GLACIAL ISOSTATIC ADJUSTMENT: A PRECURSOR TO INTRAPLATE SEISMICITY IN THE MIDWEST

The enduring impact of the Laurentide ice sheet initiated 2.58 Ma includes continued lithospheric deformation in Canada and the northern United States. During these post glacial isostatic adjustments (GIA) the lithosphere under the ice sheet rebounds in response to the melting ice and the forebulge, caused by far-field flexure in response to the glacial load, collapses. This results in intense faulting, fracturing, and seismic activity associated with the deglaciation phase. It is believed that the rate of uplift is at its maximum during the initial period of deglaciation, with variations in uplift rates around the forebulge.

Our research focuses on unexplained seismic events in the Midwest, spanning ten states within the region of 39° to 48°N and 105° to 88°W. Building upon Calais et al. (2006), we explore the connection between intraplate earthquakes and GIA in North America. Previous work suggests non-uniform crustal deformation pattern, highlighting greater rigidity in the central United States and increased flexibility towards Canada and Gulf Coast. Adjustment of the lithosphere manifests as both extension due to far-field flexure and compression towards the adjustment boundaries. Intraplate seismic activity in the Midwest is hypothesized to be related to GIA caused by the Laurentide ice sheet.

To better understand the complexities of GIA and lithospheric response in the Midwest, we conducted a detailed analysis of intraplate earthquakes. We identified major fault structures prevalent in the study area and evaluated available focal mechanisms of recent and historic earthquakes to determine principal stress directions. Using this stress information, we created contoured tension (T)-axis plots to determine if earthquakes in the study area are kinematically compatible, i.e. formed under the same stress conditions. We find that most of the earthquakes with focal mechanisms are associated with northeast-southwest-directed extension. This finding supports our conclusion that the study area is undergoing extension associated with the migrating forebulge, related to the uplift and isostatic adjustment of the retreating Laurentide ice sheet. For future work we will expand our study area with the goal of deepening our understanding of the relationship between intraplate earthquakes in North America and GIA.