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The Nazca-South America convergent margin is marked by the presence of the Andean mountain belt, which stretches along the 8000-km long western margin of the South American plate. The subduction zone is characterized by significant along-strike changes in both upper plate structure and slab geometry that make it an ideal region to study the relationship between the subducting slab, the surrounding mantle, and the overriding plate. This dissertation summarizes the results of three finite frequency teleseismic tomography studies of the central Nazca-South America subduction zone which improve our understanding of how along-strike variations in the Andean mountain belt and the subducting Nazca plate interact with each other and with the surrounding mantle. This is accomplished by first focusing on two smaller adjacent regions of the central Andes to explore upper mantle variations and then by using a combined dataset, which covers a larger region, to image the deeply subducted Nazca slab to investigate the fate of the slab. The first study focuses on the central Andean upper mantle under the Altiplano-Puna Plateau where normally dipping subduction of the Nazca plate is occurring (18° to 28°S). The shallow mantle under the Eastern Cordillera is generally fast, consistent with either underthrusting of the Brazilian cratonic lithosphere from the east or a localized "curtain" of delaminating material. Additional evidence for delamination is seen in the form of high amplitude low velocities under the Puna Plateau, consistent with proposed asthenospheric influx following lithospheric removal. In the second study, we explore the transition between normal and flat subduction along the north edge of the Altiplano Plateau (8° to 21°S). We find that the Peruvian flat slab extends further inland along the projection of the Nazca Ridge than was previously proposed and that when re-steepening of the slab occurs, the slab dips very steeply (~70°) down through the mantle transition zone (MTZ). We also tentatively propose a ridge parallel tear along the north edge of the Nazca Ridge. Both of these observations imply that the presence of the Nazca Ridge is at least locally influencing the geometry of the flat slab. The final study investigates along-strike variations in the deeply subducted Nazca slab along much of the central Nazca-South America subduction zone (6° to 32°S). Our results confirm that the Nazca slab continues subducting into the lower mantle rather than remaining stagnant in the MTZ. Thickening of the slab in the MTZ north of 16°S is interpreted as folding or buckling of the slab in response to the decreased slab sinking velocities in the lower mantle.

The upper mantle structure of Swedish part of Baltic Shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves (i.e. P waves and shear waves) recorded by the Swedish National Seismological Network (SNSN). Nonlinear high-resolution P and SV and SH wave isotropic tomographic inversions reveal velocity perturbations of ± 3 % down to at least 470 km below the network. Separate SV and SV models indicate several consistent major features, many of which are also consistent with P-wave results. A direct cell by cell comparison of SH and SV models reveals velocity differences of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially caused by noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. Shear-wave splitting and P-travel time residual analyses also detect anisotropic mantle structure. Distinct back-azimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Joint inversion using both the P and S data reveals 3D self-consistent anisotropic models with well-defined mantle lithospheric domains. These domains of differently oriented anisotropy most probably retain fossil fabric since the domains' origin, supporting the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the Archean. The possible disturbing effects of anisotropy on seismic tomography studies are investigated, and found to be potentially significant. P-wave arrival times were adjusted based on the estimates of mantle anisotropy, and re-inverted. The general pattern of the velocity-perturbation images was similar but changed significantly in some places, including the disappearance of a slab-like structure identified in the inversion with the original data. Thus the analysis demonstrates that anisotropy of quite plausible magnitude can have a significant effect on the tomographic images, and should not be ignored. If, as we believe, our estimates of anisotropy are reasonably correct, then the model based on the adjusted data should give a more robust and correct image of the mantle structure.

Cette thèse traite de l'étude de la profondeur des séismes modérés (4 < M < 5.5), à travers l'utilisation de données telesismiques, c'est à dire à partir d'enregistrements de sismographes situes a des distances de 3000 jusqu'à 9000 km de la source. L'estimation de la profondeur d'un séisme s'obtient en comparant les temps d'arrivée de l'onde directe (P) générée par ce séisme, avec ceux des ondes réfléchies sur la surface de la Terre, au-dessus du foyer du séisme (les phases de profondeur pP, sP). Nous utilisons le réseau du CTBTO (Comprehensive Test-Ban-Treaty-Organization, composé de mini-réseaux (10-20 sismographes), nous permettant de développer deux nouvelles méthodes d'estimation de la profondeur. La première est une inversion complète du mécanisme au foyer, simultanément a la profondeur. La seconde est une méthode d'analyse spectrale : le cepstre. A l'aide d'exemples de séismes en zone intracontinentale, nous montrons que ces deux méthodes sont complémentaires et qu'elles apportent une information nouvelle sur l'estimation de la profondeur pour les séismes de régions peu instrumentées. Dans un second temps, une optimisation de la méthode cepstrale a permis d'étendre l'estimation de la profondeur a l'utilisation de stations isolées provenant du réseau mondial IRIS. Cette nouvelle méthode, complètement automatique, a permis de localiser en profondeur les séismes de magnitude supérieure à 4.5 pour la zone de subduction de Guerrero (Mexique). Nous avons ainsi mis en évidence une répartition homogène des profondeurs des séismes dans la zone du gap de Guerrero. En combinant ces estimations de la profondeur de l'interface avec celles obtenues a l'aide d'une relocalisation de l'ensemble de la sismicité (par la méthode de l'ISC-Locator), nous proposons une imagerie de la géométrie de la subduction. Nous avons ensuite étudié l'influence de la profondeur sur les propriétés sismiques des séismes. En particulier, nous avons évalué la relation entre la profondeur et la chute de contrainte pour les séismes récents et modérés de la plaine du Pô, en Italie. Pour cela, nous nous sommes appuyés sur le réseau accéléromètrique régional de l'INGV Milan pour estimer les spectres sources de ces séismes, puis en déduire les fréquences coins et les chutes de contraintes associées. Ces spectres sources s'obtiennent à l'aide d'une méthode d'inversion qui sépare simultanément l'effet de l'atténuation, les effets de sites et l'effet de la source sur les spectres des ondes S, générées par les séismes et enregistrées en surface par les accéléromètres. Après inversion, nous trouvons une faible augmentation de la chute de contrainte avec la profondeur. / This thesis deals with depth estimations of moderate earthquakes (4 < M < 5.5), observed through the use of teleseismic data. At teleseismic distances (from 3000 to 9000 km), estimations of earthquake depths come from the estimation of the delays between the arrival time of the direct P-wave and the arrival times of the waves which have been reflected on the surface above the source (known as : depth phases pP and sP). The CTBTO (Comprehensive Test-Ban- Treaty-Organization) monitoring system allows the detection of these teleseismic phases for weak events, with magnitudes below 5, since this global network is composed by arrays (10-20 close single stations, in the same location). Using this network, two methods were developed for teleseismic depth estimation. First, a depth-phase recognition method is applied, based on a new improved cepstral analysis. In addition, we have developed a focal mechanism genetic algorithm inversion. We have applied these two methods for earthquakes occurring in intracontinental areas and we have proved that these new depth determinations provide new and complementary information about the source for barely instrumented areas. We have also developed another method, completely blind and automatic, which consists in an improvement of the cepstral analysis. The reliability of this method to improve depth estimation has been proved by relocating the recent moderate seismicity of the Guerrero subduction area (Mexico). In the Guerrero area, this cepstral analysis efficiently clusters event locations. We then use teleseismic waves reflected on the subduction interface to evaluate the depth of the interface above the focal hypocenter, which allows to specify the subduction interface geometry. We have finally conducted an exhaustive analysis of the depth distributions, based on a relocalization of the ISC catalogue using the new ISC-Locator algorithm and we provide an improved image of the subduction. Finally, we have conducted a study of the relationship between earthquake depths and seismic properties. In particular, we have focused on the burning issue about the links between stress drops and focal depths. We have found an interesting case study with the Po Plain recent seismicity (Northern Italy). Brune's stress drops and magnitudes have been estimated from a generalized parametric inversion using the regional accelerometric network from INGV Milano. We have shown that there is only a slight dependency between stress drops and earthquake depths in the Po Plain.

xv, 108 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Studies on seismic attenuation are an important complement to those on seismic velocity, especially when interpreting results in terms of temperature. But estimation of attenuation (t*) is more computationally involved and prone to contamination by noise, especially signal-generated noise. We have examined the effects of various forms of synthetic noise on t* estimation using time and frequency domain methods with varying window lengths and data frames of reference. We find that for S-waves, error due to noise can be reduced by rotating the data into the estimated polarization direction of the signal, but unless the exact nature of the noise is known, no method or window size is clearly preferable. We recommend the use of multiple estimation methods including a careful assessment of the uncertainty associated with each estimate, which is used as a weight during inversion for 1/Q. Our synthetic tests demonstrate that the misfit between actual and estimate-predicted traces or spectra correlates with t* error, and a similar relationship is suggested for real data. Applying this approach to data from the Yellowstone Intermountain Seismic Array, we employ two important constraints during inversion. First, we scale the misfit values so that the resulting weights are comparable in magnitude to the squares of the eventual data residuals. Second, we smooth the model so that the maximum attenuation (1/Q) does not exceed a value which would totally explain the observed velocity anomaly. The tomographic models from all the estimation methods are similar, but in the vicinity of the Yellowstone mantle plume, S-wave models show greater attenuation than do P-wave models. We attribute this difference to greater focusing by the plume of S-waves. All models show relatively high attenuation for the plume at depth, but above 250 km attenuation in the plume drops rapidly to values less than those of the surrounding mantle. We attribute this drop to the onset of partial melting, which dehydrates the olivine crystals, suppressing dislocation mobility and thereby attenuation. These attenuation models suggest excess plume temperatures at depth which are too low to support a plume origin in the lower mantle. This dissertation includes unpublished co-authored material. / Committee in charge: Eugene Humphreys, Chairperson, Geological Sciences; Emilie Hooft Toomey, Member, Geological Sciences; Douglas Toomey, Member, Geological Sciences; James Isenberg, Outside Member, Mathematics

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.

An der Universität Potsdam wird seit 2008 ein automatisiertes Verfahren angewandt, um Bruchparamter großer Erdbeben in quasi-Echtzeit, d.h. wenige Minuten nachdem sich das Beben ereignet hat, zu bestimmen und der Öffentlichkeit via Internet zur Verfügung zu stellen. Es ist vorgesehen, das System in das Deutsch-Indonesische Tsunamifrühwarnsystem (GITEWS) zu integrieren, für das es speziell konfiguriert ist. Wir bestimmen insbesondere die Dauer und die Ausdehnung des Erdbebens, sowie dessen Bruchgeschwindigkeit und -richtung. Dabei benutzen wir die Seismogramme der zuerst eintreffenden P Wellen vom Breitbandstationen in teleseimischer Entfernung vom Beben sowie herkömmliche Arrayverfahren in teilweise modifizierter Form. Die Semblance wir als Ähnlichkeitsmaß verwendet, um Seismogramme eines Stationsnetzes zu vergleichen. Im Falle eines Erdbebens ist die Semblance unter Berücksichtigung des Hypozentrums zur Herdzeit und während des Bruchvorgangs deutlich zeitlich und räumlich erhöht und konzentriert. Indem wir die Ergebnisse verschiedener Stationsnetzwerke kombinieren, erreichen wir Unabhängigkeit von der Herdcharakteristik und eine raum-zeitliche Auflösung, die es erlaubt die o.g. Parameter abzuleiten. In unserem Beitrag skizzieren wir die Methode. Anhand der beiden M8.0 Benkulu Erdbeben (Sumatra, Indonesien) vom 12.09.2007 und dem M8.0 Sichuan Ereignis (China) vom 12.05.2008 demonstrieren wir Auflösungsmöglichkeiten und vergleichen die Ergebnisse der automatisierten Echtzeitanwendung mit nachträglichen Berechnungen. Weiterhin stellen wir eine Internetseite zur Verfügung, die die Ergebnisse präsentiert und animiert. Diese kann z.B. in geowissenschaftlichen Einrichtungen an Computerterminals gezeigt werden. Die Internetauftritte haben die folgenden Adressen: http://www.geo.uni-potsdam.de/arbeitsgruppen/Geophysik_Seismologie/forschung/ruptrack/openday http://www.geo.uni-potsdam.de/arbeitsgruppen/Geophysik_Seismologie/forschung/ruptrack

Dans cette thèse, un algorithme d'inversion de formes d'ondes (FWI) est développé pour l'imagerie 3D des paramètres élastiques de la lithosphère à partir des enregistrements télésismiques dans le but d'accroître la résolution des images lithosphériques. La modélisation sismique est effectuée par un méthode hybride d'injection de champ d'ondes. Une première modélisation est effectuée dans une Terre globale avec le logiciel AxiSEM pour déterminer les champs d’ondes aux bords de la cible lithosphérique. Ces solutions sont ensuite propagées dans cette cible par une méthode aux éléments finis spectraux. Le problème inverse est résolu avec un algorithme d’optimisation locale de type quasi-Newton (l-BFGS). La sensibilité de la méthode à la configuration expérimentale (paramétrisation du milieu, modèle initial, géométrie et échantillonnage du dispositif de capteurs) est tout d’abord analysée avec un modèle synthétique réaliste des Alpes Occidentales. L’algorithme est finalement appliqué à neuf événements de la campagne CIFALPS dans les Alpes occidentales jusqu’à une fréquence de 0.2Hz. Les modèles de vitesses P et S et de densité révèlent les grandes structures lithosphériques de la chaîne alpine, en particulier le corps d’Ivrée et la géométrie des Moho européen et adriatique. Plus profondément, deux anomalies de vitesses lentes sont imagées dans le manteau et sont interprétées comme la signature d’une remontée asthénosphérique et la localisation du détachement du panneau plongeant européen. Ces résultats corroborent l’hypothèse d’une subduction continentale de la croûte européenne et d’une éventuelle déchirure du panneau plongeant européen lors de la phase de collision. / In this thesis, a full-waveform inversion (FWI) algorithm is developed with the aim to image the elastic properties (Vp, Vs and density) of 3D lithospheric models from teleseismic recordings with a spatial resolution of the order of the wavelength. Seismic modeling is performed with a wavefield injection hybrid approach. A first simulation is performed in a global radially symmetric Earth with the AxiSEM code to compute the wavefields on the borders of the lithospheric target. Then, these wavefields are propagated in the target with the spectral finite-element method. After linearization, the inverse problem is solved with a quasi-Newton (1-BFGS) optimization algorithm. The sensitivity of the teleseismic FWI to the experimental setup (subsurface parameterization, initial model, sampling and geometry of the station layout) is first assessed with a realistic synthetic model of the Western Alps. The method is finally applied to nine events of the CIFALPS experiment carried out in the Western Alps, up to a frequency of 0.2Hz. Reliable models of P and S wave speeds and density reveal with an unprecedented resolution the crustal and lithospheric structures of the Alpine Belt, in particular the geometry of the Ivrea body, and the European and Adriatic Mohos. Deeper, two slow velocity anomalies beneath the Western Alps are imaged in the mantle. The first, to the west of the chain, is interpreted as the signature of an asthenospheric upwelling, the second near the location of the Ivrea body indicates the European slab break-off. The study supports the hypothesis of the European continental crust subduction and confirms the possible tearing of the European slab.

Abstract The crustal and upper mantle structures of the Shield on the regional scale were investigated using the data of the POLENET/LAPNET passive seismic array and the previously published models of active and passive seismic experiments in the study area. This area is centred in northern Finland and it extends to surrounding areas in Sweden, Norway and northwestern Russia. The bedrock there is mostly of the Archaean origin and the lithosphere of the region was reworked by two orogenies during Palaeoproterozoic. One of the results of the thesis was a new map of the Moho depth of the study area, for which new estimates of the crustal thickness were obtained using receiver function method and complemented by published results of receiver function studies and controlled source seismic profiles. The map differs from the previously published maps in two locations, where we found significant deepening of the Moho. The 3D structure of the upper mantle was studied using teleseismic traveltime tomography method. The resulting model shows high seismic velocities below three cratonic units of the study area, which may correspond to non-reworked fragments of cratonic lithosphere and a low velocity anomaly separating these cratonic units from each other. The regional scale studies were complemented by two smaller scale studies in upper crust level using combined interpretation of seismic profiling and gravity data. These studies were centred on Archaean Kuhmo Greenstone Belt in eastern Finland and central Lapland in northern Finland located in the crust reworked during Palaeoproterozoic. Both areas are considered as prospective ones for mineral exploration. Both studies demonstrate the advantage of gravity data inversion in studying 3D density structure of geologically interesting formations, when the Bouguer anomaly data is combined with a priori information from petrophysical and seismic datasets.