page_title: Harrisburg, Illinois
section_title: Cottage Grove Fault System
hierarchical_section_title: Harrisburg, Illinois / Geography and cityscape / Cottage Grove Fault System
caption_attribution_description: This map of the New Madrid and Wabash Valley seismic zones shows  earthquakes as circles. Red circles indicate earthquakes that occurred from 1974 to  2002 with magnitudes larger than 2.5 located using modern instruments (University  of Memphis). Green circles denote earthquakes that occurred prior to 1974 (USGS  Professional Paper 1527). Larger earthquakes are represented by larger circles. The Cottage Grove Fault is indicated by the green line.
context_page_description: Harrisburg is a city in and the county seat of Saline County, Illinois, United States. It is located about 57 miles southwest of Evansville, Indiana and 111 mi southeast of St. Louis, Missouri. The 2010 population was 9,017, and the surrounding Harrisburg Township had a population of 10,790, including the city residents. Harrisburg is included in the Illinois–Indiana–Kentucky tri-state area and is the principal city in the Harrisburg Micropolitan Statistical Area with a combined population of 24,913.
  Located at the concurrency of U.S. Route 45, Illinois Route 13, Illinois Route 145, and Illinois Route 34, Harrisburg is known as the "Gateway to the Shawnee National Forest", and is also known for the Ohio River flood of 1937, the old Crenshaw House, the Tuttle Bottoms Monster, prohibition-era gangster Charlie Birger, and the 2012 EF4 tornado. A Cairo and Vincennes Railroad boomtown, the city was one of the leading bituminous coal mining distribution hubs of the American Midwest between 1900 and 1937.
  At its peak, Harrisburg had a population that reached 16,000 by the early 1930s. The city had one of the largest downtown districts in Southern Illinois.
context_section_description: After the 5.5 Richter Scale magnitude 1968 Illinois earthquake, scientists realized that there was a previously unknown fault under Saline County, just north of Eldorado, Illinois near Harrisburg. This fault is called the Cottage Grove Fault, a small tear in the Earth's rock running west–east, in the Southern Illinois Basin. The fault is connected to the north–south trending Wabash Valley Fault System at its eastern end. Seismographic mapping completed by geologists reveal that monoclines, anticlines, and synclines are present within the region; these signs suggest deformation during the Paleozoic era coincident to strike-slip faulting nearby.
  A focal mechanism solution of the earthquake confirmed two nodal planes both striking north–south and dipping approximately 45 degrees to the east and to the west. This faulting suggests dip-slip reverse motion, and to a horizontal east–west axis of confining stress. The rupture also occurred partially on the New Madrid Fault, responsible for the great New Madrid earthquakes in 1812, consisting of the most powerful earthquakes to hit the contiguous United States.
caption: Map showing the Cottage Grove Fault in green
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