A Reevaluation of the Intensities of the Northern Illinois Earthquake of 1909

The largest earthquake in northern Illinois is the m b ∼5.2 so‐called “Aurora” earthquake of 26 May 1909. Reports from nearly 500 newspapers yielded intensities for about 400 localities published in the days following the earthquake and indicate that many damage reports (especially in Bloomington, Kenosha, Platteville, and Aurora) were not as severe as previously interpreted. Some early wire reports of damage were exaggerated and later retracted, or were not supported by local newspapers. Conversely, some local accounts of extensive damage (e.g., at Morris, Illinois) were not widely reported by syndicated news media. The felt area extends further than that of previous studies—from the Ohio River to northern Wisconsin, and from Lansing, Michigan, to near Cedar Rapids, Iowa. The felt area centers near Sandwich, Illinois; the highest intensity, VII, is assigned to Morris, Illinois. The event was felt over an area of 350,000–400,000  km2 within a maximum radius of approximately 420 km. The distribution of felt area and intensity is similar to the m b  4.2 northern Illinois earthquake (28 June 2004), which has an instrumental epicenter near LaSalle, suggesting that the two events occurred in relatively close proximity. Our preferred epicenter for the 1909 event is near Sandwich, Illinois, based on intensity reports, and is located near the Sandwich fault, as is a potential small aftershock. This places the 1909 event into a tectonic framework and slightly further from the Chicago metropolitan area than previously proposed. Online Material: Tables of intensity reports, a list of newspapers from which intensity reports were extracted, and details of analog records and bulletins.