Carpenter, Philip J.
M.S. (Master of Science)
Department of Earth, Atmosphere and Environment
The upper midwestern United States is located within the North American Craton. This area has not undergone major deformation and metamorphism since the latest rifting events in the late Precambrian to early Cambrian (0.55 to 0.53 Ga). However, this area is not completely absent of seismicity, and major structural geologic features are present, including the Wisconsin Arch, LaSalle Deformation Belt, northern part of the Illinois basin and, deeper, the presumed transition between the Yavapai and Mazatzal tectonic provinces. Since the late 1800s, approximately 30 felt earthquakes of magnitude 2 or greater have occurred within the study area. It is likely many more unfelt microearthquakes have occurred. In this study, I use receiver functions to investigate seismic discontinuities beneath the upper midwestern United States. Data from the USArray, a continental-scale seismic observatory, is primarily used in this study. The USArray is comprised of the transportable array, the flexible array, the reference network, and the magnetotelluric array. From 2010 to 2015, the transportable array was stationed in the midwestern United States, providing dense receiver coverage throughout the study area. The goal of the USArray is to collect sufficient seismic data for studies of subsurface structure. The structure of the lower crust and upper mantle was probed using receiver functions calculated from teleseismic arrivals recorded throughout the array. The events used in this study were magnitude 5.75 or greater and between distances of 25°and 100° away from the seismic stations. Angle distances are typically used for teleseisms (earthquakes at long distances). Receiver functions are calculated to isolate the P-to-S wave conversions from discontinuities beneath the stations. The receiver functions are migrated with the best estimate of the seismic velocities to determine the depth to the Mohorovičić (Moho) and other major discontinuities. These deep (0 to 60 km) investigations provide a glimpse of the structures, processes, and conditions beneath the study area. Information derived from these studies can be used to understand the effects of large tectonic processes at depth, identify transition zones between tectonic provinces, and better understand the complex seismicity patterns that occur with mid-continental earthquakes. The Moho discontinuity is estimated to be at depths ranging from 40 to 50 km in the study area. A possible sub-Moho anomaly was observed in the southeastern part of the study area. Several low velocity zones in the upper crust have also been identified beneath Southern Wisconsin and Northern Illinois.
Guajardo, Anthony, "Seismic Imaging of The Lithospheric Structure Beneath The Upper Midwestern United States" (2021). Graduate Research Theses & Dissertations. 7083.
Northern Illinois University
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