The 1968 Illinois earthquake was the largest recorded earthquake in the U.S. Midwestern state of Illinois. It measured 5. 4 on the Richter scale, affecting 23 states over an area of 580,000 sq mi. The earthquake was felt in 23 states and affected a zone of around 300 million years ago. Scientists discovered the Cottage Grove Fault in the Southern Illinois Basin. A future earthquake is extremely likely, with a 90% chance of a magnitude 6–7 tremor before 2055.
About 1968 Illinois earthquake in brief
The 1968 Illinois earthquake was the largest recorded earthquake in the U.S. Midwestern state of Illinois. It measured 5. 4 on the Richter scale, affecting 23 states over an area of 580,000 sq mi. Scientists discovered the Cottage Grove Fault in the Southern Illinois Basin. A future earthquake is extremely likely, with a 90% chance of a magnitude 6–7 tremor before 2055. The rupture also partly occurred on the New Madrid Fault, responsible for the great New Madrid earthquakes in 1812. The first recorded Illinois earthquake is from 1795, when a small earthquake shook the frontier settlement of Kaskaskia. The earthquake was felt in 23 states and affected a zone of around 300 million years ago. It was mainly active in the Late Pennsylvanian and Early Permian epochs. The quake’s epicenter was slightly northwest of Broughton in Hamilton County, and close to the Illinois–Indiana border, about 120 miles east of St. Louis, Missouri. It occurred at a depth of 25 km. A fault plane solution for the earthquake confirmed two nodal planes striking north–south and dipping about 45° to the east and to the west.
This faulting suggests dip slip reverse motion and a horizontal east–west axis of confining stress. In 2005, seismologists and geologists estimated a 90 per cent chance of such a tremor in the region by 2055, likely originating in the Wabash Valley seismic zone on the Illinois-Indiana border. Since 1968, other earthquakes have occurred in the same region in 1972, 1974, 1984, and 2008. The event caused considerable structural damage to buildings, including the toppling of chimneys and shaking in Chicago, the region’s largest city. Although no fatalities occurred, the event caused significant damage to Chicago. Some people near the epicenter did not react to the shaking, while others panicked. Various theories were put forward for the cause of the rupture. Scientists eventually realized, though, that the cause was a then-unknown fault, the C cottage Grove Fault, a small tear in the Earth rock near the city of Harrisburg, Illinois. At the time of the earthquake, no faults were known in the immediate epicentral region.
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