SEISMICITY OF NORTHERN ILLINOIS: INSIGHTS FROM RECENT EARTHQUAKES AND A NEW REVIEW OF HISTORICAL SOURCES

Northern Illinois has experienced between 20 and 25 small earthquakes during the past 120 years. Most of these shocks were pre-instrumental and considerable uncertainty exists regarding their location and magnitude. Careful examination of over 1100 newspaper articles and historical accounts dating from the early 1800s, combined with modern instrumentally-determined epicenters and shaking patterns, clarifies the earthquake record and improves locations of pre-instrumental shocks. Historical analysis reveals several source areas with recurrent seismicity, in most cases not associated with major mapped faults. For example, several shocks over the past 130 years have occurred in Lee, LaSalle and Ogle Counties, a complex zone where the Peru monocline, Wisconsin arch, Plum River and the Sandwich fault zones converge, perhaps representing slip on basement faults below the Peru monocline. Other source areas appear near Rock Island (1934-35), the southwest Chicago suburbs near McCook (1977, 1985, 1997, 2010), and in Kane Co., northeastern Illinois. The McCook earthquakes appear to be very shallow quarry-induced events. Kane Co. shocks felt along the Fox Valley in 1944 and 47 may have occurred in the nearby source area of the magnitude 3.8 Feb. 10, 2010, Lily Lake earthquake. Basement faulting was previously proposed in this area based on gravity anomalies. A 1980s seismic reflection section that passed very close to the Lily Lake epicenter shows no faulting within Paleozoic rocks; reprocessing, however, may reveal faulting in the Precambrian basement. More recent aero-magnetic maps show a lineament extending from northern DeKalb to central DuPage Co., passing directly through the Lily Lake epicenter. The epicenter for the strongest earthquake originating in northern Illinois, the May 26, 1909 magnitude 5.1 shock, still remains elusive, with possible locations ranging from the Wisconsin border to the Sandwich fault zone. The epicenter might also have been near the Peru monocline, in the vicinity of the 1972, 1999 and 2004 shocks, as noted by other investigators. Broadband seismograms of recent earthquakes show considerable complexity, including converted waves and possible reflections from crustal layers, which should be investigated further.