The seismicity at the northern section of the Gorda Ridge has been studied with Ocean Bottom Seismographs. Nearly one hundred earthquakes were located with RMS travel time residuals less than 0.5 seconds. Most of these events lie within or near the array and they are probably associated with the dynamics of crustal formation which creates the observed bench-like features in the Gorda Ridge. An average of 5 microtremors per hour can be detected in the median valley with a lower activity outside of the ridge. Considerable clustering in time can be seen and it is typical of a main-shock sequence with the largest shock at or very near the beginning followed by a progressive decay in number. Clustering was spatial as well as temporal and the records indicate that they have similar mechanisms. A brittle zone of 18 km thickness at 42°N and 13 km at 42.5°N was found implying any possible magma chamber must be at greater depth. These thicknesses are larger than those observed in the Mid Atlantic ridge and in the East Pacific Rise. They may be explained by the depression of isotherms due to the heat lost by the contact of the ridge with the old and cold plates across the Blanco and the Mendocino Fracture Zones. Comparison of earthquakes locations using only OBS with those obtained from land stations indicate a major P-delay for the Gorda Basin. The seismic activity appears to decrease markedly to the south of 42°N. This drop in activity indicates that the southern part of the ridge is more stable than the northern part. This decrease in activity is not consistent with a southward propagating rift if we accept that higher seismicity levels are expected at the tip of the propagating ridge segments. / Graduation date: 1982

In this thesis anomalous downcore shallowing of paleomagnetic inclinations is interpreted to be caused by sediment compaction. Thus, compaction-induced inclination shallowing may influence tectonic reconstructions that are based on inclinations from deep-sea sediment cores. Progressive downcore shallowing of the remanent inclination was observed in a 120-m section of Plio- Pleistocene sediments at Deep Sea Drilling Project (DSDP) site 578 in the northwest Pacific. Near the top of the section the average inclination corresponds to the expected geocentric axial dipole value of 53° but shallows downcore by about 6° to 8°. In sediments spanning the same time interval of neighboring site 576, no inclination shallowing was observed. This second site has considerably lower sedimentation rates, and the Plio- Pleistocene is represented by a 26-m sedimentary section. The inclination shallowing at site 578 was correlated to a downhole decrease in porosity, and these results are interpreted to suggest that both the downhole inclination shallowing and decrease of porosity in site 578 were caused by sediment compaction. Microscopic models demonstrate that sediment compaction may lead to inclination shallowing of the magnetic remanence. Furthermore, it is shown that inherent initial within-sample dispersion of the magnetic moments will transform any form of microscopic mechanism to an equation of a standardized form: tan (I-ΔI) = (1-a ΔV) tan I, where I is the inclination of the ambient field, ΔI is the inclination shallowing, a is a constant and ΔV the compaction. Paleomagnetic inclinations of Cretaceous DSDP sediments from the Pacific plate are known to be systematically shallower than predicted from paleolatitudes of hot spot reconstructions. Published paleomagnetic data were reexamined and the shallow Cretaceous inclinations explained as a result of sediment compaction. The Cretaceous data are used to estimate the parameter a. The resulting a values are comparable to those of previous studies of compaction-induced inclination shallowing, both from laboratory experiments and the considerably younger deep-sea sediments at site 578. Values of the parameter a suggest that it might be controlled by sediment lithology with greater shallowing for clayey than calcareous sediments. / Graduation date: 1991

Graduation date: 1987 / Best scan available for figures.

Management strategies to prevent overfishing while achieving optimum yield vary according to the available data and life history of the fished stock. I evaluated two sets of management strategies for Pacific coast rockfish: strategies to set harvest limits for data-poor stocks, and strategies intended to protect the age structure of fished stocks. Setting Harvest Limits for Data-poor Stocks - The collapse of canary rockfish, Sebastes pinniger, in the northeast Pacific began more than two decades before the stock was officially declared overfished. The 2006 reauthorization of the Magnuson-Stevens Fishery Conservation and Management Act requires a scientifically-based harvest limit for all fished stocks, including those with data limited to catch. Two such "data-poor" methods are currently in use for the management of west coast stocks, depletion-corrected average catch (DCAC) and depletion-based stock reduction analysis (DB-SRA). To evaluate the performance of each method when challenged with catch and biological uncertainty, I retrospectively applied the methods to the catch and biological data available at the time of the first and second canary rockfish stock assessments in 1984 and 1990. In 1980 canary rockfish would be classified as "data-poor", and in 1990 as "data-rich". To evaluate the sensitivity of DCAC and DB-SRA to error in the catch data, harvest limits were estimated using both the historic catch data from each assessment, and the reconstructed catch data from the most recent stock assessment. In addition, harvest limits were estimated using simulated catch data sets for the years 1916 to 1983 with increasing variability around the true catch. DCAC and DB-SRA estimated harvest limits were significantly lower than the catch recommended in both the data-poor and data-rich stock assessments, but higher than the "true" overfishing limit. Use of current catch data improved the estimated harvest limit when the stock was data-poor, but not when the stock was data-rich. The simple methods responded to increasing error in the catch time series with decreasing mean estimates of the harvest limit, indicating that these methods are highly precautionary for this species, when the catch time series is the only source of error. Age Structure Management Strategies - In a variable oceanographic environment, a population with many reproductive age classes benefits not only from the increased fecundity of older fish; but also, in some species, an increase in larval fitness. Older females may also spawn at different times or over longer periods than younger females, increasing the probability of larvae encountering favorable environmental conditions. Despite the accumulating evidence for the importance of age structure to long-term population viability in harvested fish populations, long-lived west coast rockfish (Genus Sebastes) are managed with a biomass-based harvest control rule. I compared three strategies for age structure management, and evaluated the strategies relative to the status quo, biomass-based harvest control rules, across three rockfish life histories. I examined the tradeoff between yield and traditional management reference points, as well as performance measures that could serve as management reference points for age structure. Yield was reduced by strategies that maintain "old growth" age structure, but annual variation in the catch and the probability of becoming overfished were also reduced. The longest-lived rockfish benefited the most from strategies that maintained older fish in the population through dome-shaped selectivity. The shorter-lived rockfish benefited from adjustments in the catch limit based on the age composition of the catch one year previous. Achieving "pretty good yield" with management strategies that also decrease the potential for overexploitation is an important goal for stocks that are well-studied and those that are poorly understood; these investigations contribute to a growing literature on alternative approaches to sustainable fisheries management. / Graduation date: 2012

Thesis (M.S. in Meteorology)--Naval Postgraduate School, March 2004. / Thesis advisor(s): Patrick A. Harr, Russell L. Elsberry. Includes bibliographical references (p. 117-118). Also available online.

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The time-averaged velocity field in the North Pacific was estimated in two sets of inverse calculations. The planetary geostrophic equations were the basis for dynamical models of the flow in each case. The inverse estimates of the circulation were obtained by minimizing a positive-definite cost function, which measured the inconsistency of the model's predictions against a set of observations comprised of a large, high-quality hydrographic data set, and surface fluxes of heat, fresh water, and momentum. In the first part of this work, four solution methods for the generalized inverse of a linear planetary geostrophic model of the North Pacific are compared. A conjugate gradient solver applied to the equation for the generalized inverse, expressed in terms of a representer expansion, was the most computationally efficient solution method. The other methods, in order of decreasing efficiency, were, a conjugate gradient descent solver (preconditioned with the inverse of the model operators), a direct solver for the representer coefficients, and a second conjugate gradient descent solver (preconditioned so that the diagonal elements of the cost Redacted for Privacy function Hessian were unity). All but the last method were successful at minimizing the penalty function. Inverse estimates of the circulation based on the linear planetary geostrophic model were stable to perturbations in the data, and insensitive to assumptions regarding the model forcing and boundary condition uncertainties. A large calculation, which involved approximately 18,000 observations and 60,000 state variables, indicated that the linear model is remarkably consistent with the observations. The second part of this work describes an attempt to use a nonlinear planetary geostrophic model (which included realistic bottom topography, lateral momentum mixing, out-cropping layers, and air-sea fluxes of heat, freshwater, and momentum) to assimilate the same hydrographic data set as above. Because of the nonlinearity in the model, descent methods (rather than a representer-based method) were used to solve the inverse problem. The nonlinearity of the model and the poor conditioning of the cost function Hessian confounded the minimization process. A solver for the tangent-linearization of the planetary geostrophic system should be used as a preconditioner if calculations of this type are attempted in the future. / Graduation date: 1996

During the North Pacific cruise of the R/V Yaquina in 1966, total magnetic field intensity was measured in the Andreanof group of the Aleutian Islands. Three north-south track lines were made across the Aleutian Trench and ridge between longitudes 175° W and 180°. Three small scale magnetic profiles across the trench and ridge and one large scale profile over the crest of the ridge were constructed from the data. Total field and anomaly contour maps were drawn from the profiles. The data reveal: 1. A strong east-west trend of the contours present south of the ridge but absent north of the ridge which suggests that the Aleutian ridge is the boundary of two different magnetic provinces. 2. Large anomalies south of the trench which appear to be continuous for a distance of about 500 miles. The anomalies result from shallow structures and are considered to be ocean floor magnetic lineations. 3. Large anomalies north of the trench which result from deep-seated structures, probably strongly magnetic intrusions. 4. Short wavelength anomalies on the crest of the ridge which may result from dike intrusions parallel to the trend of the ridge. 5. A local magnetic gradient of about 800 gammas after removal of the regional geomagnetic field. The residual gradient is "low" over the trench and "high" over the ridge. The magnetic variation may be related to changes in the depth of the Curie temperature isotherm. / Graduation date: 1968