Earth and Atmospheric Sciences, Department of


First Advisor

Dr. Irina Filina

Second Advisor

Dr. Cara Burberry

Third Advisor

Dr. Mindi Searls

Date of this Version



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 Professor Irina Filina. Lincoln, Nebraska: July, 2021

Copyright © 2021 Asif Ashraf


The Cascadia subduction zone (CSZ) has a high potential for an inevitable and devastating megathrust earthquake. This margin is characterized by a complex seismicity pattern. Particularly in Oregon, there is a seismically quiescent zone bounded by high seismicity regions to the north and south. To comprehend these variations in seismicity, it is important to study the differences in crustal architectures and physical properties (densities and magnetic susceptibilities) along the CSZ. The primary objectives are to develop two plate-scale 2D integrated models through different seismicity zones and to map major tectonic structures from filtered potential fields. The Juan de Fuca oceanic crust requires a number of lower density zones with respect to adjacent oceanic crust to fit gravity data. These zones correlate to previously identified propagator wakes that are formed during spreading ridge propagation and mapped from disturbances of seafloor magnetic stripes. However, this correlation disagrees with a previous study that relates propagator wakes to denser oceanic crust. To resolve this contention, two gravity models have been developed along the same modeling lines of the previous study which show propagator wakes correlate with lower density zones. Newly mapped tectonic features termed pseudofault lineaments were traced from filtered magnetic data. These structures appear to be triggered by offsets between spreading ridge segments; they correspond to modeled lower density zones. Seamounts mapped from filtered gravity data appear to be clustered around identified pseudofault lineaments and propagator wakes, suggesting that those act as conduits for magma. This indicates that pseudofault lineaments and propagator wakes represent zones of weakened oceanic crust. Because of the oblique subduction, most of these identified features are subducting beneath Washington, resulting in greater seismicity. Furthermore, the pattern of earthquakes within the Wadati-Benioff zone is aligned with interpreted zones of crustal weakness.

Advisor: Irina Filina