Detection, Containment and Scaling Relations of Near Source Explosionsin Granite Through Moment Tensor Representations

The Source Phenomenology Experiment (SPE) was a series of nine, single-fired chemical explosions within the Morenci Copper mine in Arizona. Its purpose was to design, detonate, record and analyze seismic waveforms from these single-fired, partially and fully contained explosions. Ground motion data from the SPE are analyzed in this study to assess the uniqueness of the source representation of these explosions and its ability to resolve yield and depth when containment and geology or physical parameters of the source region may have a range of possible values. The P-wave velocities (Vp) at the test site are well constrained by seismic refraction surveys, but the accompanying shear wave velocities (Vs) are less constrained. In order to assess the effects of source depth and Vs model on the seismic moment tensors, Green’s functions were computed for different source depths as well as different Vs models, holding the Vp model constant. The Green’s functions for the 16, near-source stations were calculated using a one-dimensional velocity model developed from the SPE employing reflectivity modeling in order to include spherical wave effects, body waves and surface waves, focusing on observations in the 37-680 m range. The compensated linear vector dipole and explosion components of the Green’s functions are compared to quantify the possible effects of source depth and Vs on the source representation on expected explosion contributions. For the forward model, Green’s functions with variable depths of burial (DOB) and Vs are convolved with a time function based on the Mueller-Murphy (1971) isotropic source function produce synthetic seismograms for assessing possible tradeoffs between depth and yield in the source models. Our study suggests that the original SPE model parameter values used are most representative of the geology. Subsequently, observational data inversions are conducted within the frequency domain and moment tensors are decomposed into deviatoric and isotropic components to evaluate the effects of containment and yield on the resulting source representation. Isotropic moments are compared to those for other contained explosions as reported by Denny and Johnson (1991) and are in good agreement with their scaling results. Isotropic and Mzz moment tensor spectra are compared to Mueller-Murphy (1971), Denny-Johnson (1991) and revised Heard-Ackerman (Patton, 2012 b) models and suggest that the larger yield explosions with the most confinement fit the models best. Secondary source effects resulting from free surface interactions, including the effects of spallation, contribute to the resulting moment tensors, which include a CLVD component. Hudson diagrams, using frequency domain moment tensors, are computed as a tool to assess how these containment scenarios affect the source representation. Our analysis suggests that, within our band of interest (2-20 Hz), as the frequency increases, the source representation becomes more explosion like, peaking at around 20 Hz. These results guide additional analysis of the observational data and the practical resolution of physical phenomenology accompanying underground explosions

Development of a Python Library for Processing Seismic Time Series

Earthquakes occur around the world every day. This natural phenomena can result in enormous destruction and loss of life. However, at the same time, it is the primary source for studying Earth, the active planet. The seismic waves generated by earthquakes propagate deep into the Earth, carrying considerable information about the Earth’s structure, from the shallow depths in the crust to the core. The information transferred by seismic waves needs advanced signal processing and inversion tools to be converted into useful information about the Earths inner structures, from local to global scales. The ever­evolving interest for investigating more accurately the terrestrial system led to the development of advanced signal processing algorithms to extract optimal information from the recorded seismic waveforms. These algorithms use advanced numerical modeling to extract optimal information from the different seismic phases generated by earthquakes. The development of algorithms from a mathematical­physical point of view is of great interest; on the other hand, developing a platform for their implementation is also significant. This research aims to build a bridge between the development of purely theoretical ideas in seismology and their functional implementation. In this dissertation SeisPolPy, a high quality Python­based library for processing seismic waveforms is developed. It consists of the latest polarization analysis and filter algorithms to extract different seismic phases in the recorded seismograms. The algorithms range from the most common algorithms in the literature to a newly developed method, sparsity­promoting time­frequency filtering. In addition, the focus of the work is on the generation of high­quality synthetic seismic data for testing and evaluating the algorithms. SeisPolPy library, aims to provide seismology community a tool for separation of seismic phases by using high­resolution polarization analysis and filtering techniques. The research work is carried out within the framework of the Seismicity and HAzards of the sub­saharian Atlantic Margin (SHAZAM) project that requires high quality algorithms able to process the limited seismic data available in the Gulf of Guinea, the study area of the SHAZAM project.Terramotos ocorrem todos os dias em todo o mundo. Esta fenomeno natural pode vir a resultar numa enorme destruição e perda de vidas. No entanto, ao mesmo tempo, é a principal fonte para o estudo da Terra, o planeta activo. As ondas sísmicas geradas pelos terramotos propagam­se profundamente na Terra, levando informação considerável sobre a estrutura da Terra, desde as zonas de menor profundidade da crosta até ao núcleo. A informação transferida por ondas sísmicas necessita de processamento avançado de sinais e ferramentas de inversão para ser convertida em informação util sobre a estrutura interna da Terra, desde escalas locais a globais. O interesse sempre crescente em investigar com maior precisão o sistema terrestre levou ao desenvolvimento de algoritmos avançados de processamento de sinais para extrair informação óptima das formas de ondas sísmicas registadas. Estes algoritmos fazem uso de modelos numéricos avançados para extrair informação óptima das diferentes fases sísmicas geradas pelos terramotos. O desenvolvimento de algoritmos de um ponto de vista matemático­físico é de grande interesse; por outro lado, o desenvolvimento de uma plataforma para a sua implementação é também significativo. Esta investigação visa construir uma ponte entre o desenvolvimento de ideias puramente teóricas em sismologia e a sua implementação funcional. Com o decorrer desta dissertação foi desenvolvido o SeisPolPy, uma biblioteca de alta qualidade baseada em Python para o processamento de formas de ondas sísmicas. Consiste na mais recente análise de polarização e algoritmos de filtragem para extrair diferentes fases sísmicas nos sismogramas registados. Os algoritmos variam desde os algoritmos mais comuns na literatura até um método recentemente desenvolvido, que promove a frequência de filtragem por tempo e frequência. Além disso, o foco do trabalho é a geração de dados sísmicos sintéticos de alta qualidade para testar e avaliar os algoritmos. A biblioteca SeisPolPy, visa fornecer à comunidade sismológica uma ferramenta para a separação das fases sísmicas, utilizando técnicas de análise de polarização e filtragem de alta resolução. O trabalho de investigação é realizado no âmbito do projecto SHAZAM que requer algoritmos de alta qualidade que possuam a capacidade de processar os dados sísmicos, limitados, disponíveis no Golfo da Guiné, a área de estudo do projecto

Regional Seismic Discriminants Using Wave-Train Energy Ratios

We have examined broadband regional waveforms of recent (since 1988) Nevada Test Site (NTS) underground explosions and earthquakes throughout the southwestern United States and Baja, Mexico, recorded by TERRAscope and other IRIS stations in order to characterize seismic sources for the purposes of event identification. As expected, earthquakes tended to be richer in long-period surface-wave and short-period shear-wave energy relative to explosions of comparable P-wave strength. Also, explosions, in general, were found to be richer in 1- to 6-sec surface-wave (Rg) energy and other late-arriving coda energy than were earthquakes. Most earthquakes show relatively little long-period (T > 6 sec) Rg and surface-wave coda energy, which we attribute to their deeper source depths, whereas known shallow earthquakes do exhibit these phases. We have developed several seismic discriminants based on our observations. The most promising discriminant is the ratio of short-period (f ≧ 1.0 Hz), vertical component, P_(nl) wave-train energy (E_(spPz)) to long-period (0.05 to 0.167 Hz), three-component, surface-wave energy (E_(lp−3)). For this ratio, explosions tend to have a higher value than do earthquakes. This discriminant works on the same premise as the teleseismic m_b:M_S ratio, for which earthquakes are richer in long-period surface-wave energy relative to explosions with the same body-wave magnitude. The long-period passband was chosen to limit the effect of longer-period noise and to remove the effect of the coda surface waves. Another potential discriminant examined is the ratio of short-period (f ≧ 1.0 Hz), vertical-component, P-wave to S-wave energy (E_(spPz):E_(spSz)). We find that this criterion only yields marginal separation of the source populations but becomes more effective at higher frequency bands (f ≧ 4.0 Hz) or when looking at single-station observations. It does, however, help to quantify significant short-period waveform differences between the three test subsites, with Pahute Mesa shots generating relatively little S-wave energy compared to those of Yucca Flat for which the S wave (or Lg) is often the largest phase, while Rainier's shots are intermediate in character with distinct but less prominent S waves. This S-wave generation is thought to be caused by near-source scattering to converted phases and appears to be highly dependent on the near-source geology. These two discriminants are useful in that they are simple and fast to calculate. Using regional stations for sources 200 to 1300 km away, the magnitude threshold for the E_(spPz):E_(lp−3) discriminant is roughly M_L ≧ 4.0, the limiting factor being the signal level of the Airy phase, while that for the E_(spPz):E_(spSz) discriminant is roughly M_L ≧ 3.0 for the same distance ranges

Seismic Imaging and Salt Tectonics of the Mediterranean Salt Giant in the Central Algerian Basin

The Mediterranean Salt Giant (MSG) is a thick layer of Messinian evaporites (up to 4 km) that is thought to be deposited during an extreme paleo-environmental event known as the Messinian Salinity Crisis (MSC). After decades of research, there is not yet a consensual model explaining the emplacement and the evolution of the MSG. This is due to the absence of samples of the deep offshore MSG. Past scientific drilling operations were limited to the topmost MSG records because of the risks of intersecting zones of hazardous fluids and overpressure linked to evaporites. The European project SALTGIANT, in which the work of this thesis fits into, is dedicated to understanding the formation of the MSG and its implications for the microbial life, the drilling hazards and the geo-economics of the Mediterranean region and the history of oceanography. In that framework, this thesis aims to improve our seismic images of the offshore MSG and to use the new results to update our understanding of the Messinian salt tectonics. This work is focused on the Algerian basin, in the south-western Mediterranean Sea, where the salt was deposited in an already contractional tectonic setting. I compile, reprocess, and interpret legacy academic seismic data acquired in the central Algerian basin. The re-processing is designed to improve as much as possible the salt and pre-salt structures. It relies on an integrated approach combining geophysics and geological interpretation to iteratively build the velocity model. The new results display a better imaging of salt structures and the seismic facies variations. They shed a new light on the tectono-sedimentary evolution of the central Algerian basin, highlighting the presence of seismic fluid indicators evidencing an active fluid circulation in the basin and its margins. Interpretation of the new seismic sections is done following the most recent nomenclature of the MSC seismic markers. New isochores and thickness maps are produced and compared with the spatial distribution of the salt structures. I interpret contractional salt tectonic structures, such as buckle folds, squeezed diapirs and related salt sheets as evidence of regional thick-skinned shortening episodes. I suggest that extensional stage of the salt system (where the deformation is driven by gravity loading) was short-lived, and that many salt structures were driven by contractional tectonic loading during the Plio-Quaternary. I demonstrate that the initial shortening-related salt deformation in the late Messinian was focussed along the Algerian margin and later shifted outward toward the Balearic margin in the Plio-Quaternary. The shifting of the deformation front is interpreted to be a result of the thickening and strengthening of the overburden. The second peak of deformation may have reactivated faults along the Emile-Baudot escarpment with thick-skinned deformation. I also observe a variation in the intensity of the salt deformation along the margin from SW to NE, which I associate to variable tectonic loading applied along the Algerian margin or the pre-shortening distribution of salt. Fluid indicators are imaged within the Plio-Quaternary of the Algerian basin. They could be thermogenic or biogenic gas sourced from the Messinian Upper Unit, or from the pre-salt, migrating through a hydro-fractured salt. The new results also evidence numerous volcanic structures within the Formentera basin. The distribution of this volcanic edifice could affect fluid circulation, resulting in small-wavelength surface HF anomalies observed locally