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Earthquakes - Earthquake Near Evansville: Another Warning of Things to Come

John R. Hill

The magnitude 5.0 earthquake of June 18, 2002 had its epicenter between Mt. Vernon and West Franklin in Posey County, southwestern Indiana. The fault that produced the earthquake moved at a depth of about 14.5 km in rocks that are predominantly metamorphic and igneous. Fault movement was predominantly strike-slip motion along steeply dipping fault planes that trend due northeast or northwest according to Won-Young Kim of Lamont-Doherty Earth Observatory at Columbia University.

Although little property damage resulted from the temblor, the event should serve as yet another warning that the Lower Wabash Valley of southwestern Indiana and adjacent parts of Illinois is seismically active. As recently as December 7, 2001, the city of Evansville was shaken by a modest magnitude 3.9 earthquake and some may still remember the magnitude 5.1 quake that shook much of the Midwest on June 10, 1987. On November 9, 1968, a magnitude 5.5 earthquake with an epicenter in southeastern Illinois really shook things up. Are these isolated events that indicate that southwestern Indiana has little to fear in terms of the potential for a damaging earthquake? The answer is a resounding NO. Seismologists and geologists from the U.S. Geological Survey, various universities in Indiana, and geologists of the Indiana Geological Survey are in collective agreement that the Lower Wabash Valley of Indiana and Illinois is capable of producing large and damaging earthquakes at virtually any time.

Given the recorded history of damaging earthquakes in the Midwest, we have but two major earthquakes to draw attention to the region's earthquake hazard potential. The most notable were the series of earthquakes that shook much of the U.S. east of the Rocky Mountains during the period from December 1811 through February 1812. These quakes occurred near the town of New Madrid, Missouri and the attendant ground shaking actually rang church bells as far away as Boston. In 1895, a magnitude 6.2 quake again shook the region; this time the epicenter was near Charleston, Missouri. Since then, however, no large earthquakes have affected the region.

Nevertheless, geologic evidence exists in southwestern Indiana and southeastern Illinois that clearly documents prehistoric earthquakes of great intensity. What is the evidence for these large earthquakes, you may ask. When an earthquake occurs, energy travels through rock and soil in the form of waves. Some of the waves produce back and forth motion along the line of travel (push-pull or P-waves) while others have a side to side motion (shearing motion or S-waves). If the energy being transmitted by, for example, a shear wave is sufficiently great, sandy soils saturated by a high water table can be ejected onto the land surface. The process of transforming saturated sand into a semi-liquid is called liquefaction. During liquefaction, the sandy soils flow vertically upward along fractures in the overlying materials. These fractures are called dikes, and by studying the size of the dike and kind of sediment found within it, scientists can estimate the size of the earthquake that it took to form the feature.

Along the east bank of the Wabash River near the town of Vincennes, a major liquefaction feature was discovered in the mid 1980s by Steven Obermeier who was then working for the U.S. Geological Survey. At the time of the earthquake that formed this feature, sand and gravel from a buried terrace of the Wabash River was ejected onto the surface and simultaneously captured part of a tree that had been growing along the river bank at the time. Using radio carbon dating methods, we discovered the tree to be 6,100 years old, which means that the liquefaction feature is also approximately 6,100 years old. Steve Obermeier believes that it would have taken at least a magnitude 7 earthquake to produce the Vincennes liquefaction feature. From field evidence at this and other sites in southwestern Indiana, we know that the Lower Wabash Valley is capable of producing large and potentially damaging earthquakes.

But will another earthquake of that size ever happen again? On the basis of geophysical and geologic evidence being gathered by scientists working in the Midwest, it is safe to say that a damaging earthquake will definitely shake up our part of the world. The question that remains unanswered is when the quake will happen. The moderate magnitude earthquakes like those of June 10, 1987 and June 18, 2002, should serve as a warning to residents and public officials alike.

Preparedness is the watch word that should dictate what we do to face the inevitable "big one." The city of Evansville is currently requiring structure designs that will withstand a magnitude 7 earthquake. People living in other communities throughout southwestern Indiana would do well to follow at least the basic steps dictated by the earthquake hazard. These steps include: stocking at least a one week supply of food and water; locating rigid sites within dwellings such as door frames and under heavy tables, to which persons may move in the event of a major quake (the saying "duck and cover" means curl up under a desk or table and cover your head); strapping hot water heaters to nearby walls; removing heavy objects from overhead positions on bookshelves; and learning how to find and turn off natural gas or LPG tanks, water supplies, and electricity.

In summary, a major earthquake will cause damage in Indiana; quite possibly in your lifetime. Although fearing the inevitable is pointless, taking basic steps to prepare for an earthquake is prudent.

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