Exploring Intraplate Seismicity in the Midwest

Authors

Alexa Fernández, University of Nebraska-LincolnFollow

First Advisor

Caroline M. Burberry

Second Advisor

Mindi L. Searls

Committee Members

Harmon Maher

Date of this Version

8-2024

Document Type

Thesis

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Earth and Atmospheric Sciences

Under the supervision of Professors Caroline M. Burberry and Mindi L. Searls

Lincoln, Nebraska, August 2024

Comments

Copyright 2024, Alexa Fernández. Used by permission

Abstract

Intraplate seismicity represents a notable occurrence within the stable North American Craton. This research explores the potential sources of stresses that could reactivate older faults and influence seismic activity within this region. Among these sources, the enduring impact of the last glacial period is considered, which includes continued glacial isostatic adjustments (GIA). During GIA the lithosphere rebounds due to the retreating ice, and the forebulge caused by far-field flexure in response to the glacial load, collapses. This results in significant faulting, fracturing, and seismic activity associated with the deglaciation phase. The adjustment of the lithosphere manifests as both near surface extension due to far-field flexure and compression towards the adjustment boundaries.

Our research focuses on seismic events in the Midwest and the New Madrid Seismic Zone, where the connection between intraplate earthquakes and GIA is explored. Intraplate seismic activity in the North American craton is hypothesized to be related to GIA caused by the end of the Last Glacial Maximum (LGM). However, other stress sources influencing the deformation of the lithosphere can be attributed to the relaxation of the Appalachian Mountains.

To better understand the complexities of GIA and lithospheric response in North America, a detailed analysis of intraplate earthquakes was conducted. Major fault structures present in the study area were identified, and focal mechanisms of earthquakes were evaluated to determine principal stress directions. Using this stress information, contoured tension (T)-axis plots were created to determine whether the faults in the study area were reactivated under the same stress conditions. T-plots west of the Midcontinent Rift show a NE to SW extension attributed to GIA, while T-plots east of the MCR show an NNW to SSE extension consistent with the combined effects of GIA and orogenic collapse of the Appalachians. Additionally, our results show that the Reelfoot Rift is not actively rifting. Our findings contribute to the understanding of intraplate seismicity in this region and the broader implications of GIA.

Advisors: Caroline M. Burberry and Mindi L. Searls