Graduate Thesis Or Dissertation

Regional and Site-Specific Vulnerabilities of the Western Power Grid from a M9.3 Cascadia Subduction Zone Earthquake

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  • The likelihood of a Cascadia Subduction Zone (CSZ) earthquake is estimated between 37 to 42% in the next 50 years, leading to strong shaking, liquefaction, landsliding, and other seismic ground failure resulting in major impacts to critical lifelines such as the western power grid. Electrical power is essential for continued functionality of emergency services, economic recovery, and other basic, essential needs such as water and fuel supply, wastewater treatment, and communications. Hence, understanding the extent of damage to the power grid from a CSZ event is critical. The objective of this research, part of a broader study identifying the seismic resilience of the western power grid, is to assess the vulnerabilities of the synthetic western power grid by quantifying the likelihood and magnitude of seismically induced landslide occurrence to determine the effects of those displacements on electrical transmission poles and towers at a regional scale. To this end, this thesis presents a probabilistic method for a regional seismic landslide hazard analysis and map for the Western United States based on the USGS M9.3 Megathrust CSZ scenario earthquake with consideration of topographic, geologic, and other geospatial information. The landslide triggering analysis completed uses several empirical seismic displacement prediction models based on the Newmark sliding block method, which are calibrated using strength parameters for each geological unit based on the terrain slope at locations of previously mapped landslides within the unit. A predictive displacement regression model, LOS21 was developed using a logic tree scheme that weights the individual models based on the suitability of the model to this regional assessment. The LOS21 model was used to calculate the probability of exceedance of specific thresholds (e.g., 5%, 15% and 50%) to evaluate potential impacts to the power grid. Electrical infrastructure located west of the Cascades in Washington, Oregon, and Northern California were determined to be subjected to the highest risk of landslide-induced damage. To provide context to the broader study focusing on non-landslide-induced impacts to the western power grid (e.g., ground shaking and inertial equipment loading), an evaluation site was characterized for a detailed site-specific site response analyses to evaluate the differences in amplification between equivalent linear and nonlinear, total stress analyses using ten ground motions pairs scaled and matched to the USGS seismic scenario hazard. The results of the site-specific site response analysis are used to evaluate the potential impact to the electrical components for the substation at the evaluation site for comparison to the seismic hazard developed using the regional map. The equivalent linear and the nonlinear approach produced PGA amplification results that were lower from the USGS seismic scenario hazard by 6% and 32%, respectively. The maps created are suitable for regional resilience and planning and to guide geotechnical investigations but should not be used in place of site-specific analysis for engineering design purposes.
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