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Effect of seismic loads on water-retaining structures in areas of moderate seismicityFourie, Johanna Aletta 03 1900 (has links)
Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Water-retaining structures are commonly used in South Africa for the storage of potable water and
waste water. However, a South African code pertaining to the design of concrete water-retaining
structures do not currently exist and therefore use is made of the British Standard BS 8007 (1987).
For the design of concrete water-retaining structures in South Africa, only the hydrostatic loads are
considered while forces due to seismic activity are often neglected even though seismic excitations
of moderate magnitude occur within some regions of the country. Hence, the primary aim of this
study was to determine whether seismic activity, as it occurs in South Africa, has a significant
influence on water-retaining structures and whether it should be considered as a critical load case.
In order to assess the influence of seismic activity on the design of water-retaining structures the
internal forces in the wall and the required area of reinforcement were compared. Comparisons
were made between the seismic analyses and static analyses for both the ultimate and serviceability
limit states. In order to obtain the internal forces in the wall use was made of an appropriate Finite
element model. Three Finite element models were investigated in this study and the accuracy of
each model was assessed based on the fundamental frequency, base shear force and overturning
moment. These values were compared to the values obtained with the numerical method presented
by Veletsos (1997) which was verified with Eurocode 8: Part 4 (2006).
The results obtained indicated that seismic excitations of moderate magnitude do have a significant
influence on the reinforcement required in concrete water-retaining structures. For both the
ultimate limit state and serviceability limit state the required reinforcement increased significantly
when seismic loads were considered in the design. As in the case for static design of water-retaining
structures, the serviceability limit state also dominated the design of these structures under seismic
loading. / AFRIKAANSE OPSOMMING: Beton waterhoudende strukture in Suid-Afrika word op ‘n gereelde basis gebruik vir die stoor van
drink- sowel as afvalwater. ‘n Suid-Afrikaanse kode vir die ontwerp van hierdie strukture bestaan
egter nie en dus word die Britse kode BS 8007 (1987) hiervoor gebruik. Vir ontwerp doeleindes word
soms slegs die hidrostatiese kragte beskou terwyl kragte as gevolg van seismiese aktiwiteite nie
noodwendig in berekening gebring word nie. Seismiese aktiwiteite van gematigde grootte kom egter
wel voor in sekere dele van Suid-Afrika. Die hoofdoel van hierdie studie was dus om die invloed van
seismiese aktiwiteite, soos voorgeskryf vir Suid-Afrikaanse toestande, op beton waterhoudende
strukture te evalueer asook om te bepaal of dit ‘n kritiese lasgevalle sal wees.
Vir hierdie doel is die interne kragte asook die area staal bewapening vir elk van die statiese en
dinamiese lasgevalle vergelyk. Vergelykings is getref tussen die dinamiese en statiese resultate vir
beide die swigtoestand en die diensbaarheidstoestand. Vir die bepaling van die interne kragte is
gebruik gemaak van eindige element modelle. Tydens hierdie studie was drie eindige element
modelle ondersoek en die akkuraatheid van elk geëvalueer op grond van die fundamentele
frekwensie, die fondasie skuifkrag en die omkeermoment. Hierdie waardes was ondermeer bereken
met twee numeriese metodes soos uiteengesit in Veletsos (1997) en Eurocode 8: Part 4 (2006).
Die resultate dui daarop dat die invloed van seismiese aktiwiteite op beton waterhoudende
strukture in Suid-Afrika nie weglaatbaar klein is nie en wel in berekening gebring behoort te word
tydens die ontwerp. Die interne kragte vir beide die swigtoestand en diensbaarheidstoestand is
aansienlik hoër vir die seismiese lasgeval as vir die statiese geval. Die diensbaarheidstoestand het
deurentyd die ontwerp van beton waterhoudende strukture vir seismiese toestande oorheers.
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A relative moment tensor inversion technique applied to seismicity induced by miningAndersen, Lindsay, Marguerite 18 July 2001 (has links)
Three hybrid moment tensor inversion methods were developed for seismic sources
originating from a small source region. These techniques attempt to compensate for
various types of systematic error (or noise) that influence seismograms recorded in the
underground environment in order to achieve an accurate and robust measure of the
seismic moment tensor. The term 'hybrid' was used to distinguish between the
relative method proposed by Dahm (1995) and the methods developed in this thesis.
The hybrid methods were essentially weighting schemes designed to enhance the
accuracy of the computed moment tensors by decreasing the influence of any low
quality observations, to damp (or amplify) any signals that have been overestimated
(or underestimated) due to local site effects, and to correct for raypath focussing or
defocussing that results from inhomogeneities in the rockmass.
The weighting or correction applied to a particular observation was derived from the
residuals determined when observed data were compared with corresponding
theoretical data (for a particular geophone site, sensor orientation and wave phase) and
were calculated using a cluster of events rather than a single event. The first and
second weighting schemes were indirectly related to the mean and the median of the
residuals where the residuals were defined as the ratio of the theoretical to observed
data. In the third scheme, the residuals were defined as the difference between the
observed and theoretical data and the weights were based on the distance of a data
point (measured in standard deviations) from the mean residual. In each of the
weighting schemes, the correction was applied iteratively until the standard error of
the least-squares solution (normalised to the scalar seismic moment) was a minimum.
The schemes were non-linear because new weights were calculated for each iteration.
A number of stability tests using synthetic data were carried out to quantify the source
resolving capabilities of the hybrid methods under various extreme conditions. The
synthetic events were pure double-couple sources having identical fault-plane
orientations, and differing only in rake. This similarity in the mechanisms was chosen
because the waveforms of tightly grouped events recorded underground often show
high degrees of similarity. For each test, the results computed using the three hybrid
methods were compared with one another and with those computed using the single
event, absolute method and two relative methods (with and without a reference
mechanism). In the noise-free situation, it was found that the relative method without
reference mechanism showed the highest resolution of mechanisms, provided that the
coverage of the focal sphere was not too sparse (> 3 stations). The hybrid method
using a median correction was found to be the most robust of all the methods tested in
the most extreme case of poor coverage (2 stations) of the focal sphere.
When increasing levels of pseudo-random noise were applied to the data, the absolute
moment tensor inversion method, the hybrid method using a median correction, and
the hybrid method using a weighted mean correction all showed similar robustness
and stability in extreme configurations concerning network coverage of the focal
sphere and noise level. When increasing levels of systematic noise were added to the
data, the hybrid methods using a median correction and weighted mean correction
were found to exhibit similar robustness and stability in extreme configurations
concerning network coverage of the focal sphere and systematic noise. In all
situations investigated, these two hybrid methods outperformed the relative and
absolute methods.
The hybrid moment tensor inversion methods using a median and weighted mean
correction were applied to a cluster of 14 events, having remarkably similar
waveforms, recorded at Oryx Gold Mine. For comparative purposes, the absolute
method was also applied. The inputs to the inversion methods consisted of the spectral
plateaus of both P- and S-waves at frequencies below the comer frequency of the
time-integrated displacement traces. The polarities of dominant motion were used as
an additional constraint and were determined from cross-correlation of observed with
synthetic P- or S-waves. The solutions computed using the hybrid moment tensor
inversion using a median correction displayed a distinct improvement after the
iterative residual correction procedure was applied. The radiation patterns and faultplane
solutions showed a high degree of similarity, and are probably more accurate
reflections of reality than those computed using the absolute moment tensor inversion
methods. These observations are very encouraging and point towards the method's
potential for use as a standard processing tool for mine seismicity.
The implications of this work are a better understanding of the focal mechanisms of
seismic events induced by mining activities, ultimately leading to improved safety
underground.
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Quelle prédictibilité pour les éruptions volcaniques ? De l'échelle mondiale au Piton de la Fournaise / How predictable are the volcano eruptions? from global scale to Piton de la Fournaise volcano.Schmid, Agathe 20 October 2011 (has links)
Ce travail de thèse a porté sur différents aspects de la prédiction des éruptions. La première partie, basée sur une approche statistique de l'information, nous a permis d'aborder la prédictabilité des éruptions de façon globale, et d'avoir une vision intégrée, à différentes échelles, de la dynamique éruptive. La dernière partie basée sur une approche déterministe du signal, nous a emmené de façon plus indirecte vers la prédiction des éruptions par le biais de l'analyse et la modélisation des comportements éruptifs d'un unique volcan à travers la sismicité. Ce travail de thèse nous a donc permis de traverser plusieurs échelles de temps et d'espace, en partant de la plus large, la Terre dans sa globalité, pour se concentrer ensuite sur le cas d'un volcan unique, le Piton de la Fournaise et finalement se focaliser sur une propriété spécifique de la sismicité du Piton de la Fournaise à travers la détermination des mécanismes source. Dans un premier temps, une analyse descriptive des propriétés statistiques de la dynamique éruptive à l'échelle mondiale et l'apparition de lois puissances à différentes échelles nous ont permis de relier la dynamique des éruptions à celle d'un système critique auto-organisé, justifiant de la difficulté rencontrée à prédire les éruptions de façon déterministe et de la nécessité de se tourner vers les méthodes statistiques. L'étude à l'échelle mondiale des lois d'Omori retrouvées pour la sismicité autour des éruptions, ainsi que l'analyse des propriétés de ces lois d'Omori en fonction de la taille des éruptions, suggèrent que l'énergie d'endommagement et de relaxation dans la croûte est distribuée en loi puissance autour des éruptions, mais avec des coefficients de chargement et de relaxation différents de la sismicité tectonique classique, et donc dépendant des processus qui génèrent l'endommagement. En zoomant sur trois observables du Piton de la Fournaise (sismicité, déformation et variations de vitesse sismique), nous avons montré que les comportement respectifs des observables imagent les différentes étapes du processus éruptif, pour différentes échelles de temps et d'espace. L'analyse des comportements pré-éruptifs et l'utilisation de méthodes de "pattern recognition" ont permis de mettre en évidence l'apport de la combinaison des informations des différents précurseurs dans l'amélioration des résultats de prédiction. Enfin, l'analyse plus spécifique et déterministe des mécanismes source au Piton de la Fournaise, à partir d'une méthode développée pour le contexte particulier d'une sismicité de magnitude très faible en milieu fortement hétérogène, a permis de mettre en évidence l'apparition de trois familles d'événements similaires. Deux de ces familles peuvent être interprétées comme l'ajustement du milieu au passage du dyke, la troisième famille correspondant à la signature de l'interaction entre les fluides et l'édifice volcanique au niveau du toit de la chambre magmatique. / This PhD work focused on several aspects of the eruptions prediction. It allowed us to consider different space scales from global scale to a single volcano, Piton de la Fournaise, through statistical approaches. We then focused more specifically on the volcano-seismicity associated to its eruptive processes, using deterministic methods. First, we analyzed the statistical properties of the eruptive dynamics at the global scale. The emergence of recurrent power laws allowed us to relate the eruptive dynamics to the SOC systems, thereby attesting for the difficulty to predict eruptions by means of deterministic methods. At the global scale, the study of the Omori-law found for eruptions foreshocks and aftershocks, along with the analysis of the parameters of those Omori laws given the size of eruptions, suggests that the damage and relaxation energy in the crust around eruption time and location is power-law distributed. However, the loading and relaxation coefficients are found different than for classical tectonic seismicity, thereby attesting their dependency to the damage processes. When zooming on three observables of Piton de la Fournaise volcano (seismicity, deformation and seismic velocity changes), me showed in a third part that the respective behaviors of the three observables respond to the different steps of the eruptive process, at different time and space scales. The analysis of the pre-eruptive behaviors of those forerunners under pattern recognition techniques enhanced the optimization of the prediction results by combining simultaneously their pre-eruptive information. Lastly, we performed a specific and deterministic study of the volcano-tectonic seismicity of Piton de la Fournaise through the assessment of the source mechanisms of the volcano-tectonic seismicity. Given the complexity of volcanic context, we developed a method to retrieve the source mechanisms of small and high-frequency seismic signals. Using this method three main families of events with different locations and times of occurrence emerge. Two of them may be interpreted as the readjustment of the crust during dyke opening whereas the third family is interpreted as the signature of fluids-rock interactions at the top of the magma Storage zone.
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Rôle des transferts multiples de contraintes, déficit de sismicité et caractéistiques physiques des ruptures sismiques / Role of multiple stress transfers, seismicity deficit and physical property of seismic ruptureKariche, Jugurtha 21 September 2018 (has links)
Cette thèse comprend six chapitres qui décrivent les caractéristiques de la tectonique active et du transfert de contrainte liées aux séismes majeurs. L’objectif étant une meilleure estimation de l’aléa et du risque sismique du nord de l’Algérie et du Maroc. Après un chapitre introductif, le chapitre II présente la méthodologie adoptée pour l’élaboration des modèles de transfert de contrainte. Le chapitre III traite de l’interaction entre failles dans le Tell Atlas algérien. Le chapitre IV développe les aspects de transfert de contrainte et de déformation poroélastique dans le Rif marocain et la mer d’Alboran. Le chapitre V présente la déformation poroélastique sur un plan plus large et les caractéristiques physiques des ruptures sismiques. Le chapitre VI consiste en une conclusion générale avec présentation des principaux résultats incluant les perspectives et suites pour cette recherche. / This thesis consists in six chapters that describe the characteristics of active tectonics and stress transfer related to major earthquakes. The aim of this thesis is a better estimate of the seismic hazard and risk in northern Algeria and Morocco. After an introduction, Chapter II presents the methodology adopted for the development of stress transfer models. Chapter III deals with the interaction between faults in the Algerian Tell Atlas. Chapter IV develops the aspects of stress transfer and poroelastic deformation in the Rif and the Alboran Sea. Chapter V presents a large development of the poroelastic deformation and the physical characteristics of seismic ruptures. Chapter VI consists of a general conclusion with presentation of the main results including the perspectives and futur researchs.
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Ne pas grever l'avenir au bénéfice du présent : Une histoire environnementale de l’extraction du charbon de la fin du 18e siècle à l’Entre-deux-guerres : un développement non soutenable. : L’exemple du Couchant de Mons et du Valenciennois / Ne pas grever l'avenir au bénéfice du présent : an environmental history of coal mining from the end of the 18th century to the inter-war period : an unsustainable development. The examples of the Couchant de Mons and the Valenciennois basins.Troch, Kevin 02 February 2018 (has links)
Ce travail de recherche étudie l’histoire des impacts environnementaux de l’extraction du charbon dans les bassins miniers du Couchant de Mons et du Valenciennois du 18e siècle jusqu’à l’Entre-deux-guerres ainsi que l’émergence de la logique extractiviste en Belgique et en France. Il met en lumière les fondements culturels et les bases scientifiques et législatives qui ont permis l’expansion de l’extraction du charbon dans ces deux pays, notamment au point de vue de la régulation des dégâts miniers. Ensuite les réactions des États, des entreprises minières et des habitants des bassins aux dégradations environnementales causées par l’extraction du charbon sont abordées. Les processus de négociation, les jeux de pouvoir et les mouvements d’opposition au charbon sont au cœur de cette interrogation. Le poids important des charbonnages dans la régulation des dégâts miniers, la volonté des gouvernements à permettre l’extraction du charbon et le développement d’une « guerre contre le charbon » par les habitants des bassins houillers sont analysés à partir de plusieurs situations exemplatives. Enfin, la thèse envisage l’influence des géologues et des ingénieurs des mines dans la création d’une « science des dégâts miniers » à travers trois controverses scientifiques : la séismicité induite, les théories des affaissements miniers et les inondations engendrées par l’extraction de la houille. / This is a study on the history of the environmental impacts of coal mining in the « Couchant de Mons » and the « Valenciennois » basins and the development of extractivism in Belgium and France from the 18th century to the Inter-war Period. It highlights the cultural foundations and the scientific and legal basis explaining the expansion of coal mining in these two countries, especially regarding the regulation of mining damages. Reactions of the States, mining companies and the inhabitants of the basins to the environmental damages caused by coal extraction are also analyzed. Processes of negotiation, power strategies and movements against coal mining are at the heart of this thesis. The heavy weight of collieries in the regulation system of mining damages, the willingness of governments to allow the extraction of coal and the development of a « War against coal » by the inhabitants are analyzed from several exemplary situations. Finally, this work considers the influence of geologists and mining engineers in the creation of a « science of mining damages » through three scientific controversies : induced seismicity, theories on mining subsidence and flooding engendered by coal mining.
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Investigation Of Source Parameters Of Earthquakes In Northern SwedenGonzález-Caneda, María January 2019 (has links)
By studying the frequency domain of seismic signals generated by earthquakes, the source parameters can be recovered, i.e., the seismic moment (M0) and the stress drop (Δσ). This method is an advantage especially since if the source parameters are calculated from the time domain a full waveform inversion is needed, therefore this procedure facilitates the computation. Besides, the moment magnitude (Mw) can be calculated from the seismic moment and, in turn, the local magnitude (ML) can be obtained by using an algorithm that matches different ranges of moment magnitude with their corresponding local magnitude. In the present thesis, small to moderate earthquakes in Northern Sweden have been used to develop a code that calculates the source parameters through the fitting of five different spectral models and, this way, discerns which model obtains the best determination of the parameters. These models have been chosen in a way that we can also extract information about the attenuation. The different models are; the Brune spectral model, Boatwright spectral model, Boatwright spectral model with a fixed fall-off rate, a general form of the spectral model with quality factor equal to 1000 and a general form of the spectral model with quality factor equal to 600. Among these models, the Boatwright model with fixed fall-off rate equal to 2, has been found to give the best fit to the data used in this thesis. This might be due to the regional conditions which are the low attenuation in the crust of northern Fennoscandia and the short hypocentral distances of the studied earthquakes. The earthquakes studied in the present thesis have shown a range of magnitudes from ML 4.2 to -0.2 with radius of an assumed circular fault ranging from 269 m to 66 m.
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Rock mass mechanical behavior in deep mines : in situ monitoring and numerical modelling for improving seismic hazard assessment / Comportement mécanique des massifs rocheux dans les mines profondes : surveillance in situ et modélisation numérique pour l’amélioration de l'évaluation de l'aléa sismiqueDe Santis, Francesca 05 February 2019 (has links)
Afin de mieux comprendre les interactions entre les modifications des contraintes induites par l'exploitation minière et la génération d'activité sismique, une zone profonde de la mine de Garpenberg (Suède) a été instrumentée par l’Ineris avec de sondes microsismiques et de cellules géotechniques. L’analyse spatio-temporelle des événements microsismiques enregistrés entre 2015 et 2016, ainsi que leurs paramètres à la source, ont mis en évidence deux types de réponses sismiques : une locale et courte dans le temps directement induite par les tirs de production, l’autre plus persistante et distante des excavations étant principalement contrôlée par des hétérogénéités géologiques. L’analyse des données géotechniques a montrée l’occurrence de déformations asismiques, ainsi que de phénomènes de fluage induits par l’exploitation. De plus l'activité sismique décroît proportionnellement au taux de diminution des déformations mesurées. Cette dernière observation implique que le fluage peut être un autre mécanisme menant à la sismicité, s’ajoutant au changement de contrainte immédiat induit par les tirs de production. Dans la dernière partie de cette thèse, un modèle géomécanique élasto-plastique 3D a été réalisé et ses résultats ont été comparés aux données géophysiques. Cette comparaison a montré que les modèles numériques à l'échelle de la mine peuvent être des outils puissants pour étudier la sismicité induite à grande échelle. Cependant, il y a certains aspects de la sismicité induite que le modèle ne peut expliquer entièrement. Cela est le cas pour la sismicité déclenchée à distance des excavations, alors que de meilleures corrélations sont trouvées lorsque l'on considère la sismicité à proximité des zones de production. Les résultats de cette thèse ont démontré qu'une approche combinée associent les données sismiques et géotechniques à la modélisation numérique peut améliorer considérablement notre compréhension de la réponse des massifs rocheux à l'exploitation minière. La combinaison de ces méthodologies dans une approche intégrée peut réduire considérablement leurs limites explicites qui sont évidents lorsque ces instruments sont considérés séparément. / With the aim of better understanding interactions between stress modifications induced by mining and the generation of seismic activity, a deep area of Garpenberg mine (Sweden) was instrumented by Ineris with microseismic probes and geotechnical cells. Spatiotemporal analysis of recorded seismicity between 2015 and 2016, as well as seismic source parameters, have highlighted two types of seismic rock mass responses: one local and temporally short directly induced by production blasts, the other long-lasting over time and remote from excavations being mainly controlled by geological heterogeneities. Geotechnical data analysis showed the occurrence of aseismic deformations, as well as creep phenomena induced by mining exploitation. In addition, seismic activity decays proportional to the decaying rate of measured strains. This latter observation implies that creep may be another mechanism driving seismicity, in addition to the immediate stress change induced by blasting. In the last part of this thesis, a 3D elasto-plastic geomechanical model has been realized and its results have been compared with geophysical data. This comparison showed that mine-wide numerical models can be suitable for the analysis of mining induced seismicity at large-scale. However, there are some aspects of the induced seismicity that the model cannot fully explain. This is particularly true for remote seismicity occurring at a distance from excavations, while better correlations are found when considering seismicity close to production areas. Results of this thesis demonstrated that a combined approach which associates seismic and geotechnical data with numerical modelling can significantly improve our understanding of the rock mass response to mining. The combination of these methodologies in an integrated approach can significantly reduce their straightforward limitations, which appears evident when these instruments are considered separately.
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Apport des données hydroacoustiques pour l'étude de la sismicité de la dorsale médio-Atlantique nord / Hydroacoustic data contribution for the long term seismicity study of the northern Mid-Atlantic RidgeGiusti, Marion 08 March 2019 (has links)
Les dorsales océaniques sont caractérisées par une sismicité de faible magnitude, induite par une succession d’épisodes volcaniques et tectoniques. Le déploiement de réseaux d’hydrophones autonomes le long de grandes sections de dorsales permet d’enregistrer cette sismicité peu détectable par les stations terrestres. Dans l’océan Atlantique Nord, de nombreuses expériences hydroacoustiques ont été conduites depuis 1999, donnant accès à des séries d’observations temporelles sur plusieurs années. La présente thèse s’inscrit dans ce contexte, avec l’acquisition et le traitement de nouvelles données. L’analyse de catalogues télésismiques et hydroacoustiques, couplés à des données bathymétriques et gravimétriques, a contribué à la caractérisation des processus d’accrétion actifs le long de la dorsale médio-Atlantique Nord. Les travaux réalisés lors de cette thèse ont permis de : (1) rechercher des paramètres responsables d’une différence de sismicité entre des groupes de segments de la dorsale médio-Atlantique ; (2) proposer une nouvelle limite sud d’influence du point chaud des Açores ; (3) identifier et caractériser différents types de processus d’accrétion à partir de la recherche de crises sismiques ; (4) mettre en évidence une crise magmatique de grande ampleur et une intrusion magmatique à travers une discontinuité non-transformante ; et (5) proposer un schéma de répartition des processus d’accrétion dominants le long de l’axe de la dorsale médio-Atlantique Nord, à partir d’une analyse spatiotemporelle de la sismicité. L’ensemble de ces résultats montre l’importance des catalogues de sismicité à grande étendue spatiale et temporelle pour approfondir notre connaissance : sur la dynamique des segments de dorsale et les interactions entre l’axe de la dorsale et le point chaud, et d’autre part, sur la récurrence des processus d’accrétion. / The mid-ocean ridge is caracterised by a seismicity of low magnitude generated by multiple volcanic and tectonic episodes. Autonomous hydrophones arrays along large ridge sections are efficient to record low seismicity contrary to landbased stations. In the northern Atlantic Ocean, several hydroacoustic experiments have been realised since 1999 that supply long term records.This thesis is based on the acquisition and the treatment of new data. The analysis of teleseismic and hydroacoustic catalogues combined with bathymetric and gravimetric data, have contributed to the characterisation of accretionary processes along the northern Mid-Atlantic Ridge. This PhD work allowed : (1) to research parameters responsable of a seismicity difference between groups of Mid-Atlantic Ridge segments; (2) to propose a new southern limit of the Azores hotspot influence ; (3) to identify and define different types of seafloor spreading processes based on seismic crisis interpretations ; (4) to highlight a large magmatic crisis and a magmatic intrusion across a non-transform discontinuity ; and (5) to propose dominant seafloor spreading processes from a spatio-temporal seismicity analysis along the northern Mid-Atlantic Ridge. The entire study shows the value of large spatial and temporal seismicity catalogues to broaden our knowledge on : the dynamic of the ridge segments, the interactions between the ridge axis and the hotspot and the recurrence of the seafloor spreading processes.
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Tectonic History and Present-Day Deformation in the Zagros Fold-Thrust BeltHessami, Khaled January 2002 (has links)
<p>This thesis uses various approaches such as observation of satellite images, field investigations, analogue modeling and GPS measurements to constrain deformation of the basement and sedimentary cover of the Zagros fold-thrust belt in time and space.</p><p>Focal mechanism solutions of most earthquakes indicate that deformation in the Zagros basement is due to shortening and thickening through numerous thrust faults. However, observations of strike-slip faulting recognized on satellite images imply that N-S trending faults in the Zagros, inherited from Pan-African basement, rotated about vertical axes to accommodate the convergence between Arabia and central Iran.</p><p>Field studies suggest that southwestward advance of the Zagros front has been recorded by syn-sedimentary structures. These structures indicate that deformation started as early as end Eocene in the northeast of the Simply Folded Zone and propagated progressively to the southwest. The deformation front drove the foreland basin to its present position along the Persian Gulf and Mesopotamia. </p><p>Scaled analogue models suggest that the seismicity due to orogenic shortening depends largely on the friction between the cover and its basement. Models show that fold-thrust belts with low tapers shortened above low friction ductile decollements involve several long-lived thrust faults generating low to moderate earthquakes over wide areas at the same time. By contrast, earthquakes with larger magnitudes are expected to occur along a few short-lived thrust ramps in fold-thrust belts with larger tapers shortened above high-friction decollments.</p><p>GPS-derived velocities across and along the Zagros suggest that only about one third (10 ± 3 mm/yr) of the current convergence between Arabia and Eurasia is accommodated within the Zagros by thickening to the east of the Kazerun Fault and thickening and lateral movement to the west. The remaining (21 ± 3 mm/yr) is transferred beyond the Zagros suture to central Iran and the northern Iranian mountains.</p>
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Critical states of seismicity : modeling and data analysisZöller, Gert January 2005 (has links)
The occurrence of earthquakes is characterized by a high degree of
spatiotemporal complexity. Although numerous patterns, e.g. fore- and aftershock sequences, are well-known, the underlying mechanisms are not observable and thus not understood. Because the recurrence times of large earthquakes are usually decades or centuries, the number of such events in corresponding data sets is too small to draw conclusions with reasonable statistical significance. Therefore, the present study combines both, numerical modeling and analysis of real data in order to unveil the relationships between physical mechanisms and observational quantities. The key hypothesis is the validity of the so-called "critical point concept" for earthquakes, which assumes large earthquakes to occur as phase transitions in a spatially extended many-particle system, similar to percolation models. New concepts are developed to detect critical states in simulated and in natural data sets. The results indicate that important features of seismicity like the frequency-size distribution and the temporal clustering of earthquakes depend on frictional and structural fault parameters. In particular, the degree of quenched spatial disorder (the "roughness") of a fault zone determines whether large earthquakes occur quasiperiodically or more clustered. This illustrates the power of numerical models in order to identify regions in parameter space, which are relevant for natural seismicity. The critical point concept is verified for both, synthetic and natural seismicity, in terms of a critical state which precedes a large earthquake: a gradual roughening of the (unobservable) stress field leads to a scale-free (observable) frequency-size distribution. Furthermore, the growth of the spatial correlation length and the acceleration of the seismic energy release prior to large events is found. The predictive power of these precursors is, however, limited. Instead of forecasting time, location, and magnitude of individual events, a contribution to a broad multiparameter approach is encouraging. / Das Auftreten von Erdbeben zeichnet sich durch eine hohe raumzeitliche
Komplexität aus. Obwohl zahlreiche Muster, wie Vor- und Nachbeben bekannt
sind, weiß man wenig über die zugrundeliegenden Mechanismen, da diese sich
direkter Beobachtung entziehen. Die Zeit zwischen zwei starken Erdbeben in
einer seismisch aktiven Region beträgt Jahrzehnte bis Jahrhunderte. Folglich
ist die Anzahl solcher Ereignisse in einem Datensatz gering und es ist
kaum möglich, allein aus Beobachtungsdaten statistisch signifikante Aussagen
über deren Eigenschaften abzuleiten. Die vorliegende Arbeit nutzt daher
numerische Modellierungen einer Verwerfungszone in Verbindung mit
Datenanalyse, um die Beziehung zwischen physikalischen Mechanismen und
beobachteter Seismizität zu studieren. Die zentrale Hypothese ist die
Gültigkeit des sogenannten "kritischen Punkt Konzeptes" für Seismizität,
d.h. starke Erdbeben werden als Phasenübergänge in einem räumlich ausgedehnten
Vielteilchensystem betrachtet, ähnlich wie in Modellen aus der statistischen
Physik (z.B. Perkolationsmodelle). Es werden praktische Konzepte entwickelt,
die es ermöglichen, kritische Zustände in simulierten und in beobachteten
Daten sichtbar zu machen. Die Resultate zeigen, dass wesentliche
Eigenschaften von Seismizität, etwa die Magnitudenverteilung und das
raumzeitliche Clustern von Erdbeben, durch Reibungs- und
Bruchparameter bestimmt werden. Insbesondere der Grad räumlicher Unordnung
(die "Rauhheit") einer Verwerfungszone hat Einfluss darauf, ob starke
Erdbeben quasiperiodisch oder eher zufällig auftreten. Dieser Befund zeigt
auf, wie numerische Modelle genutzt werden können, um den Parameterraum für
reale Verwerfungen einzugrenzen.
Das kritische Punkt Konzept kann in synthetischer und in
beobachteter Seismizität verifiziert werden. Dies artikuliert sich auch in
Vorläuferphänomenen vor großen Erdbeben: Die Aufrauhung des
(unbeobachtbaren) Spannungsfeldes führt zu einer Skalenfreiheit der
(beobachtbaren) Größenverteilung; die räumliche Korrelationslänge
wächst und die seismische Energiefreisetzung wird beschleunigt. Ein starkes
Erdbeben kann in einem zusammenhängenden Bruch oder in einem unterbrochenen
Bruch (Vorbeben und Hauptbeben) stattfinden. Die beobachtbaren Vorläufer
besitzen eine
begrenzte Prognosekraft für die Auftretenswahrscheinlichkeit starker
Erdbeben - eine
präzise Vorhersage von Ort, Zeit, und Stärke eines
nahenden Erdbebens ist allerdings nicht möglich. Die
genannten Parameter erscheinen eher vielversprechend als Beitrag zu einem
umfassenden Multiparameteransatz für eine verbesserte zeitabhängige
Gefährdungsabschätzung.
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