Electronic Theses and Dissertations

Arushi Saxena

Date

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Earth Sciences

Committee Chair

Eunseo Choi

Committee Member

Christine Powell

Committee Member

Charles Langston

Committee Member

Robert Smalley, Jr.

Abstract

Earthquakes far from the plate boundaries pose a significant hazard to human life, and damage to property. Intraplate seismicity has piqued interest among many researchers to understand the risks associated with it. Several models have been proposed to explain seismicity in regions far from the influence of tectonic stresses, the nature of which depends on the depth and time scale of the geological process invoked in the model. However, these mechanisms are debatable and often require revision compatible with new observations.The Central and Eastern US (CEUS) is an ideal location for investigating intraplate seismicity. The CEUS hosts numerous seismic zones, including the largest intraplate seismic zone of the US, the New Madrid Seismic Zone (NMSZ), and has undergone extensive deformation from past episodes of the Wilson cycle. The recent deployment of a dense seismic network by EarthScope Transportable Array in the CEUS has revealed complex upper-mantle heterogeneity of unknown physical origin. The link between these current upper-mantle observations and the ongoing crustal seismicity in the CEUS is not yet understood and needs attention for a comprehensive model of seismicity within the CEUS. Additionally, the network coverage has presented an opportunity to image the structure beneath the CEUS better and answer new research questions in this region.The presented work provides new and updated models that can illuminate better than before on the factors responsible for the seismicity and the unexplained anisotropy within the CEUS, that are consistent with the seismological observations. Lithospheric layering is also constrained using receiver functions in this region. A systematic analysis of the possible origins of large negative Vp and Vs anomaly found beneath the NMSZ shows that in addition to temperature effects, an increase in orthopyroxene contents is required to explain the observed Vp and Vs magnitudes. Viscoelastic models based on all of the possible scenarios show concentration of differential stress from the upper mantle into the upper crust of the NMSZ. However, it is demonstrated that among the three factors considered---temperature, water content, and orthopyroxene content---, the temperature is most sensitive to the computed differential stress. Another model, which includes all the major CEUS seismic zones and the upper mantle extends down to 660 km depth, employs diffusion and dislocation creep with temperatures inverted from a different P-wave tomography study. Results from this model show stress concentration at all the seismic zones due to the buoyancy flow arising from the upper-mantle heterogeneity. The third work on the upper mantle within the CEUS using S-wave receiver functions shows a continuous low-velocity mid-lithospheric layer starting at depths of 60 to 100 km in the Mississippi Embayment. A detailed investigation using the synthetic seismograms points to both temperature and compositional variations to explain the origin of this layer. Lastly, a numerical model is found that incorporates an ancient mafic pluton and a low-velocity structure beneath the NMSZ, to explain the observed spiral-like Pn anisotropy.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest

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