REEVALUATION OF THE JANUARY 2, 1912 NORTHERN ILLINOIS EARTHQUAKE: USING GIS AND COMPREHENSIVE MICROFILM ANALYSIS TO ASSESS FELT AREA, INTENSITY, AND EPICENTER

The January 2, 1912 Aurora, IL earthquake is one of the largest earthquakes (~ 4.7 m_b) to occur in northern Illinois in the past 100 years. It occurred in close proximity to the largest northern Illinois earthquake (May 26, 1909; ~ 5.1 m_b), and was widely felt in northern Illinois and several locations in Indiana, Iowa, Michigan, and Wisconsin. A 1912 study places the epicenter between Aurora and Morris IL, assigns a maximum Rossi-Forel Intensity of nearly 7 (MMI = VI), and estimates a total felt area of 40,000 mi² (103,600 km²). No instrumental records of this earthquake are known to survive, and the original 1912 study serves as the basis for the reported location and intensity of this event in the modern seismicity literature.

As a follow-up to studying the 1909 event, we have reexamined previous location, total felt area, and intensity estimates for the Jan. 2, 1912 earthquake. More than 300 newspapers from 124 locations, published in the days following the earthquake, were collected and evaluated. These indicate that some damage reports are not as severe as previously interpreted (e.g. Aurora and Dixon, IL), and some wire reports of damage were not supported by local newspapers (e.g. Geneva, IL). The total felt area is larger than previously reported, extending from southern Wisconsin and eastern Iowa to outliers in Fort Wayne, Indiana and Monroe, Illinois (near St. Louis, MO). The area constrained by the MMI > V isoseismal is also larger than previous estimates and includes Fort Wayne, southern WI, and eastern and central IL. Peak intensity (MMI = VI) is assigned to Morris, IL. The intensity distribution pattern is similar to other earthquakes in the area, with higher intensities to the north and west.

Geographic Information Systems (GIS) provides an excellent means of documenting data collected for this earthquake, and provides a tool for comparisons to known bedrock structures, instrumentally recorded earthquakes, and other historical earthquakes in the region. We use GIS to estimate a total felt and to reevaluate the epicenter of this event. Results are preliminary and research is ongoing, however we calculate a total felt area of approximately 181,300 km² (70,000 mi²). Felt reports from Fort Wayne and other outlying areas place the epicenter further south and east than previous estimates, and near the Sandwich Fault Zone.