Thesis (M.S.)--Joint Program in Oceanography/ Applied Ocean Science and Engineering, Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution, 2003. / Includes bibliographical references (p. 37-40).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1956. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 176-181).

Thesis (M.S.)--Humboldt State University, 1992. / Includes bibliographical references (leaves 59-63).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1971. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Seismic data from the intermediate-spreading Endeavour segment of the Juan de Fuca Ridge reveal several crustal-level, low-velocity, high-attenuation regions on the eastern and western ridge flanks 7 to 16 km from the neovolcanic zone. I examine Pg amplitude anomalies for a wide variety of source-receiver azimuths in the Endeavour active source seismic tomography data. I use finite difference waveform forward modeling to estimate the dimensions, depth, and seismic properties of the best-observed inferred anomalous regions. The attenuating regions extend 10-15 km beneath axis-parallel bathymetric highs and from 2 to 4 km below the seafloor. The velocity reduction is small (~8%) and the attenuation large (QP ≈ 8-40) suggesting the presence of partial melt. I infer that melt focusing toward the neovolcanic zone is incomplete and that tectonic interactions with the Heckle seamount chain and/or the large segment-bounding overlapping spreading centers may promote off-axis melt delivery at the Endeavour segment.

Using joint P-wave seismic tomography, receiver functions, and ambient noise we image the magmatic structure beneath Newberry Volcano, located near Bend, Oregon. Use of active source and teleseismic events in a joint tomographic inversion provides the ray crossings necessary to resolve a low velocity body around 4 km depth. Receiver functions show large lateral heterogeneity and are consistent with the location of a low velocity body derived from the tomography but require a larger low velocity anomaly. Ambient noise autocorrelations are used to image a low velocity reflector, located at ~3 km depth, shallower than the imaged low velocity body recovered using tomography and receiver functions. Ultimately, our results reveal a magma chamber at 3-4 km depth beneath Newberry caldera, with an overlying partially molten sill at ~3 km depth. These results show the usefulness of dense seismometer deployments over volcanoes.

We determine the crustal and upper mantle structure within the northern Canadian Cordillera using two complementary Rayleigh wave analysis techniques: ambient noise and teleseismic two-station interferometry. These methods are used to measure Rayleigh waves propagating between all available pairs of seismic stations in northwestern Canada, which are processed to obtain phase velocity dispersion curves. These curves provide information on the inter-station, path-averaged phase velocity as a function of frequency (or period). These inter-station phase velocities are then inverted to produce phase velocity maps. Phase velocity maps for periods between 8 and 80 s show to first-order high velocities within the Shield and low velocities within the Cordillera, supporting the thermal isostasy model for the region. Smaller scale features are observed throughout the Cordillera with high velocities west of the Tintina Fault reflecting the mafic composition of the accreted terranes of the Intermontane belt, and low velocities east of the Tintina fault reflecting the sedimentary rocks of the Selwyn Basin. High velocities extending west past the Cordilleran Deformation Front provide evidence for the existence of regions within the eastern Cordillera underlain by cool cratonic lithosphere. Anisotropy within the upper crust and mantle shows fast-axis orientations in line with the major faults within the region, providing evidence for a shear-zone that extends to lithospheric mantle depths. Lower crustal anisotropy shows an increase in heterogeneity of fast-axis orientations, which provides limited support for the existence of a weak shear zone and detachment within the lower crust. Results of a 1D inversion show approximate Moho depths of 35 km for the region west of the Tintina Fault, 36 km for the Mackenzie Mountains region, and 37 km for the shield. Reduced lower crust velocities observed throughout the Cordillera, provide support for the existence of a weak lower crustal layer.

Deducing the structure and origin of the Rocky Mountain Trench has proven to be a difficult problem in the past. To understand this feature more fully and to obtain information about the entire crustal section, an unreversed seismic refraction profile has been recorded in the southern Rocky Mountain Trench from 50°N to 53°N. Using blasts from two open pit coal mines, forty-four useful recordings were obtained over a distance of 540 km. Three components of short period ground motion were recorded by tape recording systems; the vertical component was also recorded by elements of the Mica array. Careful attention to amplitude scale factors results in the formation of a record section in which the energy pattern varies uniformly along the profile. A geometric ray theory interpretation involving Weichert-Herglotz integration of p-delta curves is used to obtain a velocity-depth structure. Approximate synthetic seismograms are then calculated using modified ray theory. Refractors with apparent P-wave velocities of 6.5 - 6.6 km/s and 8.22 ± 0.04 km/s are interpreted as the surface of the Precambrian basement and the Moho discontinuity, respectively. A velocity gradient is present in the lower crustal section. The depth to basement beneath the western Rocky Mountains at 50°30'N is calculated to be 6.5 ± 1 km. Near Radium, a significant anomaly in the seismic data is best interpreted as a northeasterly-trending normal fault with a downthrow of 5.6 ± 1 km to the northwest. The directions are inferred from gravity and magnetic trends in the region. Alternatively, the anomaly could represent a disappearance of the basement surface west of the east wall of the Trench. An anomalously thick crustal section is inferred from the data. A preferred model gives a depth of 51 ± 2 km southeast of Radium and 58 ± 2 km to the northwest. Study of a converted phase leads to the conclusion that there may be a discontinuity on the Moho surface beneath the Trench near 52°N. Analysis of arrivals shortly after the Pn phase is consistent with the interpretation of a low velocity zone, approximately 7 km thick, 8 km beneath the Moho. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The history of diffraction theory, exact frequency domain solutions and selected past time domain solutions are briefly reviewed. Exact time domain solutions for scattering of plane, cylindrical and spherical acoustic waves by a half plane are derived by inverse Fourier transforming the frequency domain integral solutions. The solutions consist of two diffraction terms, a reflected term and a direct term. The diffracting edge induces step function discontinuities in the direct and reflected terms at two shadow boundaries. At each boundary, the associated diffraction term reaches a maximum amplitude of half the geometrical optics term and has a signum function discontinuity, so that the total field remains continuous. A physical interpretation is developed in terms of Huygen's principle near the diffracting edge. Solutions for practical point source configurations are evaluated by numerically convolving the impulse diffraction responses with a wavelet. The numerical problems presented by convolution with a singular, truncated operator are solved by analytically derived correction techniques, which are favourably compared to those used by earlier authors. The diffraction solution collapses into a compact discretized formulation. The half plane is shown to be a limiting form of wedge solutions, which can thus be computed using similar algorithms. Two zero offset sections are produced and compared to approximate Kirchhoff integral solutions. The exact diffraction hyperbola is noticeably non-symmetric, with higher amplitudes on the reflector side of the edge. Near the apex of the hyperbola the Kirchhoff solution is nearly equivalent to the exact diffraction term symmetric in amplitude about the reflection shadow boundary but fails to describe the other, low amplitude, term equivalent to half the response of a line scatterer. The differences are more noticeable on the flanks of the hyperbola, where the two terms are comparable in amplitude, and at shallow depths, due to an aperture effect. Increasing either the depth of the edge or the angle of the seismic line to the normal to the edge results in a flatter diffraction hyperbola showing little amplitude variation with moveout. As the seismic line becomes parallel to the edge the diffraction curve becomes flat and is indistinguishable from a reflection event. At great depth diffraction events may be mistaken for reflection events as well. Examples of CDP and CSP gathers, when compared to the Common Offset (CO) gathers, demonstrate that CO gathers are optimal for diffraction processing. Also, since the diffraction moveout and reflection moveout curves differ widely except for depth points near the edge, normal moveout stacking will distort the diffractions and diffraction stacking is essential to retain diffraction information. Strips of varying width are modelled by superposition of half planes to demonstrate resolution effects and show that the limit of a strip is a line scatterer. A dipping strip and an offset half plane model are produced and added for later comparison with wedge solutions. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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