This thesis examines the seismic behavior of sediments in shallow subduction zones. In the traditional view of the seismogenic zone, the upper stability limit is controlled by a transition to velocity-strengthening (frictionally stable) clay-rich sediments at shallow depths in the accretionary prism. However, recent observations have emphasized that these shallow sediments can host a wide range of seismic behaviors. On one end of the seismic spectrum, the March 2011 Mw9.1 Tohoku-oki earthquake demonstrated that peak slip in a megathrust rupture can be hosted at the shallowest depths. At the other end of the spectrum, observations at the Hikurangi trench off the North Island of New Zealand have revealed that spontaneous, periodic slow slip events (SSEs) can nucleate in the shallowest portions of a subduction zone. The Japan Fast Trench Drilling Project (JFAST, IODP Expedition 343) drilled through the plate boundary faults in the Japan Trench to investigate the structure that hosted the Tohoku-oki earthquake. In Chapter 2, I use a trace element-based stratigraphy to identify several large displacement faults within the bottom ~15 m of the JFAST core. This work highlights that there are multiple candidate structures that could host a megathrust rupture and that not all displacement is accommodated along a weak pelagic clay layer recovered in the JFAST core. However, this method is incapable of determining which of these faults experienced significant seismic slip. In Chapter 3, I develop a novel paleoseismic indicator appropriate for faults hosted in seafloor sediments. This tool takes advantage of the fact that organic material (molecular biomarkers) in sediments degrades as a function of time and temperature. In this study, I determine the kinetics of thermal maturation for alkenones (coccolithophore-derived molecules) and n-alkanes (plant leaf wax-derived molecules) found in western Pacific sediments. In Chapter 4, I apply these kinetics to measured biomarker anomalies in JFAST samples to determine which faults recovered in the JFAST core could have hosted a megathrust event such as the Tohoku-oki earthquake. This approach reveals that multiple faults in the plate boundary region have likely hosted megathrust events and that the occurrence of seismic slip is not confined to a particular lithology. This implies that small differences in frictional behavior in subducting sedimentary lithologies are not the primary control on the occurrence of shallow seismic slip. In Chapter 5, I turn to a different type of shallow seismic behavior and focus on SSEs in the shallowest portion of the Hikurangi trench. In this study, I measure friction and velocity-dependence of the input sediments for this subduction zone at a range of pressures and temperatures relevant to the shallow portion of the slab where SSEs have been observed. These experiments demonstrate that the sediment here becomes frictionally weak at effective stresses expected deeper than ~2 km. At the same effective stresses, the sediment becomes less velocity strengthening, and under some conditions exhibits velocity neutral behavior. A plate-rate experiment exhibits velocity-weakening behavior and two spontaneous SSEs, indicating that at slow velocities, the sediment subducting at the Hikurangi trench is capable of unstable frictional behavior required to promote shallow SSEs. These results demonstrate that subducting sediments can exhibit a variety of frictional properties that can support unstable behavior in the shallowest reaches of the subduction zone.

We address questions related to the parameterization of two distinct types of seismic sources: earthquakes and underground nuclear explosions. For earthquakes, we focus on the improvement of location parameters, latitude and longitude, using relative measurements of spatial cluster of events. For underground nuclear explosions, we focus on the seismic source model, especially with regard to the generation of surface waves. We develop a procedure to improve relative earthquake location estimates by fitting predicted differential travel times to those measured by cross-correlating Rayleigh- and Love-wave arrivals for multiple earthquakes recorded at common stations. Our procedure can be applied to populations of earthquakes with arbitrary source mechanisms because we mitigate the phase delay that results from surface-wave radiation patterns by making source corrections calculated from the source mechanism solutions published in the Global CMT Catalog. We demonstrate the effectiveness of this relocation procedure by first applying it to two suites of synthetic earthquakes. We then relocate real earthquakes in three separate regions: two ridge-transform systems and one subduction zone. In each scenario, relocated epicenters show a reduction in location uncertainty compared to initial single-event location estimates. We apply the relocation procedure on a larger scale to the seismicity of the Eltanin Fault System which is comprised of three large transform faults: the Heezen transform, the Tharp transform, and the Hollister transform. We examine the localization of seismicity in each transform, the locations of earthquakes with atypical source mechanisms, and the spatial extent of seismic rupture and repeating earthquakes in each transform. We show that improved relative location estimates, aligned with bathymetry, greatly reduces the localization of seismicity on each of the three transforms. We also show how improved location estimates enhance the ability to use earthquake locations to address geophysical questions such as the presence of atypical earthquakes and the nature of seismic rupture along an oceanic transform fault. We investigate the physical basis for the mb-MS discriminant, which relies on differences between amplitudes of body waves and surface waves. We analyze observations for 71 well-recorded underground nuclear tests that were conducted between 1977-1989 at the Balapan test site near Semipalatinsk, Kazakhstan in the former Soviet Union. We combine revised mb values and earlier long-period surface-wave results with a new source model, which allows the vertical and horizontal forces of the explosive source to be different. We introduce a scaling factor between vertical and horizontal forces in the explosion model, to reconcile differences between body wave and surface wave observations. We find that this parameter is well correlated with the scaled depth of burial for UNEs at this test site. We use the modified source model to estimate the scaled depth of burial for the 71 UNEs considered in this study.

An abnormally high number of explosion quakes were noted during the monitoring effort for the 2007 eruption of Pavlof Volcano on the Alaskan Peninsula. In this study we manually counted the explosion quakes from their characteristic ground-coupled air waves. This study makes an effort at better quantifying the number of explosion quakes and how the characteristic ground-coupled air waves are affected by wind direction and wind speed. Additionally this study investigates how the ground coupled air waves might be used in a monitoring or analysis effort by calculating energy release and gas mass release. Over 3.2x104 quakes were recorded. It was found that wind direction affects the travel time of the air wave by up to 0.7 seconds depending on station location and wind direction. Wind direction and speed, however, are demonstrated not to cause an appreciable difference in ground-coupled air wave frequencies or amplitude ratios. The energy release from the explosions is calculated to be 3.04x1011 J. and the total gas mass (assuming 100% water) released was 729 metric tons. These values are compared to other volcanoes in the literature and found to be somewhat lower. Nevertheless, the tracking of explosion quakes has the potential to become a valuable member of the seismic monitoring arsenal.

Development of a hydrocarbon reservoir requires information about the type of fluid that saturates the pore space, and the permeability distribution that determines how the fluid can be extracted. The presence of fractures in a reservoir can be useful for obtaining this information. The main objectives of this thesis are to investigate how fracturing can be detected remotely using exploration seismology. Fracturing will effect seismic data in a number of ways. Firstly, if the fractures are aligned preferentially in some direction, the medium will exhibit long wavelength anisotropy. In turn, if wave propagation is not aligned with one of the symmetry axes of the effective medium then shear wave splitting will depend upon the properties of the fracture filling fluid. Secondly, elastic waves will experience attenuation and dispersion due to scattering and wave-induced fluid flow between the fractures and matrix porosity. This occurs because the fractures are more compliant than the background medium and therefore there will be a pressure gradient formed during passage of the wave, causing fluid to flow between fractures and background. If the direction of shear-wave propagation is not perpendicular or parallel to the plane of fracturing, the wave polarized in the plane perpendicular to the fractures is a quasi-shear mode, and therefore the shear-wave splitting will be sensitive to the fluid bulk modulus. / The magnitude of this sensitivity depends upon the extent to which fluid pressure can equilibrate between pores and fractures during the period of the deformation. In this thesis I use the anisotropic Gassmann equations and existing formulations for the excess compliance due to fracturing to estimate the splitting of vertically propagating shear-waves as a function of the fluid modulus for a porous medium with a single set of dipping fractures and with two conjugate fracture sets dipping with opposite dips to the vertical. This is achieved using two alternative approaches. In the first approach it is assumed that the deformation taking place is quasi-static. That is, the frequency of the elastic disturbance is low enough to allow enough time for fluid to flow between both the fractures and the pore space throughout the medium. In the second approach I assume that the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space, but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between different fracture sets to occur. It is found that the second approach yields a much stronger dependency of shear-wave splitting on the fluid modulus than the first one. This is a consequence of the fact that at higher wave frequencies there is not enough time for fluid pressure to equilibrate and therefore the elastic properties of the fluid have a greater effect on the magnitude of the shear-wave splitting. I conclude that the dependency of the shear-wave splitting on the fluid bulk modulus will be at its minimum for quasi-static deformations, and will increase with increasing wave frequency. / In order to treat the problem of dispersion and attenuation due to wave-induced fluid flow I consider interaction of a normally incident time-harmonic longitudinal plane wave with a circular crack imbedded in a porous medium governed by Biot’s equations of dynamic poroelasticity. The problem is formulated in cylindrical coordinates as a system of dual integral equations for the Hankel transform of the wave field, which is then reduced to a single Fredholm integral equation of the second kind. It is found that the scattering that takes place is predominantly due to wave induced fluid flow between the pores and the crack. The scattering magnitude depends on the size of the crack relative to the slow wave wavelength and has its maximum value when they are of the same order. I conclude that this poroelastic effect should not be neglected, at least at seismic frequencies. Using the solution of the scattering problem for a single crack and multiple-scattering theory I estimate the attenuation and dispersion of elastic waves taking place in a porous medium containing a sparse distribution of such cracks. I obtain from this analysis an effective velocity which at low frequencies reduces to the known static Gassmann result and a characteristic attenuation peak at the frequency such that the crack size and the slow wave wavelength are of the same order. / When comparing with a similar model in which multiple scattering effects are neglected I and that there is agreement at high frequencies and discrepancies at low frequencies. I conclude that the interaction between cracks should not be neglected at low frequencies, even in the limit of weak crack density. Since the models only agree with each other at high frequencies, when the time available for fluid diffusion is small, I conclude that the interaction between cracks that takes place as a result of fluid diffusion is negligible at high frequencies. I also compare my results with a model for spherical inclusions and find that the attenuation for spherical inclusions has exactly the same dependence upon frequency, but a difference in magnitude that depends upon frequency. Since the attenuation curves are very close at low frequencies I conclude that the effective medium properties are not sensitive to the shape of an inclusion at wavelengths that are large compared to the inclusion size. However at frequencies such that the wavelength is comparable to or smaller than the inclusion size the effective properties are sensitive to the greater compliance of the flat cracks, and more attenuation occurs at a given frequency as a result.

We have determined the seismic crustal structure of the northern part of the Gorda Ridge using signals generated by explosive charges and recorded on Ocean Bottom Seismometers. The shot pattern forms two parallel lines, one on the east flank and the other along the median valley. Inversion of the travel time data and synthetic modelling of the signals resulted in two compressional velocity structures: the model for the flank indicates a 1.6 km thick upper crust characterized by high velocity gradients and 3.6 km thick lower crust characterized by a low gradient. A sharp mantle transition exists at 5.2 km depth with an upper mantle velocity of 7.6 km/sec. The median valley velocity model has a thicker high gradient upper crust of 3.0 km and a lower crust of at least 3.5 km thickness. No upper mantle velocities were detected beneath the median valley. We have also monitored the seismicity of the ridge during 15 days with two arrays of OBS and detected ~4 events/hour. Epicentral coordinates were determined for 140 earthquakes. Most of them lie within the median valley and show spatial clustering. Intraplate seismicity was also detected in the Gorda Basin with three of the earthquakes big enough to be reported by land stations. They suggest that the Gorda Plate is presently undergoing deformation. Good control over the focal depth was possible for ~80 earthquakes occurring on the ridge, and there the seismic activity appears to be pervasive throughout the upper 20 km suggesting that the the brittle lithosphere is at least this thick. From the earthquake shear-wave data we have obtained a value of 1.73 for the Vp/Vs ratio. Moments of the well constrained events derived from the spectra of the waveforms are of the order 10²⁰ dyne-cm and suggest an average fault width of 300 m. The refraction data is consistent with the earthquake results, and all the evidence suggests that a large magma chamber underlying the axis of spreading does not presently exist at shallow depths. / Graduation date:1985

Graduation date: 1997

The Cascadia subduction zone has been characterized as a typical Chilean-type subduction zone based on qualitative comparisons of plate age and convergence rate, with simple forearc structure. However, the discovery of unusual structural styles of deformation, variations in the morphology of the forearc, and its absence of seismic activity suggest differences from the Chilean analog. The manuscripts presented here (McNeill et al., 1997, in press, submitted) illustrate this complexity and provide examples of contrasting deformation throughout the offshore forearc. The Washington and northern Oregon shelf and upper slope are characterized by extension in the form of listric normal faults. These faults have been active since the late Miocene and are driven by detachment and extension of the underlying overpressured m��lange and broken formation. This region of the forearc is partly to wholly decoupled from convergence-driven compression which dominates deformation elsewhere in the forearc. One exception to convergence-driven compression is a region of N-S compression of the inner shelf and coastal region which reflects the regional stress field. N-S compressional structures apparently influence the positions of coastal lowlands and uplands and may contribute to the record of coastal marsh burials interepreted as the result of coseismic subsidence during subduction zone earthquakes. Modeling of subduction zone earthquake characteristics based on marsh stratigraphy is likely to be inaccurate in terms of rupture zone position, magnitude, and recurrence interval. The Cascadia shelf and upper slope are underlain by a sequence of deformed basinal strata which reflects the tectonic evolution of the margin. The surface of a regional late Miocene angular unconformity (7.5-6 Ma: a global hiatus) indicates deformation by uplifted submarine banks and subsided synclines (coincident with low recent uplift onshore), which control the current shelf break position. The basin is currently filled behind a N-S-trending outer-arc high, which uplifted in the early-middle Pliocene following truncation and erosion of the seaward edge of the basin. Breaching of the outer-arc high occurred in the early Pleistocene leading to the formation of the Astoria Submarine Fan and increased growth rates of the accretionary wedge. / Graduation date: 1999

Hydroacoustic tertiary (T-) waves are seismically generated acoustic waves that propagate over great distances in the ocean sound channel with little loss in signal strength. Hydrophone recorded T-waves can provide a lower earthquake detection threshold and an improved epicenter location accuracy for oceanic earthquakes than land-based seismic networks. Thus detection and location of NE Pacific ocean earthquakes along the Blanco Transform Fault (BTFZ) and Gorda plate using the U.S. Navy's SOSUS (SOund SUrveillance System) hydrophone arrays afford greater insight into the current state of stress and crustal deformation mechanics than previously available. Acoustic earthquake information combined with bathymetry, submersible observations, earthquake source-parameter estimates, petrologic samples, and water-column chemistry renders a new tectonic view of the southern Juan de Fuca plate boundaries. Chapter 2 discusses development of seismo-acoustic analysis techniques using the well-documented April 1992 Cape Mendocino earthquake sequence. Findings include a hydrophone detection threshold estimate (M~2.4), and T-wave propagation path modeling to approximate earthquake acoustic source energy. Empirical analyses indicate that acoustic energy provides a reasonable magnitude and seismic moment estimate of oceanic earthquakes not detected by seismic networks. Chapters 3 documents a probable volcanogenic T-wave event swarm along a pull-apart basin within the western BTFZ during January 1994. Response efforts yielded evidence of anomalous water-column ��He concentrations, pillow-lava volcanism, and the first discovery of active hydrothermal vents along an oceanic fracture zone. Chapter 4 discusses the detection of a NE-SW trending microearthquake band along the mid-Gorda plate which was active from initiation of SOSUS recording in August 1991 through July 1992, then abruptly ceased. It is proposed that eventual termination of the Gorda plate seismicity band is due to strain reduction associated with the Cape Mendocino earthquake sequence. Chapter 5 combines bathymetric, hydro-acoustic, seismic, submersible, and gravity data to investigate the active tectonics of the transform parallel Blanco Ridge (BR), along the eastern BTFZ. The BR formation mechanism preferred here is uplift through strike-slip motion (with a normal component) followed by formation and intrusion of mantle-derived serpentinized-peridotite into the shallow ocean crust. The conclusion considers a potential link between the deformation patterns observed along the BTFZ and Gorda plate regions. / Graduation date: 1998

Seismic first-arrival tomography is a technique currently experiencing a renaissance in popularity due to the simplicity of implementation and promising results for delineating a variety of subsurface targets. The purpose of this study is to investigate seismic first-arrival tomography in a variety of settings and applications, and thus to provide a solid framework for future work. The study largely consists of two separate themes, hydrogeophysics and low-velocity anomaly detection. Hydrogeophysics is an emerging field whereby measured geophysical properties are used as proxies for physical properties of the subsurface. This study represents one of the first high-resolution hydrogeophysical investigations in the upper three meters of the subsurface using seismic first-arrival tomography, and consists of detecting shallow high-velocity zones that are interpreted to be perched water bodies on the basis of geophysical and hydrologic evidence. The delineation and imaging of the perched water bodies is further advanced using trend-analysis techniques. A second theme of this dissertation is the optimization of methods for delineating low-velocity anomalies at depth using seismic first-arrival tomography. In order to locate a low-velocity zone at Oak Ridge, Tennessee, multiple seismic lines were collected and correlated with site-wide geology. The integration of geologic and geophysical data-sets assisted in developing a comprehensive transport conceptual model, and provided a predictive framework for future geophysical investigations at Oak Ridge. As a second component of this theme, a systematic methodology for detecting and delineating shallow low-velocity zones is developed.

Karst environments are unique landscapes that contain important resources, including freshwater aquifers and specialized ecosystems, which are easily disturbed due to the interconnected nature of the surface and subsurface. The anthropogenic impacts on karst are deleterious to the ecosystems that are dependent on the karst environment and also to groundwater supplies. The Karst Disturbance Index (KDI) is a holistic tool used to measure anthropogenic impacts associated with karst environments, which has been applied and refined through studies performed in Florida and Italy, yet still remains untested and susceptible to modification for other areas. Application of the KDI in Arecibo, Puerto Rico, which is geographically isolated, and highly vulnerable due to its sensitive karst resources, provides an opportunity to test the index in an island setting. This research resulted in two KDI scores for the study area using both the original and recently modified methods. The scores reflect a significant to severe disturbance to the municipality’s karst environment of 0.54 and 0.68, respectively. Issues regarding the KDI were found from the application and comparison of these methodologies and revealed the need for adding additional indicators, including Mogote Removal and Coastal Karst, as well as several additional refinements and recommendations pertaining to scale, weighting, and incorporating the two methods together to create a single, more practical KDI tool. The disseminated results of the assessment of the area using the KDI will educate and help to foster stewardship of this vital resource in Puerto Rico.