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Drolet, Doriane

University of British Columbia

Large earthquakes often dominate attention due to their significant impact, but the majority of seismic activity consists of smaller and more frequent earthquakes. These smaller earthquakes, when properly analyzed, can produce enormous datasets and greatly enhance ourunderstanding of Earth’s dynamic processes. Through three research projects, this thesis focuses on developing and applying methods to analyze small earthquakes (M < 4) using the relative moment tensor inversion, addressing the challenges of small seismic sources. The first project examines two significant M7.1 intraslab earthquakes in southern Alaska: the 2016 Iniskin (24/01/2016) and the 2018 Anchorage (30/11/2018) earthquakes. The study provides insights into rupture processes and stress evolution by relocating aftershocks, recovering moment tensors, and inverting for stress variation. The aftershock distribution of the Iniskin event suggests a rupture along a steeply dipping fault in the Pacific Plate, with evidence of pre-existing fault reactivation (likely an outer-rise fault) and fluid migration. The Anchorage sequence reveals complex aftershock patterns, likely due to simultaneous rupture on both nodal planes. The second project tackles the challenges of determining moment tensors for small earthquakes recorded by sparse regional networks. Building upon previous work, a simultaneous multiple-event relative moment tensor inversion scheme is developed, leveraging constraints from P-wave polarities, relative P- and S-wave amplitudes, and local magnitude estimates without requiring prior knowledge of a reference moment tensor. This method is validated with synthetic examples and applied to a seismic cluster near San Juan Island, Washington, revealing insights into the local stress regime and fluid migration from the subducting Juan de Fuca Plate. The final project focuses on the Haida Gwaii incipient subduction zone in Canada. The study uses the novel relative moment tensor inversion technique to analyze earthquake groups in this sparsely instrumented area. Moment tensor results for events near the expected Queen Charlotte Fault and thrust zone display reverse mechanisms due to underthrusting of the Pacific Plate. Overall, this thesis advances the understanding of seismic source processes and stress distributions across different tectonic settings, offering valuable insights into fault mechanics, role of geologic structure, fluid presence and regional seismotectonics through innovative inversion methodologies.

Text

2025-01-03

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/90032

Geophysics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/