A RE-EVALUATION OF THE AURORA, ILLINOIS, EARTHQUAKE OF 1909

The largest earthquake to be located in northern Illinois is the mb ~ 5.1 Aurora earthquake of May 26, 1909. Its epicenter remains ambiguous with estimates ranging from south of Aurora to the Wisconsin border and from Du Page County to the Rock River valley. As the strongest earthquake presumed to be close to Chicago, determination of the epicenter and intensity of this event is critical for hazard assessment in northern Illinois.

Previous epicenter estimates have been constrained by anomalously high intensities reported from Platteville, Wisconsin, and Bloomington, Illinois, where structural damage was purported to have occurred to buildings, and to reports of massive chimney damage in the Aurora area.

Recent examination of more than 400 newspaper reports published in the days following the earthquake, indicate that the damage in Bloomington and Platteville was not as severe as reported, and the amount of damage to structures in the Aurora area is significantly less than reported. Many early reports of greater damage were retracted in later papers, were exaggerated, or are not supported by local newspapers. Conversely, local newspaper reports of extensive chimney damage, and changes in the local water table in Morris, Illinois, were not reported by larger news organizations.

The felt area is larger than previously reported, potentially extending from the Ohio River to northern Wisconsin and from Lansing, Michigan, to Des Moines, Iowa. The area affected by intensities of V-VI covers most of northern Illinois, parts of eastern Iowa, and southern Wisconsin. The highest intensity, VII, is assigned to Morris, Illinois. The center of the felt area, based on simple geometry, falls to the southwest of Aurora, near LaSalle, Illinois, between the Fox and Illinois Rivers.

Elevated intensities appear to follow river valleys, glacial moraines, and the shores of Lake Michigan. The distribution of felt area and intensity is extremely similar to the mb 4.2 northern Illinois earthquake of June 28, 2004, which has an instrumental epicenter also near LaSalle. This leads us to suggest that the 1909 and 2004 events occurred along the same fault and that both are associated with the Peru monocline as were other instrumental events in 1999 and 1972. This places the event into a known tectonic framework and further from the Chicago metropolitan area.