Graduate Thesis Or Dissertation
 

Subducting Seamounts and Heceta Bank: An Amphibious Seismic Tomography Experiment of the Central Oregon Forearc

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w3763g401

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  • Studies reveal differences in slip segmentation and plate coupling along the Cascadia subduction zone. The segment between 44.0°N and 45.0°N exhibited reduced slip in the 1700 megathrust earthquake (Wang et al., 2013) and corresponds with previous rupture boundaries inferred from paleoseismic data (Leonard et al., 2010). Notably, this segment of the forearc has a lower slip deficit in multiple locking models (Schmalzle et al., 2014; Lindsey et al., 2021) and has hosted small and medium-sized earthquakes over the last two decades (Williams et al., 2011; Morton et al., 2018; Stone et al., 2018). Structures in the downgoing and overriding plates have been proposed as mechanisms for the margin-wide segmentation patterns. Examination of potential field data and recorded earthquakes suggest that this region may contain multiple subducting seamounts. It is agreed that seamount subduction plays a critical role in subduction zone dynamics; however, it is not clear whether seamounts nucleate large earthquakes or if they prevent or arrest megathrust ruptures. In this study, we use controlled-source amphibious seismic data from four experiments to explore the P-wave velocity structure of the Oregon forearc between 43.6°N and 45.2°N. We invert travel times for refracted seismic waves to construct a high-resolution model of the subsurface. This model is used to identify structural heterogeneities in this region and identify topography on top of the subducting slab. We identify a total of five subducting seamounts within this region of the forearc including two that were not previously identified based on gravity or magnetic data. We suggest that three of these seamounts are causing uplift within the accretionary wedge as they subduct. Additionally, we find a seamount located on the seaward edge of Siletzia that we propose is responsible for the southern earthquake cluster in this region. We hypothesize that a cluster of subducting seamounts will have a different impact on plate coupling than a single seamount. This rough portion of the subducting Juan de Fuca plate may be responsible for the decrease in plate coupling observed in this segment of the margin.
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