Direct P-wave anisotropy measurements at Homestake Mine: implications for wave propagation in continental crust

We measured anisotropic seismic properties of schists of the Homestake Formation located at a depth of 1478 m in the Sanford Underground Research Facility (SURF) in the Black Hills of South Dakota, USA. We deployed a 24-element linear array of three-component geophones in an area in the Homestake Mine called 19-ledge. An airless jackhammer source was used to shoot two profiles: (1) a walkaway survey to appraise any distance dependence and (2) a fan shot profile to measure variations with azimuth. Slowness estimates from the fan shot profile show a statistically significant deviation with azimuth with the expected 180° variation with azimuth. We measured P-wave particle motion deviations from data rotated to ray coordinates using three methods: (1) a conventional principal component method, (2) a novel grid search method that maximized longitudinal motion over a range of search angles and (3) the multiwavelet method. The multiwavelet results were computed in two frequency bands of 200–600 and 100–300 Hz. Results were binned by azimuth and averaged with a robust estimation method with error bars estimated by a bootstrap method. The particle motion results show large, statistically significant variations with azimuth with a 180° cyclicity. We modelled the azimuthal variations in compressional wave speed and angular deviation from purely longitudinal particle motion of P-waves using an elastic tensor method to appraise the relative importance of crystalline fabric relative to fracturing parallel to foliation. The model used bulk averages of crystal fabric measured for an analogous schist sample from southeast Vermont rotated to the Homestake Formation foliation directions supplied by SURF from old mine records. We found with average crustal crack densities crack induced anisotropy had only a small effect on the observables. We found strong agreement in the traveltime data. The observed amplitudes of deviations of P particle motion showed significantly larger variation than the model predictions and a 20° phase shift in azimuth. We attribute the inadequacies of the model fit to the particle motion data to inadequacies in the analogue rock and/or near receiver distortions from smaller scale heterogeneity. We discuss the surprising variability of signals recorded in this experimental data. We show clear examples of unexplained resonances and unexpected variations on a scale much smaller than a wavelength that has broad implications for wave propagation in real rocks

Fault Zone Structure and Rupture Behavior with Fiber-Optic Sensing and Second Moments

The structure of fault zones and the behavior of ruptures are indivisible. Fault structure is molded by the continued overprinting of slip events, and rupture propagation is highly sensitive to fault zone parameters. Observational constraints on both fault zone characteristics and the behavioral response of ruptures to fault variability are thus needed to understand earthquakes. Fault zones are narrow structures that are difficult to image in detail, particularly at depth. This means that fault structure is often oversimplified in rupture models and inversions. Earthquake source descriptions are frequently high dimensional. Fault slip distributions are often complicated and nonunique and seismicity catalogs can contain hundreds of thousands of events. This complexity can be difficult to reduce for the purpose of making clear conclusions on earthquake phenomenology. In this sense, observations of fault structure may benefit from a dimensionality expansion and observations of earthquakes may benefit from a dimensionality reduction. In Chapters 2-5 of this thesis I address the former problem. I show how an emergent technology, distributed acoustic sensing (DAS), that transforms fiber optic cables into dense arrays of strainmeters can be used to resolve fault zone characteristics with astonishing detail. This applies to small and large faults at shallow and deep depths. I define a framework for partitioning the seismic wavefield and show that scattered phases in earthquake wavefields encode information on the location and dimensions of small faults. I then investigate the Garlock Fault with an intersecting DAS array and uncover structural variability across the fault at shallow and seismogenic depths. I also use this array to investigate Moho topography, and find that the Garlock Fault offsets and, by extension, penetrates the mantle over a narrow width. In Chapters 6-8 of this thesis I address the latter problem. I show that second moments, both of source and seismicity distributions, can improve the clarity of observations and make drawing meaningful conclusions about rupture behavior possible. I first develop a framework to probabilistically invert for the second moments of source distributions and use it to investigate all large strike-slip events of the past three decades. These solutions show several patterns between faults and rupture behavior. I also create a seismicity catalog for the Ridgecrest earthquake sequence and use a second moment measure to constrain the evolution of fault orientation and the stress state

New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus

Enantiornithines were the most diverse group of birds during the Cretaceous, comprising over half of all known species from this period. The fossil record and subsequently our knowledge of this clade is heavily skewed by the wealth of material from Lower Cretaceous deposits in China. In contrast, specimens from Upper Cretaceous deposits are rare and typically fragmentary, yet critical for understanding the extinction of this clade across the K-Pg boundary. The most complete North American Late Cretaceous enantiornithine is Mirarce eatoni, a member of the diverse clade Avisauridae. Except for Mirarce, avisaurids are known only from isolated hindlimb elements from North and South America. Here we describe three new enantiornithines from the Maastrichtian Hell Creek Formation, two of which represent new avisaurid taxa. These materials represent a substantial increase in the known diversity of Enantiornithes in the latest Cretaceous. Re-examination of material referred to Avisauridae through phylogenetic analysis provides strong support for a more exclusive Avisauridae consisting of six taxa. Exploration of the functional morphology of the avisaurid tarsometatarsus indicates potential strong constriction and raptorial attributes. The lower aspect ratio of the tarsometatarsus facilitates a more biomechanically efficient lever system which in extant birds of prey equates to lifting proportionally heavier prey items. In addition, the proportional size and distal position of the m. tibialis cranialis tubercle of the tarsometatarsus is similar to the morphology seen in extant birds of prey. Together with the deeply-grooved metatarsal trochlea facilitating robust and likely powerful pedal digits, morphologies of the hindlimb suggest avisaurids as Late Cretaceous birds of prey

A second Cretaceous ornithuromorph bird from the Changma Basin, Gansu Province, Northwestern China

Finely-bedded lacustrine deposits of the Aptian (Lower Cretaceous) Xiagou Formation exposed in the Changma Basin of Gansu Province, northwestern China, have yielded numerous fossil vertebrate remains, including approximately 100 avian specimens. Though the majority of these birds appear referable to the ornithuromorph Gansus yumenensis, a number of enantiornithine fossils have also been recovered. Here we report on a specimen consisting of a complete, three-dimensionally preserved sternum, furcula, and sternal ribs that represents a second ornithuromorph taxon from the Xiagou Formation at Changma. The fossil exhibits morphologies that distinguish it from all previously-known Xiagou birds and demonstrate that it represents a derived non-ornithurine member of Ornithuromorpha. Though it is morphologically distinct from the equivalent elements of all other described ornithuromorphs, the material is too incomplete to justify the erection of a new taxon. Nonetheless, it increases the taxonomic diversity of the Xiagou avifauna, thereby expanding our knowledge of Early Cretaceous avian diversity and evolution

The most complete enantiornithine from North America and a phylogenetic analysis of the Avisauridae

The most complete known North American enantiornithine was collected in 1992 but never formally described. The so-called “Kaiparowits avisaurid” remains one of the most exceptional Late Cretaceous enantiornithine fossils. We recognize this specimen as a new taxon, Mirarce eatoni (gen. et sp. nov.), and provide a complete anatomical description. We maintain that the specimen is referable to the Avisauridae, a clade previously only known in North America from isolated tarsometatarsi. Information from this specimen helps to clarify evolutionary trends within the Enantiornithes. Its large body size supports previously observed trends toward larger body mass in the Late Cretaceous. However, trends toward increased fusion of compound elements across the clade as a whole are weak compared to the Ornithuromorpha. The new specimen reveals for the first time the presence of remige papillae in the enantiornithines, indicating this feature was evolved in parallel to dromaeosaurids and derived ornithuromorphs. Although morphology of the pygostyle and (to a lesser degree) the coracoid and manus appear to remain fairly static during the 65 million years plus of enantiornithine evolution, by the end of the Mesozoic at least some enantiornithine birds had evolved several features convergent with the Neornithes including a deeply keeled sternum, a narrow furcula with a short hypocleidium, and ulnar quill knobs—all features that indicate refinement of the flight apparatus and increased aerial abilities. We conduct the first cladistic analysis to include all purported avisuarid enantiornithines, recovering an Avisauridae consisting of a dichotomy between North and South American taxa. Based on morphological observations and supported by cladistic analysis, we demonstrate Avisaurus to be paraphyletic and erect a new genus for “A. gloriae,” Gettyia gen. nov

A Geometric Morphometric and Histological Study of the Role of Development in Avialan Evolution

In order to fully understand the processes that influence phenotypic evolution, it is necessary to explore the role that developmental channeling plays in shaping morphology. In my doctoral research, I have investigated how an increased understanding of ontogenetic patterns gives insight into the evolution of birds. Here, I present the results of an integrative study combining material from multiple levels of the biological hierarchy (clade, individual organism, and cell), using both paleontological and neontological data. Specifically, I studied post-natal gross morphological change and cellular growth in a phylogenetically broad sampling of Cretaceous and modern avians, using ontogenetic series when available. A three-dimensional geometric morphometric analysis of ontogenetic cranial shape change indicates that a relatively high level of developmental constraint governs the evolution of the skull, although shape at onset of growth is a more variable characteristic, in large part because hatchling chicks already have cranial morphology very similar to that of adults. Histological study of extant bird skeletons also indicates high variability in onset of growth among taxa and reveals a common pattern of greater skeletal maturity of the femur relative to the humerus at the time of hatching. This is interpreted as an example of a ‘spandrel,’ later exapted by semi-precocial taxa in which pelvic limbs appear to achieve functional maturity before pectoral limbs for adaptive reasons. Histological analysis of Cretaceous avians indicates this trait of differential maturity between limb elements may have a very deep evolutionary origin, and could explain substantial microstructural differences that persist into adulthood in some extinct taxa. I focused in particular on growth in members of the Enantiornithes, and further conclude that this group exhibits a greater diversity of growth strategies than originally hypothesized, although with no apparent relationship to body size or geological age, and that this clade had a unique developmental mode not directly comparable that of to any crown-group birds. Finally, phylogenetic and functional constraints appear to have a much greater influence on cellular growth of long bones than on cranial shape change, for which developmental channeling is a primary influence