centuries 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 sand blows (see Figures 3 and 4) that suggest the occurrence of at least 6 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 3. 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 sand blow (A). The photo above (B) shows a cross-sectional view of an ancient sand blow exposed in the bank of the Wabash River near Vincennes. After the sand blow formed, it was covered by layers of silt deposited during floods.

Figure 4. Map of southern two-thirds of Indiana showing sites where ancient sand blows have been found, and showing areas of liquefaction for six major prehistoric earthquakes. Modified from Munson, Obermeier, Munson, and Hajic, 1997.*3

The ages of the sand blows were determined using radiocarbon dating on organic materials found in soil layers below, above, or at the same level as the tops of the sand blows. Included in the organic materials is charcoal from campfires made by people living in Indiana at that time. Other artifacts, such as projectile points, were found at many sites and helped to date the earthquakes *3. On the basis of this information, the earthquake that formed the Vincennes sand blow happened 6,100 years ago and its magnitude may have been as great as 7.5*3.

The presence of large, albeit prehistoric, liquefaction features throughout the Lower Wabash Valley suggests the possibility that the region is capable of producing very large earthquakes. Long return periods and the infrequency of major events makes impossible at present the prediction of the next damaging earthquake. Scientists from the Indiana, Illinois and Kentucky geological surveys are presently working with seismologists and structural geologists of the U.S. Geological Survey to learn more about the mechanisms that create earthquakes in the Lower Wabash Valley and to use modern statistical probability models to better define the seismic hazard within the tri-state region.

References
1. Zuppann, Charles W., 1988, Geologic Setting of the Illinois Basin, in Geology and Petroleum Production of the Illinois Basin Volume 2: Illinois and Indiana-Kentucky Geological Societies, p. 3

2. Kolata, D.R. and Hildenbrand, T.G., 1997, Structural Underpinnings and Neotectonics of the Southern Illinois Basin: an Overview: Seismological Research Letters, Vol. 68, No. 4, p. 499-510, (July/August 1997)

3. Munson, P.J, Obermeier, S.F., Munson, C.A., and Hajic, E.R., 1997, Liquefaction Evidence for Holocene and Latest Pleistocene Seismicity in the Southern Halves of Indiana and Illinois: A Preliminary Overview; in Seismological Research Letters, Volume 68, Number 4 (July/August 1997)

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