THE SEISMICITY OF SOUTHWESTERN MICHIGAN

Although Michigan is generally considered aseismic, a few earthquakes with magnitudes up to 5 have occurred in the southwestern part of the state, most recently near Scotts (M 4.2; often called the Kalamazoo earthquake) and Burlington (M 3.3) in 2015. In addition, a M 4.7 earthquake occurred near Coldwater in 1947. These events lie along geophysical anomalies that have a strike like that of the Fort Wayne Rift, which may be an extension of the Keweenawan-aged Mid-Continent Rift in central Michigan. Focal mechanism analyses of the 2015 earthquakes indicate that they both represent left-lateral strike slip faulting along a northwest striking plane, consistent with other regional events. The focal depth estimates of 4-12 km, although poorly constrained, suggest the event was likely in the Precambrian basement. Two small aftershocks were noted for the Scotts event by cross-correlation analysis; one may have been felt based on analysis of social media. This event does not show the typical indicators of induced seismicity and only a few oil wells and one low-volume injection well were operating near the area. Thus, the earthquakes appear to be of tectonic origin. Analysis of macroseismic data from the 1947 event indicates an epicenter ~18 km WNW of Coldwater, consistent with the S-P time noted at Ann Arbor. Many other reports of seismic activity have been reported from the Battle Creek area, including a widely reported event in 2021; however, it and others can be associated with sonic booms or similar events. Other earthquakes have occurred off this trend in the Lansing area in 1994 (M 3.4) and near Prairie Lake in 2001 (M 2.9). All events, like those in northern Illinois and northwest Ohio, are likely associated with NW-SE striking basement fractures, which may be regional or related to rifting. However, no events have been noted along the axis of the Mid-Continent rift in central Michigan. The stress orientation from the 2015 events, and those in adjacent areas, are WNE-WSW and consistent with the regional stress field in the northeastern and central US; the overall geometry suggests that the stress field is not due to pure deglaciation. The recent deployment of a broadband sensor near Athens, MI (NHBP) may help the detection and analysis of future earthquakes in the southern Michigan.