A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, for the degree Master of Science / The recording of seismic signals in remote areas requires a portable, low power recording system that can be left in the field for a few weeks at a time. Three components of ground motion are generally measured, and some form of event recording, rather than continuous recording should be available. [Abbreviated abstract. Open document to view full version] / AC2017

Thesis advisor: John E. Ebel / Thesis advisor: John C. Hepburn / Using the seismic refraction technique with a least squares inversion methodology, arrival time data from 1985 to the present are analyzed to delineate, with improved spatial resolution, the upper mantle P-velocity structure throughout northeastern North America (NENA). A total of one hundred and sixty-eight earthquakes are analyzed utilizing over one hundred seismic stations throughout NENA. Seismic data analyzed between 200 - 400 km, 400 - 600 km, and 600+ km throughout NENA are used to study the increase in velocity with depth in the upper mantle. A jackknife analysis was carried out to put constraints on the uncertainties of the velocity measurements. The P-wave velocity of the upper mantle through the New England Appalachians is found to be uniformly 7.94 - 8.07 km/s at depths down to 75 km. Upper mantle Pn velocities throughout the southeastern Grenville Province show velocities ranging from 8.15 km/s to 8.54 km/s as epicentral distances increase. Uncertainties of P velocities range from 0.01- 0.12 km/s. Based on laboratory measurements of simulated upper mantle conditions and the orogenic history of the Grenville Province and northern Appalachians, upper mantle mineral compositions of eclogite (Grenville Province) and pyroxenite (northern Appalachians) are proposed to be the factor controlling seismic velocity variation in the upper mantle. Variations in upper mantle temperatures between the Grenville Province and northern Appalachians are ruled out as affecting the difference in upper mantle velocities between southeastern Canada and New England. / Thesis (MS) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Geology and Geophysics.

Thesis advisor: John E. Ebel / Large magnitude offshore passive margin earthquakes are rare, making small magnitude events (M < 4) the predominant data available to study the mechanisms of seismicity along passive margins. This study is focused on a swarm of events (M2.1-M3.9) that occurred from 2012-2013 located in the Gulf of Maine (GM) along the Atlantic Passive Margin (APM) shelf break, a region with previously minimal recorded seismic activity. Relative locations were calculated for the earthquakes of the GM swarm and a moment tensor inversion method was used to calculate focal mechanisms for the two largest events in the swarm. The results of the relative location method constrained a fault orientation to a strike of 243° ± 3° and a dip of 25° ± 3°. The focal mechanisms for the two largest events were determined to be oblique normal faults with steeply dipping planes at depths between 12-18 km. For the largest event (M3.9), the strike is 235° ± 1°, with a dip of 77.7° ± .8° and a rake of -116.5° ± 3°, and for the second largest event (M3.7) the strike is 259° ± 3°, with a dip of 78° ± 2° and a rake of -58.8° ± 7°. By mapping the spatial extent of the relative hypocenters, I infer a potential fault size of 2.7 km by 2.4 km. If this entire area were to rupture at once in the future, an earthquake of M4.9-M5.0 could occur, a magnitude not large enough to be tsunamigenic in the GM. Based on Gutenberg-Richter relations from the eastern APM, if a M7 can occur in the GM, its estimated mean repeat time is 2,120-22,800 years, and it could be tsunamigenic depending on the event’s proximity to the continental slope. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Thesis advisor: Alan Kafka / Thesis advisor: Seth Kruckenberg / The western United States (WUS) is an area of high seismic activity. The Juan de Fuca, Pacific, and North American plates all meet in this area, resulting in zones of subduction and strike-slip faulting, as well as other styles of faulting, all of which make it prone to frequent, as well as large magnitude earthquakes. In this study the WUS encompasses the area between 30° to 52°N and 110° to 131°W. The diverse seismicity and tectonics of the area makes the study of seismo-tectonic processes in the WUS important not only in terms of basic geoscience, but also in terms of earthquake hazards. Understanding earthquake processes in this region is critical because of the potential for devastating earthquakes to occur along the Pacific-Juan de Fuca-North American plate boundary system. Large WUS earthquakes do not, however, only occur along these plate boundaries. They can also happen in intraplate environments within the WUS. The WUS includes three distinct tectonic regions for which this study compares and contrasts characteristics of seismic activity: the Cascadia subduction region, the San Andreas strike-slip region, and a continental extension/intraplate region to the east of the major plate boundaries referred to here as the “Western Intraplate Hinterland Region”. To help make these comparisons, the method of “Cellular Seismology” (CS; Kafka, 2002, 2007), is used here to investigate similarities and differences in the extent to which past earthquakes delineate zones where future earthquakes are likely to occur in the WUS and its various tectonic sub-regions. The results of this study show that while there seems to be a “signal” of CS predictability being dependent on tectonic region, that signal is subtle in most cases, meaning that there is not a significant difference in the level of CS predictability between the regions stated here. This means we can apply CS predictability studies widely across different regions, however, it also counterintuitively suggests that tectonic understanding of a region does not necessarily elucidate how well past seismicity predicts spatial patterns of earthquakes in a region.

In this dissertation, I use seismic imaging techniques to constrain the physical state of the upper mantle beneath regions of young oceanic lithosphere. Mantle convection is investigated beneath the Galápagos Archipelago and then beneath the Juan de Fuca (JdF) plate, with a focus on the JdF and Gorda Ridges before turning to the off-axis asthenosphere. In the Galápagos Archipelago, S-to-p receiver functions reveal a discontinuity in seismic velocity that is attributed to the dehydration of the upper mantle. The depth at which dehydration occurs is shown to be consistent with prior constraints on mantle temperature. A comparison between results from receiver functions, seismic tomography and petrology shows that mantle upwelling and melt generation occur shallower than the depth of the discontinuity, despite the expectation of high viscosities in the dehydrated layer. Beneath the JdF and Gorda Ridge, low Vs anomalies are too large to be explained by the cooling of the lithosphere and are attributed to partial melt. The asymmetry, large Vs gradients, and sinuosity of the anomalies beneath the JdF Ridge are consistent with models of buoyancy-driven upwelling. However, deformation zone processes appear to dominate mantle flow over seafloor spreading beneath the Explorer and Gorda diffuse plate boundaries. Finally, S-to-p receiver functions reveal a seismic discontinuity beneath the JdF plate that can only be attributed to seismic anisotropy. Synthesis of the receiver function results with prior SKS splitting results requires heterogeneous anisotropy between the crust and the discontinuity. Models of anisotropy feature increasing anisotropy before the decrease at the discontinuity, but well below the base of the lithosphere, and a clockwise rotation of the fast direction with increasing depth. In these results and even in the SKS splitting results, additional driving mechanisms for mantle flow such as density or pressure anomalies are required.

In this dissertation, I use seismic imaging methods to constrain the evolution of the oceanic upper mantle across the Juan de Fuca (JdF) and Gorda plates. This work begins by studying the geometry of the mantle magmatic system and patterns of mantle flow beneath the northern JdF ridge in relation to ridge-parallel changes in accretionary processes. I find that the dynamics of lithospheric rifting exert the primary control on the distribution of shallow mantle melts and variations in crustal thickness and composition. The orientation of mantle divergence beneath the JdF ridge, as inferred from seismic anisotropy, is oblique to the overlying plate divergence direction. Similar observations made at the East Pacific Rise and Mid-Atlantic ridge suggest plate motions alone do not control mantle flow patterns. On the contrary, stresses exerted at the base of the plate by the asthenospheric flow field may contribute to changes in plate motion prompting a reorientation of oceanic spreading segments. The mantle anisotropic fabric of the JdF plate interior is then investigated to identify whether the rotated mantle flow field observed beneath the JdF ridge persisted throughout the recent geologic past. However, observations suggest that the anisotropic structure created at the ridge partially reorganizes off-axis obscuring the paleo-flow geometry. Next, I focus on how the physical state of the oceanic lithosphere evolves with time. Using local earthquake arrival times I test whether the seismic velocity structure of the upper mantle lithosphere is thermally controlled or dominated by heterogeneities introduced upon accretion at the ridge or by subsequent deformation off axis. Despite extensive surficial evidence of faulting across the Gorda plate, deformation appears to be restricted to crustal depths and mantle velocities are explained by conductive cooling. In contrast, the velocity structure of the JdF plate is inconsistent with conductively-cooled mantle. Hydration of the mantle lithosphere associated with tectonic discontinuities is invoked to explain anomalously slow P-wave speeds. Lastly, a joint inversion of teleseismic body and surface wave data is proposed to image the geometry of mantle upwelling and melt production beneath the JdF and Gorda Ridges. This dissertation includes previously published and unpublished coauthored material.

A large data set comprising normal-mode eigenfrequencies, quality factors and splitting functions, Earth's mass and moment of inertia, surface-wave phase anomalies and dispersion curves, body-wave arrivals and traveltime curves, as well as long-period waveforms is inverted to obtain the distribution of elastic properties, shear attenuation and density in the Earth's interior. We address three fundamental aspects of global seismology by reconciling and modeling data sets with several methodological improvements, such as accounting for radial and azimuthal anisotropy, development of better methods for crustal corrections, and devising novel regularization and parameterization schemes. In the first contribution, we incorporate normal-mode splitting functions with other seismological data sets to examine the variation of anisotropic shear-wave velocity in the Earth's mantle. Our preferred anisotropic model, S362ANI+M, has strong isotropic velocity anomalies in the transition zone while the anisotropy is restricted to the upper 300~km in the mantle. When radial anisotropy is allowed throughout the mantle, large-scale anisotropic patterns are observed in the lowermost mantle with v_SV > v_SH beneath Africa and South Pacific and v_SH > v_SV beneath several circum-Pacific regions. However, small improvements in fits to the data on adding anisotropy at depth leave the question open on whether large-scale radial anisotropy is required in the transition zone and in the lower mantle. We demonstrate the utility of mode-splitting data in reducing the tradeoffs between even-degree variations of isotropic velocity and anisotropy in the lowermost mantle. We then devise a methodology to detect seismological signatures of chemical heterogeneity using scaling relationships between shear velocity, density and compressional velocity in the Earth's mantle. Several features reported in earlier tomographic studies persist with the inclusion of new and larger data sets; anti-correlation between bulk-sound and shear velocities in the lowermost mantle as well as an increase in velocity scaling (nu=dlnv_S/dlnv_P) with depth in the lower mantle are found to be robust. Many spheroidal and toroidal modes are largely incompatible with perfect correlations between density and shear-velocity variations in the lowermost mantle. A way to fit concurrently the various data sets is by allowing independent density perturbations in the lowermost mantle. Our preferred joint model consists of denser-than-average anomalies (~1% peak-to-peak) at the base of the mantle roughly coincident with the low-velocity superplumes. The relative variation of shear velocity, density and compressional velocity in this study disfavors a purely thermal contribution to heterogeneity in the lowermost mantle. In the third contribution, we introduce an approach to construct a 1-D reference model that is consistent with crustal heterogeneities and various asphericities in the Earth's mantle. We demonstrate that the crust contributes substantially to fundamental-mode dispersion curves when the nonlinear effects of its thickness and velocity variations are taken into consideration. We apply appropriate crustal corrections and perform several iterations to converge to our preferred radial model NREM1D, which is anisotropic in the upper mantle and smooth across the 220-km discontinuity for all physical parameters. Radial anisotropy in the shallowest mantle, with a maximum at ~150~km depth, is required to fit global averages of fundamental-mode Rayleigh and Love wave dispersion (25--250s). NREM1D also predicts arrival times of major mantle and core phases in agreement (+/- 0.5s) with a recent isotropic velocity model that was optimized for earthquake location. The new reference Earth model NREM1D introduced here is easily extendable due to its modular construction as a linear combination of radial basis functions and can be used for earthquake location, spherical-earth normal mode calculations, and as a starting model in studies of lateral heterogeneity.

The loss of ice from the Greenland ice sheet is an important contributor to current and future sea level rise occurring due to ongoing changes in the global climate. A significant portion of this ice mass loss comes through the calving of large icebergs at Greenland’s many marine-terminating outlet glaciers. However, the dynamics of calving at these glaciers is currently not well understood, complicating projections of future behaviour of these glaciers and mass loss from the Greenland ice sheet. The use of seismological tools has shown promise as a means of both monitoring and better understanding the dynamics of the calving process at these glaciers. On the global scale, data from the long-standing global seismic network has recorded the occurrence of glacial earthquakes, large long period earthquakes that occur during large calving events at near-grounded outlet glaciers. The occurrence and source parameters of these earthquakes provide insight into the link between glacier calving and climatic and oceanic forcings, as well as information on the large-scale glacier-dynamic conditions under which these major calving events occur. On the more local scale, a deployment of seismometers around an individual glacier has provided insights on the seismic environment of a calving glacier, as well as the more immediate, short-term external drivers of calving events. We consider both local and global seismic data in order to further understanding of the dynamics of the calving process at Greenland outlet glaciers, and find that glacial earthquake production is indicative of a near-grounded terminus at the source glacier. We find that the locations derived from these events are accurate and are sensitive to changes in the calving-front position of the source glacier, and that the active-force azimuths are representative of the orientation of the glacier at the time of calving. We also find that these glaciers are the source of abundant small icequakes, which are strongly tied to the occurrence of major calving events. The small icequakes that occur at Helheim glacier are modulated by semi-diurnal variations in tide height, and potentially control the timing of major calving events by progressively damaging the glacier tongue.

The largest earthquakes and the majority of the seismic energy are released on megathrust faults in subduction zones. The goal of this dissertation is to characterize the seismic behavior, structural and physical properties of the megathrust fault, so that we can better understand the controls on slip behavior and large earthquakes. To address this goal, I analyzed seismicity data collected by a local seismic network deployed in southern Alaska and multi-channel seismic (MCS) data from an active-source survey offshore of the Alaska Peninsula. This dissertation work revealed seismicity patterns associated with a large asperity, downdip transitions in megathrust fault structure, and along-strike variations in the properties of subducting sediment on the shallow part of the subduction zone. All of these observations have important implications for seismic behavior of the megathrust.

A new onshore-offshore seismic dataset from the Cascadia subduction zone was used to characterize mantle lithosphere structure from the ridge to the volcanic arc, and plate interface structure offshore within the seismogenic zone. The Cascadia Initiative (CI) covered the Juan de Fuca plate offshore the northwest coast of the United States with an ocean bottom seismometer (OBS) array for four years; this was complemented by a simultaneous onshore seismic array. Teleseismic data recorded by this array allows the unprecedented imaging of an entire tectonic plate from its creation at the ridge through subduction initiation and back beyond the volcanic arc along the entire strike of the Cascadia subduction zone. Higher frequency active source seismic data also provides constraints on the crustal structure along the plate interface offshore. Two seismic datasets were used to image the plate interface structure along a line extending 100 km offshore central Washington. These are wide-angle reflections from ship-to-shore seismic data from the Ridge-To-Trench seismic cruise and receiver functions calculated from a densely spaced CI OBS focus array in a similar region. Active source seismic observations are consistent with reflections from the plate interface offshore indicating the presence of a P-wave velocity discontinuity. Until recently, there has been limited success in using the receiver function technique on OBS data. I avoid these traditional challenges by using OBS constructed with shielding deployed in shallow water on the continental shelf. These data have quieter horizontals and avoid water- and sediment-multiple contamination at the examined frequencies. The receiver functions are consistently modeled with a velocity structure that has a low velocity zone (LVZ) with elevated P to S-wave velocity ratios at the plate interface. A similar LVZ structure has been observed onshore and interpreted as a combination of elevated pore-fluid pressures or metasediments. This new offshore result indicates that the structure may persist updip indicating the plate interface may be weak. To focus more broadly on the entire subduction system, I calculate phase velocities from teleseismic Rayleigh waves from 20-100 s period across the entire onshore-offshore array. The shear-wave velocity model calculated from these data can provide constrains on the thermal structure of the lithosphere both prior to and during subduction of the Juan de Fuca plate. Using OBS data in this period band requires removal of tilt and compliance noise, two types of water-induced noise that affect long period data. To facilitate these corrections on large seismic arrays such as the CI, an automated quality control routine was developed for selecting noise windows for the calculation of the required transfer functions. These corrections typically involve either averaging out transient signals, which requires the assumption of stationarity of the noise over the long periods of time, or laborious hand selection of noise segments. This new method calculates transfer functions based on daily time series that exclude transient signals, but allows for the investigation of long-term variation over the course of an instrument’s deployment. I interpret these new shoreline-crossing phase velocity maps in terms of the tectonics associated with the Cascadia subduction system. Major findings include that oceanic plate cooling models do not explain the velocities observed beneath the Juan de Fuca plate, that slow velocities in the forearc appear to be more prevalent in areas modeled to have experienced high slip in past Cascadia megathrust earthquakes, and along strike variations in phase velocity reflect variations in arc structure and backarc tectonics.