Abstract

A small earthquake, magnitude 3.5, struck northern Illinois at 16:17 UTC (11:17 am local) on 2 September 1999. The epicenter was located about 130 km west of Chicago at latitude 4l.72°N and longitude 89.43°W, with an estimated focal depth of 5 km. The earthquake was felt within an area of about 6,500 km2, including parts of eleven counties in Illinois and adjacent parts of southern Wisconsin and eastern Iowa. Within two weeks of the event, two separate intensity surveys were initiated to investigate the effects of this earthquake. The surveys showed that Modified Mercalli Intensity reached a maximum of V.

Although small in magnitude, this earthquake is significant because it is the second instrumentally documented earthquake located on, or very near, the Peru monocline. In 1972, a magnitude 4.5 earthquake occurred about 16 km southeast of the 1999 earthquake. A focal mechanism solution from the 1972 earthquake is consistent with strike-slip movement parallel to the trend of the Peru monocline. A third small earthquake occurred about 40 km to the southeast in 1881. The size and location of this earlier event are poorly constrained, but it appears to have originated along the same structural trend as the more recent events. Although one earthquake epicenter near the Peru monocline might be considered coincidental, taken together, these three earthquakes may suggest the possible reactivation of a Paleozoic structure within the North American midcontinent.

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