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Thesis advisor: John E. Ebel / Passive teleseismic data were collected with a 17-station broadband seismic array deployed from Vermont to Massachusetts. The purpose of the array was to detect changes in crustal seismic velocity structure related to the regional tectonostratigraphic terranes using receiver functions. Ps conversions from the Moho and mid-crust were observed and a cross-section of the crustal structure beneath the seismic array was produced. The crustal cross-section reveals a synclinal structure related to the Taconic orogeny, a remnant Iapetan oceanic slab, a plausible surface-location of the Red Indian Line, and several terrane boundaries that can be projected from their proposed surface locations into the deeper crust based on crustal-horizon offsets observed in the receiver function data. / Thesis (MS) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

A major change in seismic velocities between Earth’s crust and mantle is known as the Mohorovicic discontinuity (Moho). The depth of the Moho plays an important role in characterizing the overall structure of the crust and can be related to the tectonic setting of a region. Teleseismic P-wave receiver function techniques can provide estimates of the depth of the Moho and therefore crustal thickness under a broadband station. In this research we are interested in the structure of the crust and mantle across the northern Canadian cordillera, described by various tectonic settings. The teleseismic data recorded by broadband three-component seismic stations are used to perform receiver function analysis to determine the lateral variations of Moho depth under northern Canadian cordillera and map out the crustal thickness under the broadband stations. Based on visual inspection of receiver function results in the region, we find evidence of anisotropy or dipping reflectors in the crustal structure of the northern cordillera observed in back-azimuthal variations of transverse component receiver functions. We further provide a quantitative interpretation of receiver function in terms of anisotropy or dipping structure by decomposing the azimuthal variations of depth migrated receiver functions into back-azimuthal harmonics. This technique can be used to map out the orientation of anisotropy that may be related to cracks and/or rock texture caused by deformation. We resolve the Moho at an average depth of ~35 km along the western profile of the study area. Harmonic decomposition along the study area yields crustal anisotropy at depth 5-20 km, which does not extend in the lower crust. This can be the result of complex deformation at a detachment zone like a quasi-rigid displacement of the upper crust over a lower crust. The detected anisotropy over the study area is not coherent as the slow symmetry directions detected by harmonic decomposition are highly variable.

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.

Temporal variation of crustal structure is key to our understanding of Earth processes on human timescales. Often, we expect that the most significant structural variations are caused by strong ground shaking associated with large earthquakes, and recent studies seem to confirm this. Here we test the possibility of using P receiver functions (PRF) to isolate structural variations over time. Synthetic receiver function tests indicate that structural variation could produce PRF changes on the same order of magnitude as random noise or contamination by local earthquakes. Nonetheless, we find significant variability in observed receiver functions over time at several stations located in northeastern Honshu. Immediately following the Tohoku-oki earthquake, we observe high PRF variation clustering spatially, especially in two regions near the beginning and end of the rupture plane. Due to the depth sensitivity of PRF and the timescales over which this variability is observed, we infer this effect is primarily due to fluid migration in volcanic regions and shear stress/strength reorganization. While the noise levels in PRF are high for this type of analysis, by sampling small data sets, the computational cost is lower than other methods, such as ambient noise, thereby making PRF a useful tool for estimating temporal variations in crustal structure.

Advances in seismology allow us to obtain "high-resolution" images of the Earth's subsurface. This dissertation summarizes the results of three seismic studies on three different continents, with the aim of better understanding the crust and upper mantle structure of seemingly disparate yet ultimately related regions. The seismic techniques of Pn tomography and P-wave receiver function (RF) analysis are applied to central Turkey (Pn tomography), western Argentina and southwestern Wyoming, USA (RF analysis). These studies look at both a present-day convergent margin (Andean subduction zone, Argentina) and two ancient ones (Bitlis-Zagros collision zone of Arabia-Africa with Eurasia, Turkey; Farallon subduction zone, Wyoming).Using Pn tomography, we were able to detect the limit of the slab rupture edge along the Central Anatolian Fault Zone, Turkey. Slab break-off is an important process that modifies the mantle in tectonically active regions, and the limit of the oceanic Arabian slab break-off along the Bitlis-Zagros Suture Zone, thought to have begun at 11 Ma, was previously undetermined.Using RF analysis, we obtained high-resolution images of the subducting slab beneath the Sierras Pampeanas, Argentina. Continental Moho contours roughly follow terrane boundaries, suggesting that ancient terranes continue to exert control over present-day continental deformation. Overthickened oceanic crust is often cited as a cause of flat slab subduction; our RF results indicate that the crust is moderately overthickened, around 11-16 km. Further, we image offsets in the RF arrivals that indicate the subducted slab is broken or offset in along trench-subparallel fractures.The crustal structure beneath southwestern Wyoming, the location of ancient Farallon flat slab subduction, was studied using RF analysis. Looking at regional crustal structure, results include a new depth to Moho map. Coherency of the seismic signal across the dense LaBarge array (55 stations, ~250 m spacing) was investigated, with results showing that complicated shallow structure can greatly impact the resulting RF signal. Modeling of RFs using synthetics helped to separate the complex signal containing multiple primary conversions and their reverberations, which interact constructively and destructively. The dense spacing of the LaBarge array allowed unique opportunities to investigate coherency of waveforms across very short distances.

The Icelandic mantle plume has had a profound influence on the development of the North Atlantic region over its 64 Myr existence. Long-wavelength free-air gravity anomalies and full waveform tomographic studies suggest that the planform of the plume is highly irregular, with up to five fingers of hot asthenosphere radiating away from Iceland beneath the lithospheric plates. Two of these fingers extend beneath the British Isles and southern Scandinavia, where departures from crustal isostatic equilibrium and anomalous uplift have been identified. In this study, the spatial extent of present-day dynamic support associated with the Icelandic plume is investigated using receiver function analysis. Teleseismic events recorded at nine temporary and 59 permanent broadband, three-component seismometer stations are used to calculate 3864 P-to-S crustal receiver functions. The amplitude and arrival time of particular converted phases are assessed, and H-k stacking is applied to estimate bulk crustal properties. Sub-selections of receiver functions are jointly inverted with Rayleigh wave dispersion data to obtain crustal VS profiles at each station. Both inverse- and guided forward modelling techniques are employed, as well as a Bayesian, trans-dimensional algorithm. Moho depths thus obtained are combined with seismic wide-angle and deep reflection data to produce a comprehensive crustal thickness map of northwestern Europe. Moho depth is found to decrease from southeast (37 km) to northwest (26 km) in the British Isles and from northeast (46 km) to southwest (29 km) in Scandinavia, and does not positively correlate with surface elevation. Using an empirical relationship, crustal shear wave velocity profiles are converted to density profiles. Isostatic balances are then used to estimate residual topography at each station, taking into account these novel constraints on crustal density. Areas of significant residual topography are found in the northwestern British Isles (1400 m), southwestern Scandinavia (464 m) and Denmark (620 m), with convective support from the Icelandic plume as its most likely source. Finally, the irregular planform of the Icelandic plume is proposed to be a manifestation of radial viscous fingering due to a Saffman-Taylor instability. This fluid dynamical phenomenon occurs when less viscous fluid is injected into a layer of more viscous fluid. By comparing the thermal and convective characteristics of the plume with experimental and theoretical results, it is shown that viscous fingering could well explain the present-day distribution of plume material.

The aim of this thesis is to develop procedures for the modelling and inversion of teleseismic P and S waveforms which are as flexible as possible. This flexibility is necessary in order to obtain accurate source depth and mechanism estimates for small to moderate size events, such as those that are relevant in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT). ¶ The main challenge for extending source depth and mechanism inversion methods to smaller events is to ensure that sufficiently accurate synthetic seismograms are available for comparison with observed records. An accurate phase-adaptive reflectivity method has therefore been developed, against which the performance of less computationally intensive approximations can be judged. The standard reflectivity method has been modified to allow for different crustal and upper mantle structures at the source and receiver, and the full effects of reverberations and conversions in these structures can be allowed for. Core reflections and refractions can also be included; these phases can become important at certain distance ranges. A slowness bundle approach has been developed, where a restricted slowness integration about the geometric slowness for the direct wave is undertaken at each frequency, allowing accurate results to be obtained whilst avoiding the expense of a full reflectivity technique. ¶ Inversion using the neighbourhood algorithm (NA) is performed for source depth, mechanism and time function, by modelling direct P and S and their surface reflections (pP, sP and pS, sS) at teleseismic distances. Both SV and SH data are exploited in the inversion, in addition to P data, in order to obtain improved constraints on the source mechanism, including any isotropic component. Good results are obtained using a simple generalised ray scheme, however, the use of a flexible derivative-free inversion method means that more accurate synthetics are able to be used in the inversion where appropriate. The NA makes use of only the rank of the data misfits, so that it is possible to employ any suitable misfit criterion. In the few cases where control on the source mechanism is limited, good depth resolution is still usually obtained. ¶The structures near the source and receiver play an important role in shaping the detail of the teleseismic waveforms. Although reasonable results can be achieved with simple synthetics and a standard velocity model, significant improvement can be made by modifying the representation of structure near the source and receiver. In the case of sub-oceanic events it is important to allow for the effects of water reverberations. The crustal structure near the receiver can also have quite a large influence on the waveforms through reverberations and conversions. This is exploited in receiver function inversion, which is again accomplished using the NA approach.

Receiver functions are a good tool to investigate the seismotectonic structure beneath the a seismic station. In this study we apply the method to stations situated on or near Sumatra to find constraints on a more detailed velocity model which should improve earthquake localisation. We estimate shallow Moho-depths (~ 21 km) close to the trench and depths of ~30 km at greater distances. First evidences for the dip direction of the slab of ~60° are provided. Receiver functions were calculated for 20 stations for altogether 110 earthquakes in the distance range between 30° and 95° from the receiver. However the number of receiver functions per station is strongly variable as it depends on the installation date, the signal-to-noise-ratio of the station and the reliability of the acquisition. / Receiver Funkttion stellen eine gut Methode zur Untersuchung von Seismotektonischen Strukturen unterhalb einer seismischen Station dar. In dieser Arbeit wenden wir die Methode auf Station auf oder nahe Sumatra an um Hinweise für ein detaillierteres Geschwindigkeitsmodell zu erhalten, welches die Lokalisierung von Erdbeben verbessern sollte. Wir ermitteln flache Moho-Tiefen (~21 km) in der Nähe des Trenchs und Tiefen um die 30 km in größeren Distanzen. Erste Hinweise für eine Einfallsrichtung des Slabs von ~60° konnten gefunden werden. Receiver Funktionen wurden für 20 Stationen für insgesamt 110 Erdbeben im Distanzbereich zwischen 30° und 95° berechnet. allerdings ist die Anzahl von Receiver Funktionen pro Station sehr variabel, da sie vom Installationszeitpunkt, dem Signal-Rausch-Verhältnis und der Zuverlässigkeit der Datenaufnahme an der Station abhängt.

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