A reduced gravity model that incorporates the geometry of western North America has been used to study the dynamics of the California Current System (CCS). Three experiments were implemented: first the model was run using 19 years of wind stress from the Comprehensive Ocean-Atmosphere Data Set (local model); a second experiment (remote model) consisted of forcing the model through its southern boundary using the results of a similar reduced gravity equatorial model; in a third experiment, both forcings were used simultaneously (local + remote model). The main objective of this work was to analyze the low frequency variability on the CCS in terms of its contributions from remote and local forcing. / Away from the coast, the basic state of the model is determined by the predominantly negative wind curl through an Sverdrup balance. The general seasonal cycle (eg. set-up of Davidson Current, formation and position of Southern California eddy, etc.) is in agreement to what has been described by other authors. Through cross correlation and cross spectral analysis between the model results and observed sea-level data, it was established that most of the interannual variability in sea-level height at the coast is due to disturbances of equatorial origin that propagate into the region in the form of coastally trapped Kelvin waves. For the annual frequency variability, on the other hand, it was found that both local, as well as remotely forced variability, contribute to the total variance. / Source: Dissertation Abstracts International, Volume: 49-03, Section: B, page: 0677. / Major Professor: James J. O'Brien. / Thesis (Ph.D.)--The Florida State University, 1987.

This thesis includes three major parts, (1) Body wave analysis of mantle structure under the Calabria slab, (2) Spatial Average Coherency (SPAC) analysis of microtremor to characterize the subsurface structure in urban areas, and (3) Surface wave dispersion inversion for shear wave velocity structure. Although these three projects apply different techniques and investigate different parts of the Earth, their aims are the same, which is to better understand and characterize the subsurface Earth structure by analyzing complex seismic waveforms that are recorded on the Earth surface. My first project is body wave analysis of mantle structure under the Calabria slab. Its aim is to better understand the subduction structure of the Calabria slab by analyzing seismograms generated by natural earthquakes. The rollback and subduction of the Calabrian Arc beneath the southern Tyrrhenian Sea is a case study of slab morphology and slab-mantle interactions at short spatial scale. I analyzed the seismograms traversing the Calabrian slab and upper mantle wedge under the southern Tyrrhenian Sea through body wave dispersion, scattering and attenuation, which are recorded during the PASSCAL CAT/SCAN experiment. Compressional body waves exhibit dispersion correlating with slab paths, which is high-frequency components arrivals being delayed relative to low-frequency components. Body wave scattering and attenuation are also spatially correlated with slab paths. I used this correlation to estimate the positions of slab boundaries, and further suggested that the observed spatial variation in near-slab attenuation could be ascribed to mantle flow patterns around the slab. My second project is Spatial Average Coherency (SPAC) analysis of microtremors for subsurface structure characterization. Shear-wave velocity (Vs) information in soil and rock has been recognized as a critical parameter for site-specific ground motion prediction study, which is highly necessary for urban areas located in seismic active zones. SPAC analysis of microtremors provides an efficient way to estimate Vs structure. Compared with other Vs estimating methods, SPAC is noninvasive and does not require any active sources, and therefore, it is especially useful in big cities. I applied SPAC method in two urban areas. The first is the historic city, Charleston, South Carolina, where high levels of seismic hazard lead to great public concern. Accurate Vs information, therefore, is critical for seismic site classification and site response studies. The second SPAC study is in Manhattan, New York City, where depths of high velocity contrast and soil-to-bedrock are different along the island. The two experiments show that Vs structure could be estimated with good accuracy using SPAC method compared with borehole and other techniques. SPAC is proved to be an effective technique for Vs estimation in urban areas. One important issue in seismology is the inversion of subsurface structures from surface recordings of seismograms. My third project focuses on solving this complex geophysical inverse problems, specifically, surface wave phase velocity dispersion curve inversion for shear wave velocity. In addition to standard linear inversion, I developed advanced inversion techniques including joint inversion using borehole data as constrains, nonlinear inversion using Monte Carlo, and Simulated Annealing algorithms. One innovative way of solving the inverse problem is to make inference from the ensemble of all acceptable models. The statistical features of the ensemble provide a better way to characterize the Earth model.

Beneath many glaciers and ice sheets, hydrology influences or controls a variety of basal processes. Glaciohydraulic supercooling is a process whereby water freezes englacially or subglacially because its internal temperature is below the bulk freezing temperature. Water supercools when it is at its freezing point and flows from an area of higher pressure (lower ambient temperature) to an area of lower pressure (higher ambient temperature) without equilibrating its internal energy. The process is dependent on the configuration of the water flow path relative to the pressure gradient driving flow. I formulate the governing equations of mass, linear momentum, and internal energy for time-dependent clear water flow based on previous work (Clarke, 2003; Spring & Hutter, 1981, 1982). Because field evidence and steady-state theory point to water distributing laterally across the bed, I modify this theory to account for an aperture that is much wider than deep, which I refer to as a sheet. Ice accretion terms are formulated with porosity because accreting ice has residual porosity. Ice intrusion into such a water sheet is not described in the literature, and I formulate intrusion based on previous work as well as ideas gained from subglacial cavity formation. In addition, I modify the clear water equations to include erosion and deposition of sediment along the glacier bed and incorporation of sediment into the accreted ice. Furthermore, water may leave the ice-bed interface and flow through the glacier pore space because subglacial water pressure is relatively high when supercooling occurs. To this end, I develop an englacial waterflow model that incorporates changes in ice porosity based on creep closure and ice melt or accretion. Simulations reveal behavior that cannot be inferred from simplified models. For example, while total ice accretion is comparable to field estimates, locations of simulated ice accretion along the ice-bed interface conflict with steady state models, which tend to overpredict accretion amounts. Simulations also indicate that much sediment deposition occurs prior to water being supercooled. Sediment deposition tends to smooth subglacial topography rather than enhance it. Additional results and implications of numerical simulations are discussed.

We address questions relating to the velocity structure of the Earth in three ways: mapping the phase-velocity structure of the western United States, examining deviations of wave paths due to lateral variations in velocity, and demonstrating that Love wave fundamental-mode phase measurements from array methods can be significantly contaminated by overtone interference, dependent on differences in fundamental-mode and first-overtone phase-velocity structure. All of the studies presented in this work use USArray Transportable Array data, which allow for dense, high-quality measurements at an unprecedented level. To image the uppermost mantle beneath the western US, we improve upon single-station phase measurements by differencing them to produce a baseline data set of phase measurements along inter-station paths, for both Love and Rayleigh waves from 25--100 s. Additional measurements of the arrival angle and local phase velocity are made using a mini-array method similar to beamforming. The arrival-angle measurements are used to correct the two-station baseline measurements and produce a corrected data set. Both the baseline and corrected data sets are separately inverted, producing phase-velocity maps on a 0.5°-by-0.5° grid. We select the corrected maps as the preferred models for Rayleigh waves, with better fits to the data and more consistent measurements. We find that arrival-angle measurements for Love waves may be biased by overtone interference, and hence select the baseline maps as the preferred models for Love waves. The final set of phase-velocity maps is consistent with expectations from known geologic features, and is useful for both calculation of phase for regional paths and studies of radial anisotropy within the region. We use the mini-array method to make observations of the deviations of waves from the great-circle path. Measured arrival angles vary from 0° to ±15°. We compile results from earthquakes in small source regions, allowing the observation of bands of arrival-angle anomalies crossing the footprint of the USArray Transportable array in the propagation direction. These bands of deviations may result from heterogeneous velocity structure within the array, or on the larger source-to-array path. We use two global tomographic models to predict arrival-angle anomaly patterns, with both ray-theory-based prediction methods and measurements on synthetic waveforms calculated using SPECFEM3D Globe, a finite element package. We show that both models predict well the long-wavelength patterns of anomalies observed, but not the short-wavelength variations. Experiments with crustal structure indicate that greater heterogeneity is needed in the models. Predictions from the spectral-element-method synthetic waveforms contain the type of complexity seen in the observed patterns, and not obtained with the ray-theoretical methods, indicating that full synthetics are needed to compare model predictions to observed arrival-angle anomalies. We further examine possible overtone interference in the mini-array arrival-angle and local phase-velocity measurements for Love waves at long periods. Love wave fundamental-mode and higher-mode waves at the same period travel with similar group velocity, making them difficult to separate; the waves have different phase velocities, resulting in a beating interference pattern that oscillates with distance. We show this interference pattern for single-station, two-station, and mini-array phase-velocity measurements. Using measurements on synthetic waveforms calculated using both mode summation and SPECFEM3D Globe, we show that contamination of single-station measurements can largely be explained by interference between the fundamental and first-higher mode only. Interference causes small variations in the single-station phase velocity, up to 1%, and the oscillations about the expected values are asymmetric. The two array-based measurement techniques can be thought of as a spatial gradient over the single-station phase measurements, and consequently much larger variations are observed, 10--20%, and the results are biased to higher phase velocities. We conclude that overtone contamination must be carefully considered prior to attributing array-based Love wave phase-velocity anomalies to Earth structure.

This dissertation focuses on imaging the crustal and upper mantle seismic velocity structure beneath oceanic spreading centers. The goals are to provide a better understanding of the crustal magmatic system and the relationship between mantle melting processes, crustal architecture and ridge characteristics. To address these questions I have analyzed ocean bottom geophysical data collected from the fast-spreading East Pacific Rise and the back-arc Eastern Lau Spreading Center using a combination of ambient noise tomography and seafloor compliance analysis. To characterize the crustal melt distribution at fast spreading ridges, I analyze seafloor compliance - the deformation under long period ocean wave forcing - measured during multiple expeditions between 1994 and 2007 at the East Pacific Rise 9º - 10ºN segment. A 3D numerical modeling technique is developed and used to estimate the effects of low shear velocity zones on compliance measurements. The forward modeling suggests strong variations of lower crustal shear velocity along the ridge axis, with zones of possible high melt fractions beneath certain segments. Analysis of repeated compliance measurements at 9º48'N indicates a decrease of crustal melt fraction following the 2005 - 2006 eruption. This temporal variability provides direct evidence for short-term variations of the magmatic system at a fast spreading ridge. To understand the relationship between mantle melting processes and crustal properties, I apply ambient noise tomography of ocean bottom seismograph (OBS) data to image the upper mantle seismic structure beneath the Eastern Lau Spreading Center (ELSC). The seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC, representing a zone of partial melt. As the ridge migrates away from the volcanic arc, the LVZ becomes increasingly offset and separated from the sub-arc low velocity zone. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Therefore these results confirm a previous prediction that the changing interaction between the arc and back-arc magmatic systems is responsible for the abrupt change in crustal properties along the ELSC. I further investigate the crustal structure along and across the ELSC using seafloor compliance. Compliance measurements are inverted for local crustal shear velocity structure as well as sediment thickness at 30 OBS locations using a Monte Carlo method. Sediment increases asymmetrically with seafloor age, with much a higher rate to the east of the ridge. Along the ELSC, upper crustal velocities increase from south to north as the ridge migrates away from the volcanic arc front, consistent with a less porous upper crust with possibly less subduction input. Furthermore, average upper crust shear velocities for crust produced at past ELSC when it was near the volcanic arc are considerably slower than crust produced at present day northern ELSC. I show that the implications of previous active seismic studies in the axial ELSC can be extended much farther off-axis and back in time. I also address a challenge of ocean bottom seismology and develop a new method for determining OBS horizontal orientations using multi-component ambient noise correlation. I demonstrate that the OBS orientations can be robustly estimated through maximizing the correlation between the diagonal and cross terms of the noise correlation function. This method is applied to the ELSC OBS experiment dataset and the obtained orientations are consisent with results from a conventional teleseismic method. The new method is promising for a wide range of applications.

We develop several new techniques to better retrieve Earth's structure by analyzing seismic surface waves. These techniques are applied in regional studies to understand a variety of tectonic structures and geodynamic processes in Earth's crust and upper mantle. We create an automated method to retrieve surface-wave phase velocities using dense seismic arrays. The method is based on the notion of using cross-correlation to measure phase variations between nearby stations. Frequency-dependent apparent phase velocities are inverted from the phase-variation measurements via the Eikonal equation. The multi-pathing interference is corrected using amplitude measurements via the Helmholtz equation. The coherence between nearby-station waveforms, together with other data-selection criteria, helps to automate the entire process. We build up the Automated Surface-Wave Measuring System (ASWMS) that retrieves structural phase velocity directly from raw seismic waveforms for individual earthquakes without human intervention. This system is applied on the broad-band seismic data recorded by the USArray from 2006-2014, and obtain Rayleigh-wave phase-velocity maps at the periods of 20-100~s. In total around half million seismograms from 850 events are processed, generating about 4 million cross-correlation measurements. The maps correlate well with several published studies, including ambient-noise results at high frequency. At all frequencies, a significant contrast in Rayleigh-wave phase velocity between the tectonically active western US and the stable eastern US can be observed, with the phase-velocity variations in the western US being 1-2 times greater. The Love wave phase-velocity maps are also calculated. We find that overtone interference may produce systematic bias for the Love-wave phase-velocity measurements. We apply surface-wave analysis on the data collected by a temporary broad-band seismic array near the D'Entrecasteaux Island (DI), Papua New Guinea. The array comprises 31 inland and 8 off-shore broad-band seismic sensors, and were operated from March 2010 to July 2011. We adopt the ASWMS to retrieve phase velocities from earthquake signals, and apply the ambient-noise analysis to obtain the Rayleigh-wave phase velocities at higher frequencies. The multi-band phase velocities are inverted for a three-dimensional shear-velocity model of the crust and the upper mantle. The result reveals localized lithosphere extension along a rift-like axis beneath the DI, with a shear-velocity structure similar to an adiabatic upwelling mantle. West of the DI, very slow shear velocities are observed at shallow mantle depth (30-60~km), which we interpret either as the presence of in situ partial melt due to inhibited melt extraction, or as the existence of un-exhumed felsic crustal material embedded within the surrounding mantle. Love waves contain important information to constrain the upper-mantle radial anisotropy. However, Love-wave fundamental-mode phase-velocity measurements are often contaminated by overtone interference, especially within regional-scale arrays. We evaluate this problem by analytically and numerically evaluating the behavior of synthetic wavefields consisting of two interfering plane waves with distinct phase velocities but comparable group velocities. The results indicate large phase variance due to the interference that can explain the systemic bias observed in data. We develop a procedure that utilizes amplitude measurements to correct for the interference effect. The synthetic tests show the correction can significantly reduce the phase-velocity variance and the bias generated by the interference.

Source: Dissertation Abstracts International, Volume: 38-04, Section: B, page: 1626. / Thesis (Ph.D.)--The Florida State University, 1976.

Source: Dissertation Abstracts International, Volume: 34-03, Section: B, page: 1157. / Thesis (Ph.D.)--The Florida State University, 1973.

Source: Dissertation Abstracts International, Volume: 34-09, Section: B, page: 4459. / Thesis (Ph.D.)--The Florida State University, 1973.

The Benthic Boundary Layer is a region close the ocean bottom with features distinct from the oceanic interior. Near the bottom the ocean is turbulent and the resultant mixing leads to a neutrally stratified bottom layer. Turbulent closure models have been applied to investigate how the structure of the Benthic Boundary Layer is affected by the flow and the stratification above the layer. / The object of the present research is to analyze how the benthic region affects the dynamics of the forcing flow. More specifically, a numerical model based on the level 2 1/2 closure scheme of Mellor and Yamada is developed to examine the decay of deep mesoscale eddy-like flows. / It is found that the decay of the flow occurs through conversion of kinetic to potential energy and through dissipation by bottom friction. The relative importance of both processes is expressed by the Rossby number (epsilon) = U/fR and by the stratification parameter s = N('2)H('2)/f('2)R('2) (where H is the total depth of the eddy, R the radius, U the velocity scale, N the Brunt-Vaiasala frequency, and f the Coriolis parameter). A larger Rossby number and stratification parameter lead to a larger conversion of kinetic to potential energy, but a smaller mechanical dissipation of the same energy. / Examination of the structure of the Benthic Boundary Layer indicates that a clear distinction should be made between the mixed layer, or the region neutrally stratified, and the Bottom Boundary Layer, or the region where most of the turbulent activity occurs. It is found that the structure of the Bottom Boundary Layer depends also on the magnitude of the flow above the benthic region, but the mixed layer depends also on the sign of the mesoscale activity. Under a cyclonic flow, the mixed layer is defined by vertical advection and it is usually much thicker than the Bottom Boundary Layer. The mixed layer of an anticyclonic flow is the result of both vertical advection and near bottom turbulence, and the ambiguity between the mixed layer and Bottom Boundary Layer is notably reduced. / Source: Dissertation Abstracts International, Volume: 46-06, Section: B, page: 1852. / Thesis (Ph.D.)--The Florida State University, 1985.