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New, detailed field work in the Silurian hills, southeast of Death Valley, reveals a pattern of a complex Cenozoic brittle fault overprint of rocks containing evidence of at least three episodes of Mesozoic thermal-ductile deformation. The Cenozoic brittle fault overprint of rocks containing evidence of at least three episodes of Mesozoic thermal-ductile formation. The Cenozoic faulting consists of five distinct sets of structures ordered by cross cutting relationships. The oldest, BF1, are fragments of reverse faults trending NW and dipping 45-60 SW. BF2 consist of NW trending, en echelon, sinistral strike-slip faults. BF3 is a conjugate set of N-NE trending high-angle normal faults with 100's m offset. BF4 is a complex array of conjugate strike-slip and normal faults. NNW trending dextral and NNE trending sinistral faults offset and are offset by N and NW trending conjugate sets of high-angle normal faults. The younges set, BF5, are low-angle normal faults with no consistent basal surface or transport direction. These are interpreted to be right lateral, strike-slip extension (dextral transtension, BF4 & BF5) preceded by right lateral, vertical axis rotation, and strike-slip shortening (dextral transrotation and transpression, BF2 & BF3), all related to the interaction of the Garlock fault and northwest trending dextral shear. 30° of clockwise vertical axis rotation is inferred to have affected the Silurian Hills since the Middle Miocene as the result of BF2 & BF3 deformation. The Mesozoic deformations are; 1, a greenschist facies, low-strain but tightly folded N vergent, crystalline basement through Pahrump Group section; 2, a greenschist facies, high-strain, 100's m thick, mylonite zone with stretching lineations trending NE-SW; and 3, an upper amphibolite facies (high T, low P), high-strain E-W trending, S plunging Pahrump Group section, syntectonically intruded by multiple igneous phases U-Pb dated from 177 Ma to 97 Ma. The two Pahrump Group sections are interpreted to form as the result of NW directed backthrusting in the hinterland of the Sevier orogeny and were juxtaposed by NNW normal, extensional transport along the mylonite zone after crustal thickening. K-Ar biotite cooling ages suggest the mylonite zone developed during the Late Cretaceous to Eocene time. Finally, the protolith of the tectonostratigraphic Riggs Formation to be two formations of the Pahrump Group, the Crystal Spring and Beck Spring Formations.

A project report submitted to the Faculty of Mining Engineering, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering. / A strategy to determine the probability that a mining induced seismic event will occur with magnitude which exceeds some specified value within a given time is investigated. The model allows for a non-linear frequency-magnitude relationship and a Poissonian distribution of seismic events in time. The procedure is also independent of the method of mining and of the mining geometry. The model was applied to clusters of various sizes) starting from small areas on a single reef and ending up with the entire mine as a single entity. It was shown that the model works well with large populations of events, but to be successful with small clusters, the retention of the Poisson distribution is too restrictive and a non-stationary model of seismicevent occurrence in time will have to be developed. / AC2018

The theory of plate tectonics introduced a paradigm shift in the way we view and study our planet. Many of the world's plate boundaries, however, are beneath our oceans making data collection, the key to furthering our knowledge about these zones, difficult. Recent advances in research vessels, seismic data acquisition techniques, and equipment built to withstand the temperatures and pressures within the oceans and on the seafloor, have made a huge impact in helping us understand the complicated structure and dynamics of our Earth. In the field of seismology, precise earthquake locations can illuminate regions of active seismic deformation, and help us better understand the orientation, mechanics, and kinematics of plate boundary zones. Although ocean bottom seismometers have been in use since the late 1930s, the instruments did not have the recording capacity and endurance to withstand being placed on the seafloor for a large span of time. Today, ocean bottom seismometers are deployed in densely spaced arrays that record seismic signals for approximately a year. The high-precision seismic data now available can help us redefine plate boundaries and further our understanding of the internal processes and deformation within these zones. In this dissertation, I aim to use ocean bottom seismometer data to explore the Pacific-North America plate boundary within the Gulf of California, and the internal workings of the 9º50'N East Pacific Rise high-temperature hydrothermal system. The first chapter of my dissertation uses data collected from an ocean bottom seismometer array deployed along the plate boundary within the Gulf of California from October 2005 to October 2006. In this study, I detect and locate ~700 earthquakes mainly located on the NW-SE striking oceanic transform faults that delineate the plate boundary. In addition, we calculate regional moment tensors for ~30 of these events, and find that the majority are right-lateral strike-slip events consistent with observed transtensional plate motion. Chapter 2 investigates the relationship between tides and ~3500 microearthquakes recorded on six ocean bottom seismometers deployed in the vicinity of the 9º50'N East Pacific Rise high-temperature hydrothermal vent system from October 2003 to April 2004. I find unequivocal evidence for tidal triggering of microearthquakes with maximum extensional stresses induced by the solid Earth tide at this site. Although tides are not the underlying cause of earthquake nucleation within the region, the modulation of microearthquakes by these small amplitude tidal stresses indicates that the hydrothermal system is a high-stress environment that is maintained at a critical state of failure due to on-going tectonic and magmatic processes. In Chapter 3, I further investigate the role of tides in triggering microearthquake activity at the 9º50'N East Pacific Rise high-temperature hydrothermal vent site, and observe systematic along-axis variations between peak microearthquake activity and maximum predicted tidal extension. I interpret this systematic triggering to result from pore-pressure perturbations propagating laterally through the hydrothermal system, and from this result and a one-dimensional poroelastic model, I provide an estimate of bulk permeability at this site. This observation may allow for more sophisticated investigations into the heat and chemical exchange between the newly formed oceanic crust and hydrothermal fluids, and may provide insight into the plumbing supporting the subsurface biosphere.

This dissertation investigates the link between volcanic unrest and the occurrence of moderate-to-large earthquakes with a specific type of focal mechanism. Vertical compensated-linear-vector-dipole (vertical-CLVD) earthquakes have vertical pressure or tension axes and seismic radiation patterns that are inconsistent with the double-couple model of slip on a planar fault. Prior to this work, moderate-to-large vertical-CLVD earthquakes were known to be geographically associated with volcanic centers and vertical-CLVD earthquakes were linked to a tsunami in the Izu-Bonin volcanic arc and a subglacial fissure eruption in Iceland. Vertical-CLVD earthquakes are some of the largest and most anomalous earthquakes to occur in volcanic systems, yet their physical mechanisms remain controversial largely due to the small number of observations. Five vertical-CLVD earthquakes with vertical pressure axes are identified near Nyiragongo volcano in the Democratic Republic of the Congo. Three earthquakes occur within days of a fissure eruption at Nyiragongo, and two occur several years later in association with the refilling of the lava lake in the summit crater of the volcano. Detailed study of these events shows that the earthquakes have slower source processes than tectonic earthquakes with similar magnitudes and locations. All five earthquakes are interpreted as resulting from slip on inward-dipping ring-fault structures located above deflating shallow magma chambers. The Nyiragongo study supports the interpretation that vertical-CLVD earthquakes may be causally related to dynamic physical processes occurring inside the edifices or magmatic plumbing systems of active volcanoes. Two seismicity catalogs from the Global Centroid Moment Tensor (CMT) Project are used to search for further examples of shallow earthquakes with robust vertical-CLVD focal mechanisms. CMT solutions for approximately 400 target earthquakes are calculated and 86 vertical-CLVD earthquakes are identified near active volcanoes. Together with the Nyiragongo study, this work increases the number of well-studied vertical-CLVD earthquakes from 14 to 101. Vertical-CLVD earthquakes have focal depths in the upper ~10 km of the Earth's crust, and ~80% have centroid locations within 30 km of an active volcanic center. Vertical-CLVD earthquakes are observed near several different types of volcanoes in a variety of geographic and tectonic settings, but most vertical-CLVD earthquakes are observed near basaltic-to-andesitic stratovolcanoes and submarine volcanoes in subduction zones. Vertical-CLVD earthquakes are linked to tsunamis, volcanic earthquake swarms, effusive and explosive eruptions, and caldera collapse, and approximately 70% are associated with documented volcanic eruptions or episodes of volcanic unrest. Those events with vertical pressure axes typically occur after volcanic eruptions initiate, whereas events with vertical tension axes commonly occur before the start of volcanic unrest. Both types of vertical-CLVD earthquakes have longer source durations than tectonic earthquakes of the same magnitude. The isotropic and pure vertical-CLVD components of the moment tensor cannot be independently resolved using our long-period seismic dataset. As a result, several physical mechanisms can explain the retrieved deviatoric vertical-CLVD moment tensors, including dip-slip motion on ring faults, volume exchange between two reservoirs, the opening and closing of tensile cracks, and volumetric sources. An evaluation of these mechanisms is performed using constraints obtained from detailed studies of individual vertical-CLVD earthquakes. Although no single physical mechanism can explain all of the characteristics of vertical-CLVD earthquakes, a ring-faulting model consisting of slip on inward- or outward-dipping ring faults triggered by the inflation or deflation of a shallow magma chamber can account for their seismic radiation patterns and source durations, as well as their temporal relationships with volcanic unrest. The observation that most vertical-CLVD earthquakes are associated with volcanoes with caldera structures supports this interpretation.

An analytical and numerical model are presented and applied to predict gravitydriven transport and deposition of fluid mud layers that form within the wave boundary layer on the continental shelf off the Eel River in northern California. Observations indicate that following floods of the Eel River down-slope transport of fluid mud trapped within the wave boundary layer is the dominant across-shelf transport mechanism. The models are based upon the assumption that following significant floods, an abundant supply of easily suspended fine sediment is delivered to the coastal ocean, allowing a negative feedback mechanism to maintain the near-bed Richardson number at its critical value. Thus, sediment-induced stratification effectively limits the amount of fine sediment that can be maintained in suspension, allowing the calculation of down-slope transport and deposition knowing only the appropriate near-bed velocity scale. Analytic predictions of mid-shelf mud transport and deposition are spatially and temporally consistent with field observations and provide strong evidence that gravitydriven processes control the emplacement and location of the Eel margin flood deposit. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude of the flood event in controlling the thickness of mid-shelf gravity-driven deposition following floods. Higher wave energy increases the capacity for critically-stratified gravity flows to transport sediment to the mid-shelf and results in greater deposition. The bathymetry of the Eel margin plays a critical role in gravitydriven transport and deposition. Analytic predictions indicate that gravity-driven deposition on the mid-shelf begins roughly 7-8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf gravity-driven deposition and favoring sediment bypassing. Seaward decreases in shelf slope in the vicinity of the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. The numerical model predicts gravity-driven deposition on the continental shelf for four consecutive flood seasons of the Eel River using realistic bathymetry, waves and river forcing. Results from the numerical model are consistent with observations of deposition on the mid-self and support the results of the analytical model that suggest wave intensity and bathymetry are the dominant factors controlling the location and magnitude of observed deposition. Despite significantly greater sediment input near the river mouth, little mid-shelf deposition is predicted in this region due to the increasing off-shelf slope. The numeric results suggest that gradients in the along-shelf components of bed-slope also favor gravity-driven deposition 10-30 km north of the river mouth. Including the influence of along-shelf currents had little impact on the location of midshelf deposition, providing further support for bathymetric control of flood sedimentation on the Eel margin. A significant fraction of sediment from the Eel River was predicted to leave the shelf as a gravity-driven flow during floods with large wave energy. However, in extremely large floods, gravity-driven processes were not capable of removing riverderived fine sediment from the inner-shelf.

Typescript / 141 leaves : ill., appendix in end pocket / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Experimental determination of the dispersion of Rayleigh waves across Australia has provided information about the earth's crust in this region. This technique is particularly useful in Australia, where in many areas the low level of natural seismicity prohibits the use of more conventional methods of investigation of the crust. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics, 1967

The vertical and radial components of a seismic P wave can be decomposed by a Fourier transform into two sets of nonterminating sinusoidal waves with one set for each component. The tangent of the vertical transform divided by the radial transform gives by definition he apparent angle of emergence for that frequency. The actual angle of emergence can be calculated from the apparent angle. The change of the angle with frequency can be obtained by determining the angle over the entire frequency spectrum of the pulse. The angle of emergence is only defined for a pure pulse. Just the short length of uncontaminated signal can be used to calculate the angle U the signal is interfered with by other signals. The actual angle of emergence was calculated as a function of frequency for stations near nuclear explosions. In all cases, the angle varied with frequency. / Graduation date: 1967

Graduation date: 1966

Graduation date: 1970