Earth and Atmospheric Sciences, Department of


First Advisor

Irina Filina

Date of this Version

Summer 7-27-2021


K. Al Farsi, 2021


An Undergraduate Senior Thesis, Bachelor of Science Degree in Geology, University of Nebraska-Lincoln. 2021

Copyright © 2021 Khawlh Al Farsi,


An inevitable megathrust earthquake is expected in the Cascadia subduction zone that will affect the population of the coast of southwestern British Columbia and the northwest of the United States. In this active tectonic margin, the Juan de Fuca oceanic plate is subducting beneath the North American continental plate, causing unevenly distributed seismic activity. The major goal of this geophysical research project is to study the tectonic structures of the Cascadia subduction zone in order to comprehend the geology of the region and investigate the seismic hazards. The primary objective of this project is to develop a geophysical database of published seismic refraction and reflection surveys over the Juan de Fuca plate and the Cascadia subduction zone. The resultant seismic reflection database consists of eight publicly available surveys that were acquired between 1964 and 2017. The total length of seismic reflection data covered by this project is ~13,250 km. Interpreting tectonic features over the Cascadia subduction zone using seismic reflections was challenged by the poor quality of vintage seismic images and by the lack of both vertical and horizontal scale markers. Despite that, seismic reflections allowed to interpret some shallow subsurface structures, although most of the old images did not map the depth to the Moho boundary. Two publicly available seismic refraction surveys were also included in the database. These refractions surveys consist of two transects onshore and offshore in the states of Washington and Oregon, resulting in two-dimensional seismic velocity cross-sections. Compared to reflections, seismic refractions allowed for the interpretation of several deeper and larger tectonic structures, including the Moho boundary both in the continental and in the oceanic domains. The second objective of this project focuses on developing an integrated two-dimensional geophysical model that utilized the thickness of various tectonic elements derived from seismic refraction and reflection data to model the free-air gravity anomaly. The model is 640 km long ranging from the Juan de Fuca spreading center on the west to onshore northern Oregon on the east. The model allowed to summarize the tectonic features of the entire study area, including several low-density zones in the oceanic subducting slab that were required in order to fit the observed free-air gravity anomaly.

Faculty Mentor: Irina Filina,