The bathymetry, magnetic anomalies, gravity anomalies and sediment layer is presented to support a young Western Mediterranean no older than 25 million years and an Eastern Mediterranean as old as 200 million years. A buried salt layer is detected in seismic reflection profiles lying below a sediment cover whose base is 4 to 5 million years in age. The seafloor in the eastern Mediterranean is being actively deformed by compressional folding and thrusting beneath the Mediterranean Ridge. The Western Mediterranean opened by rifting and is floored by oceanic crust.

Graduation date: 1998 / Best scan available for figures. Original is a black and white photocopy.

In this thesis, I present results that broadly fall into two themes. The first involves understanding active tectonic and magmatic processes at mid-ocean ridges. The second involves using small stress changes due to the tides to probe earthquake processes at mid-ocean ridges. The four main results of my thesis are as follow: (1) The spatiotemporal evolution of an eruption at a fast-spreading mid-ocean ridge, the East Pacific Rise, is now characterized and understood to be mainly controlled by the buildup of tectonic stress to a critical level rather than magma overpressure. (2) Microearthquakes at the East Pacific Rise are found to be strongly modulated by tides in the years before an eruption but not immediately after the eruption, suggesting the potential utility of tidal triggering strength for eruption forecasting. (3) Earthquake size-frequency distribution, often quantified using the b value, is shown to vary systematically with tidal stresses which lends support to the use of earthquake b value as an in-situ stressmeter. (4) The 2015 Axial Seamount eruption is revealed to be preceded by variable rates of melt influx into the shallow reservoir, highlighting the short-timescale variability of magmatic systems as they are primed for an eruption.

Crustal and upper mantle structures in the West Philippine Basin, along 17-18°N, have been determined using explosions as sources and ocean bottom seismometers to measure refracted compressional waves. Seismic refraction profiles out to nearly 500 km were completed. Shallow structure was measured using small shots, 1-240 lbs., and the deeper structure was probed with large explosions, 0.9-1.8 tons. A velocity-depth inversion using short range data shows the upper crust to have strong velocity gradients which gradually decrease with depth. The lower crust is characterized by a nearly constant velocity gradient of 0.24 sec⁻¹. Standard delay-time functions and a modified function accounting for lateral velocity gradients were also used in travel time inversion. Results from the two methods are comparable and yield ~1.5 km transitional zone thicknesses in the basin. Although they vary slightly in magnitude between methods, West Philippine Basin oceanic layer thicknesses are abnormally thin, by about 2 km, when compared to average crust. Total crustal thicknesses are shown to be thinner in the eastern part of the basin, approaching only 3 km. Crustal thinning toward the east is consistent with the Palau-Kyushu Ridge being a remnant transform fault connecting Philippine and Kula-Pacific ridges in the past. Predicted water depths in the basin are about 300 meters shallower than observed depths when compensated to average mantle depths found for the Western North Pacific. The depth anomaly cannot be fully reconciled by thin crust, and requires a deeper-seated anomaly to be present in the West Philippine Basin. Temperature and pressure modeling using experimental measurements from proposed mantle constituents indicate high seismic gradients in the upper mantle and may suggest that a multi-component or graded mantle exists beneath the marginal sea. / Graduation date: 1983

Marine magnetic data from the northern Yucatan continental margin were used to obtain the radially-averaged power-density spectra (RAPDS) of 34 grids, each with dimensions of 128 by 128 km, overlapping 50 percent. Depths to the tops of three magnetic horizons were estimated from the slopes of linear segments in the spectra. The depth to the base of the magnetic crust was estimated using an implicit relationship between the deepest depth estimation and the spectral peak position. The depth determinations agree with the limited drillhole data available and show some consistency with seismic refraction interpretations for the study area. Results of similar studies, based on the spectral analysis of magnetic anomalies in different tectonic settings, seem to agree with other geophysical and geologic data. In this area however, the limited depth of resolution achieved by seismic methods, and the absence of deep drillhole data and heat-flow measurements, makes it difficult to assess the validity of this interpretation. I therefore examined the simplifications implicit in the data processing techniques commonly applied in the spectral analysis of magnetic anomalies, and the assumptions upon which the method is based. Finally, I tested the method itself by interpreting synthetic anomalies generated from model source bodies. I found that depth interpretations, based upon assuming that the slope of linear segments in the RAPDS are proportional to the average depth to the top of magnetic horizons, and thickness determinations using the position of the spectral peak, are unreliable. The problem has been oversimplified. I also show that the RAPDS is independent of the direction of both the geomagnetic field and the magnetization vector. It can be represented by a Functional in terms of depth to the top of the source, its thickness and its horizontal dimensions. The problem of interpreting the RAPDS was then formulated as an inverse problem. A solution was obtained through minimizing, iteratively, the sum of squares of residuals between a real-data spectrum and a synthetic spectrum. The minimization was based on a linearized model, using the ridge-regression algorithm. This technique provides acceptable solutions for synthetic anomalies produced by model source bodies. Depth and thickness determinations, obtained using this technique on the Yucatan spectra, were used to make contour maps of the average depths to the top and the base of the magnetic crust. The top of the magnetic crust is at an average depth of 1.8 km. The base of the magnetic crust is at an average depth of 25 km. Beneath the central Campeche Bank and Campeche Terrace the depth to the base is close to the depth to the crust-mantle boundary determined by gravity modeling and mass column analysis. This suggests that in those regions the mantle is the lower magnetic boundary. / Graduation date: 1991

Controlled-Source Electromagnetics (CSEM) is a method that has been used since the 1980’s in the marine environment for determining electrical properties of the subsurface. Receivers on the seafloor collect total electric and magnetic fields which are produced as a result of interaction of the transmitter generated primary fields with the seawater and subsurface. Badea et al. (2001) coded an existing algorithm for solving Maxwell’s equations. This finite element 3D forward modeling algorithm is used to simulate CSEM experiments. The objective of the present study is to model the changes in electromagnetic response for a resistive disk and a more geometrically complex structure, which are rough approximations of hydrocarbon reservoirs. The parameters that are varied in studying these subsurface structures are the disk radius, disk depth, the transmitter frequency, the transmitter location, and the structure orientation.The results showed that a disk of finite radius behaves similar to an infinite disk at short range and grades into double half-space behavior at longer ranges. The frequency of the transmitter must be tuned to the disk depth as certain frequencies will penetrate too shallow or too deep to probe the disk. Moving the transmitter away from the receivers causes a decrease in signal strength, but exhibits a greater capacity to distinguish between the double half-space and infinite disk scenarios. The disk was then replaced by a more complex structure. To determine if the 3D nature of the structure may be located a study was undertaken to probe the structure from different perspectives using different transmitter locations and azimuths. It is determined that the 3D nature of the structure could not be observed until the structure’s thickness is sufficiently large.The goal of the study is to better understand the effect of subsurface parameters on the total fields and show the usefulness of the 3D forward modeling code. Understanding the relationships between these parameters and the resulting signals is important in terms of setting up a real experiment. Marine CSEM studies are costly and using a valuable tool such as an accurate finite element 3D forward modeling algorithm may save time and money.

Controlled-Source Electromagnetics (CSEM) is a method that has been used since the 1980’s in the marine environment for determining electrical properties of the subsurface. Receivers on the seafloor collect total electric and magnetic fields which are produced as a result of interaction of the transmitter generated primary fields with the seawater and subsurface. Badea et al. (2001) coded an existing algorithm for solving Maxwell’s equations. This finite element 3D forward modeling algorithm is used to simulate CSEM experiments. The objective of the present study is to model the changes in electromagnetic response for a resistive disk and a more geometrically complex structure, which are rough approximations of hydrocarbon reservoirs. The parameters that are varied in studying these subsurface structures are the disk radius, disk depth, the transmitter frequency, the transmitter location, and the structure orientation.The results showed that a disk of finite radius behaves similar to an infinite disk at short range and grades into double half-space behavior at longer ranges. The frequency of the transmitter must be tuned to the disk depth as certain frequencies will penetrate too shallow or too deep to probe the disk. Moving the transmitter away from the receivers causes a decrease in signal strength, but exhibits a greater capacity to distinguish between the double half-space and infinite disk scenarios. The disk was then replaced by a more complex structure. To determine if the 3D nature of the structure may be located a study was undertaken to probe the structure from different perspectives using different transmitter locations and azimuths. It is determined that the 3D nature of the structure could not be observed until the structure’s thickness is sufficiently large.The goal of the study is to better understand the effect of subsurface parameters on the total fields and show the usefulness of the 3D forward modeling code. Understanding the relationships between these parameters and the resulting signals is important in terms of setting up a real experiment. Marine CSEM studies are costly and using a valuable tool such as an accurate finite element 3D forward modeling algorithm may save time and money.

Thesis (Sc. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1983. / Includes bibliographical references (p. 229-244).

Thesis (Ph. D.)--University of California, San Diego, 1998. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of California, San Diego, 1997. / Vita. Includes bibliographical references.