Reaches dominated by debris flow scour and incision tend to greatly influence landscape form in steepland basins. Debris flow networks, despite their ubiquity, have not been exploited to develop erosion rate proxies. To bridge this gap, I applied a proposed empirical function that describes the variation of valley slope with drainage area in fluvial and debris flow reaches of steepland channel networks in the Oregon Coast Range. I calibrated a relationship between profile concavity and erosion rate to map spatial patterns of long-term uplift rates assuming steady state. I also estimated the magnitude and inland extent of coseismic subsidence in my study area. My estimates agree with field measurements in the same area along the Cascadia margin, indicating that debris flow valley profiles can be used to make interpretations from spatial patterns of rock uplift that may better constrain physical models of crustal deformation. This thesis includes unpublished co-authored material.

Graduation date: 1969

This thesis examines processes causing faulting in the North American crust in the northern Cascadia subduction zone. A combination of seismological methods, including source mechanism determination, stress inversion and earthquake relocations are used to determine where earthquakes occur and what forces influence faulting. We also determine if forces that control faulting can be monitored using seismic anisotropy. Investigating the processes that contribute to faulting in the crust is important because these earthquakes pose significant hazard to the large population centres in British Columbia and Washington State. To determine where crustal earthquakes occur we apply double-difference earthquake relocation techniques to events in the Fraser River Valley, British Columbia, and the San Juan Islands, Washington. This technique is used to identify "hidden" active structures using both catalogue and waveform cross-correlation data. Results have significantly reduced uncertainty over routine catalogue locations and show lineations in areas of clustered seismicity. In the Fraser River Valley these lineations or streaks appear to be hidden structures that do not disrupt near-surface sediments; however, in the San Juan Islands the identified lineation can be related to recently mapped surface expressions of faults. To determine forces that influence faulting we investigate the orientation and sources of stress using Bayesian inversion results from focal mechanism data. More than 600 focal mechanisms from crustal earthquakes are calculated to identify the dominant style of faulting and inverted to estimate the principal stress orientations and the stress ratio. Results indicate the maximum horizontal compressive stress (SHmax) orientation changes with distance from the subduction interface, from margin-normal along the coast to margin-parallel further inland. We relate the margin-normal stress direction to subduction-related strain rates due to the locked interface between the North America and Juan de Fuca plates just west of Vancouver Island. Further from the margin the plates are coupled less strongly and the margin-parallel SHmax relates to the northward push of the Oregon Block. Active faults around the region are generally thrust faults that strike east-west and might accommodate the margin- parallel compression. Finally, we consider whether crustal anisotropy can be used as a stress monitoring tool in this region. We identify sources and variations of crustal anisotropy using shear-wave splitting analysis on local crustal earthquakes. Results show spatial variations in fast directions, with margin-parallel fast directions at most stations and margin-perpendicular fast directions at stations in the northeast of the region. To use seismic anisotropy as a stress indicator requires identifying which stations are primarily in uenced by stress. We determine the source of anisotropy at each station by comparing fast directions from shear-wave splitting results to the SHmax orientation. Most stations show agreement between these directions suggesting that anisotropy is stress-related. These stations are further analysed for temporal variations and show variation that could be associated with earthquakes (ML 3{5) and episodic tremor and slip events. The combination of earthquake relocations, source mechanisms, stress and anisotropy is unique and provides a better understanding of faulting and stress in the crust of northern Cascadia. / Graduate

In this thesis seafloor cold vents are examined using autonomous underwater vehicle (AUV) and remotely operated vehicle (ROV) data on the Northern Cascadia margin. These data were collected in a 2009 joint cruise between the Monterey Bay Aquarium Research Institute (MBARI) and Natural Resources Canada (NRCan). High- resolution bathymetry data, acoustic reflectivity (backscatter) data, and 3.5 kHz sub bottom profiler data were examined for cold-vent-related features that include pockmarks, chemosynthetic biological communities (CBC), and authigenic carbonate. Additionally subsequent ROV observations, sediments from push cores and seafloor video/photos were used to ground truth AUV data. Numerous prolific venting sites were examined in detail and a model for the evolution of venting was generated. Vents are categorized as juvenile, intermediate, or mature depending on the presence and or absence of cold-vent-features. High near-surface reflection amplitudes are coincident with an anomalous area of seafloor backscatter. In June of 2012, NEPTUNE (North East Pacific Time-series Underwater Networked Experiment) collected a near-surface push core with their ROV ROPOS (Remotely Operated Platform for Ocean Sciences) in the high reflective area. The retrieved core showed stacked turbidites in the top 0.5 meters of the sediment column. Closely spaced high-velocity turbidite sands are highly reflective and inhibit acoustic penetration to depth. The presence of high-density, high-velocity sands in the near surface is linked to steady ocean bottom currents. These bottom currents progress northeast to southwest over the study area and differentially erode the surface sediments by removing muds and leaving heavy sands over the exposed area. / Graduate / 0373 / 0374 / jonfurlong@hotmail.com

Thesis (M.S.)--Humboldt State University, 2004. / Includes bibliographical references (leaves 59-65). Also available via the Internet.

The west coast of North America is under the threat of future great megathrust earthquakes and associated tsunamis. This dissertation addresses three urgent but unresolved issues in tsunami hazard assessment and risk mitigation at northernmost Cascadia. (1) Plate subduction is actively taking place along the Explorer segment of the northern Cascadia subduction zone and probably also its Winona fragment, and therefore their seismogenic and tsunamigenic potential should be investigated. (2) It needs to be investigated whether the shallowest portion of the Cascadia megathrust can undergo highly tsunamigenic trench-breaching rupture in great earthquakes like in the 2011 Tohoku-Oki earthquake at the Japan Trench. (3) For tsunami hazard assessment and early warning in southwestern British Columbia, high-resolution megathrust rupture models need to be systematically developed. To address the first issue, I develop finite element models for the Explorer segment to estimate thermally allowed potential seismic rupture zone of the megathrust. The results suggest a potential rupture zone of ~60 km downdip width located offshore. For the Winona fragment, where there are large uncertainties in the tectonic history and the age of the oceanic lithosphere, a preliminary estimate by considering only the thermal effect of sedimentation on a cooling lithosphere suggests a potential rupture zone of a minimum downdip width of 35 km. I address the second issue by reanalyzing seismic survey images off Vancouver Island with a focus on secondary faults around the accretionary wedge deformation front. No strong evidence suggests trench-breaching megathrust rupture being a dominant mode of fault behaviour at northern Cascadia, although the possibility cannot be excluded from tsunami hazard assessment. Buried rupture and coseismic activation of secondary faults may be more important at Cascadia. To address the third issue and also to investigate how the different secondary faults can contribute to tsunami generation, I compile a new Cascadia megathrust geometry and develop 21 tsunami sources using a three-dimensional (3D) dislocation model, including hypothetical models of frontal thrust, back-thrust, and splay faults. The dislocation models indicate that the buried rupture, splay-faulting rupture, and trench-breaching rupture can result in large seafloor uplift and coastal subsidence, and hence will lead to tsunamis that seriously affect the local coastal area. Back-thrust rupture near the deformation front is unimportant for tsunami generation. The model results also show that properly configured land-based Global Navigational Satellite System (GNSS) monitoring can distinguish between ruptures along the Cascadia megathrust and along the strike-slip Nootka fault and between megathrust ruptures of difference strike lengths and therefore can effectively contribute to real-time tsunami early warning. However, the results also reveal that these land-based measurements are not sensitive to the slip behaviour of the shallow portion of the megathrust farther offshore, demonstrating urgent need for near-trench, seafloor observations. / Graduate / 0373 / gaodawei999@126.com

Thesis advisor: Alan Kafka / Thesis advisor: Seth Kruckenberg / The western United States (WUS) is an area of high seismic activity. The Juan de Fuca, Pacific, and North American plates all meet in this area, resulting in zones of subduction and strike-slip faulting, as well as other styles of faulting, all of which make it prone to frequent, as well as large magnitude earthquakes. In this study the WUS encompasses the area between 30° to 52°N and 110° to 131°W. The diverse seismicity and tectonics of the area makes the study of seismo-tectonic processes in the WUS important not only in terms of basic geoscience, but also in terms of earthquake hazards. Understanding earthquake processes in this region is critical because of the potential for devastating earthquakes to occur along the Pacific-Juan de Fuca-North American plate boundary system. Large WUS earthquakes do not, however, only occur along these plate boundaries. They can also happen in intraplate environments within the WUS. The WUS includes three distinct tectonic regions for which this study compares and contrasts characteristics of seismic activity: the Cascadia subduction region, the San Andreas strike-slip region, and a continental extension/intraplate region to the east of the major plate boundaries referred to here as the “Western Intraplate Hinterland Region”. To help make these comparisons, the method of “Cellular Seismology” (CS; Kafka, 2002, 2007), is used here to investigate similarities and differences in the extent to which past earthquakes delineate zones where future earthquakes are likely to occur in the WUS and its various tectonic sub-regions. The results of this study show that while there seems to be a “signal” of CS predictability being dependent on tectonic region, that signal is subtle in most cases, meaning that there is not a significant difference in the level of CS predictability between the regions stated here. This means we can apply CS predictability studies widely across different regions, however, it also counterintuitively suggests that tectonic understanding of a region does not necessarily elucidate how well past seismicity predicts spatial patterns of earthquakes in a region.

In this dissertation, I use seismic imaging methods to constrain the evolution of the oceanic upper mantle across the Juan de Fuca (JdF) and Gorda plates. This work begins by studying the geometry of the mantle magmatic system and patterns of mantle flow beneath the northern JdF ridge in relation to ridge-parallel changes in accretionary processes. I find that the dynamics of lithospheric rifting exert the primary control on the distribution of shallow mantle melts and variations in crustal thickness and composition. The orientation of mantle divergence beneath the JdF ridge, as inferred from seismic anisotropy, is oblique to the overlying plate divergence direction. Similar observations made at the East Pacific Rise and Mid-Atlantic ridge suggest plate motions alone do not control mantle flow patterns. On the contrary, stresses exerted at the base of the plate by the asthenospheric flow field may contribute to changes in plate motion prompting a reorientation of oceanic spreading segments. The mantle anisotropic fabric of the JdF plate interior is then investigated to identify whether the rotated mantle flow field observed beneath the JdF ridge persisted throughout the recent geologic past. However, observations suggest that the anisotropic structure created at the ridge partially reorganizes off-axis obscuring the paleo-flow geometry. Next, I focus on how the physical state of the oceanic lithosphere evolves with time. Using local earthquake arrival times I test whether the seismic velocity structure of the upper mantle lithosphere is thermally controlled or dominated by heterogeneities introduced upon accretion at the ridge or by subsequent deformation off axis. Despite extensive surficial evidence of faulting across the Gorda plate, deformation appears to be restricted to crustal depths and mantle velocities are explained by conductive cooling. In contrast, the velocity structure of the JdF plate is inconsistent with conductively-cooled mantle. Hydration of the mantle lithosphere associated with tectonic discontinuities is invoked to explain anomalously slow P-wave speeds. Lastly, a joint inversion of teleseismic body and surface wave data is proposed to image the geometry of mantle upwelling and melt production beneath the JdF and Gorda Ridges. This dissertation includes previously published and unpublished coauthored material.

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This article provides a comprehensive reference list to the extensive studies of marine natural gas hydrate surveys and studies on the northern Cascadian margin of Western Canada. The references are divided into each of the major study methods, surveys, analyses and conclusions. A number of MSc and PhD theses are included. We first refer to the articles that address the local tectonics and sedimentary accretionary prism in which the hydrate forms, then those that describe the numerous geophysical and geological surveys and studies, and finally the articles that address the most important conclusions that have resulted from this work on the distribution , concentrations, and amounts of hydrates, and on the processes of hydrate formation and dissociation.