1968 Illinois earthquake
Isoseismal map for the event (I–III are Not felt to Weak, IV is Light, V is Moderate, VI is Strong, VII is Very strong)
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| Date | November 9, 1968 |
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| Origin time | 17:01:40.5 UTC |
| Magnitude | 5.4 Mw |
| Depth | 25 km (16 mi) |
| Epicenter | 37°57′N 88°29′W / 37.95°N 88.48°WCoordinates: 37°57′N 88°29′W / 37.95°N 88.48°W |
| Type | Dip-slip |
| Areas affected | Midwestern United States |
| Max. intensity | VII (Very strong) |
| Casualties | None |
The 1968 Illinois earthquake (a "New Madrid event") was the largest recorded earthquake in the U.S. Midwestern state of Illinois. Striking at 11:02 a.m. on November 9, it measured 5.4 on the Richter scale. Although there were no fatalities, the event caused considerable structural damage to buildings, including the toppling of chimneys and shaking in Chicago, the region's largest city. The earthquake was one of the most widely felt in U.S. history, affecting 23 states over an area of 580,000 square miles (1,500,000 km2). In studying its cause, scientists discovered the Cottage Grove Fault in the Southern Illinois Basin.
Within the region, millions felt the rupture. Reactions to the earthquake varied: some people near the epicenter did not react to the shaking, while others panicked. A future earthquake in the region is extremely likely; in 2005, seismologists and geologists estimated a 90% chance of a magnitude 6–7 tremor before 2055, likely originating in the Wabash Valley seismic zone on the Illinois–Indiana border or the New Madrid fault zone.
The first recorded Illinois earthquake is from 1795, when a small earthquake shook the frontier settlement of Kaskaskia. Data from large earthquakes—in May and July 1909, and November 1968—suggest that earthquakes in the area are of moderate magnitude but can be felt over a large geographical area, largely because of the lack of fault lines. The May 1909 Aurora earthquake affected people in an area of 500,000 square miles (1,300,000 km2); the 1968 Illinois earthquake was felt by those living in an area of about 580,000 square miles (1,500,000 km2). Contradicting the idea that the region's earthquakes are felt over a wide area, a 1965 shock was only noticed near Tamms, even though it had the same intensity level (VII) as those of 1909 and 1968. Before 1968, earthquakes had been recorded in 1838, 1857, 1876, 1881, 1882, 1883, 1887, 1891, 1903, 1905, 1912, 1917, 1922, 1934, 1939, 1947, 1953, 1955, and 1958. Since 1968, other earthquakes have occurred in the same region in 1972, 1974, 1984, and 2008.
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