The velocity structures of southern and central part of Sweden have been derived with the local tomography (LET) method. The region has been divided into two study areas and the datasets come from the P- and S-wave traveltimes recorded by the Swedish National Seismic Network (SNSN). Man-made explosions and earthquakes occurring over the period of 5 years and 10 years, respectively, within the study areas have been used. One-dimensional starting models were derived based on an a priori model obtained from the SNSN, that were later used for starting models in the inversion for the 3-D crustal structures of the study areas. Attempts were also made to invert for Moho topography in the areas. The study areas are found to have an upper-crustal thickness of approximately 20 to 25 km and the Moho boundaries vary from 42 to 46 km in depth. The Vp/Vs ratios varies from about 1.68 to 1.78. The LET method appear to resolve the different between the Sveconorgwegian and Svecofennian orogen regions, but the stations and sources are too sparsely distributed for higher resolution models. The seismicity in the study areas are distributed in two distinctive depth ranges. The focal depth of the SNSN catalogued earthquakes concentrated in approximately 5 km and 15 - 20 km depth. Relocations of the earthquakes using a global search method reduced this tendency. The results also show that using 3-D models produces less biased results than using 1-D models with the same relocation method.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of California, San Diego, 1998. / Vita. Includes bibliographical references.

The lithosphere of northwestern Canada recorded more than 2.5 Gy of complex tectonic evolution, from the formation of the ancient cores of the continental lithosphere such as the Slave craton to the Phanerozoic Cordilleran orogeny with substantial variations in crust and upper mantle structures that led to the concentration of natural resources (i.e., diamonds in cratons). Present-day northwestern Canada juxtaposes a thin and hot Cordilleran lithosphere to the thick and cold cratonic lithosphere, which has important implications for regional geodynamics. Recently, seismic station coverage has drastically increased across northwestern Canada, allowing the development of seismic tomography models and other passive-source seismic methods at high resolution in order to investigate the tectonic evolution and dynamics of the lithosphere in this region. The P- and S-wave upper mantle structures of northwestern Canada reveal that the distribution of kimberlite fields in the Slave craton correlates with the margin of fast and slow seismic mantle anomalies, which could delineate weak zones in the lithosphere. Based on our tomographic models we identify two high-velocity seismic anomalies straddling the arcuate Cordillera Deformation Front that have controlled its regional deformation, including a newly identified Mackenzie craton characterized by high seismic velocities extending from the lower crust to the upper mantle to the north of the Mackenzie Mountains. Furthermore, our P-wave tomography model shows sharp velocity contrasts beneath the surface trace of the Tintina Fault. Estimates of seismic anisotropy show a progressive rotation of fast-axis directions when approaching the fault zone. Together, they provide seismic evidence for the trans-lithospheric nature of the Tintina Fault. We further propose that the Tintina Fault has chiseled off small pieces of the Laurentian craton between the Late Cretaceous and the Eocene, which would imply that large lithospheric-scale shear zones are able to cut through small pieces of refractory cratonic mantle and transport them over several hundred kilometers.

In this thesis, a finite-frequency theory is developed to calculate Born sensitivity kernels for Rayleigh-wave phase and amplitude measurements that are valid in regions near seismic stations. Calculations of sensitivity kernels for inter-station measurements show that exact travelling-wave representation of Green tensor is necessary when station spacing is close to or smaller than the seismic wavelength. This finite-frequency theory will allow us to take advantage of dense seismic arrays to obtain high-resolution surface-wave tomography using inter-station measurements. The non-linear dependence of surface wave phase upon large perturbations in crustal thickness as well as finite-frequency effects in global surface-wave tomography are investigated using wave propagation simulations. Calculations show that non-linearity as well as finite-frequency effects can be accounted for by using 2D phase-velocity kernels for boundary perturbations. A 3D-reference tomographic approach is developed for iterative inversions of global crustal structure where Frechet kernels are calculated in 3D reference models. A global dataset of minor-arc and major-arc Rayleigh wave dispersion measurements at periods between 25 seconds and 100 seconds are built and global phase velocity maps based on the dataset are obtained using diffractional tomography. The phase velocity model confirms many general features associated with surface tectonics including the ocean-continent dichotomy and the signature of lithospheric cooling in oceanic plates. There are significant differences between the phase velocity model and calculations based on a current global model CRUST2.0+S20RTS in oceanic regions, Archean and Proterozoic cratons as well as orogenic belts. In addition, the high resolution phase velocity maps reveal a major change in the distribution of small scale anomalies in the Pacific at different wave periods. / Ph. D.

A tomografia sísmica convencional apresenta baixa resolução em regiões de baixa sismicidade, como consequência, estudos prévios realizados na América do Sul não mapearam com clareza áreas menores do continente, como a região da bacia do Paraná. Para descrever a estrutura da crosta na região da bacia do Paraná, além de dados obtidos através da análise de terremotos andinos, foram utilizadas curvas de dispersão provenientes da correlação do ruído ambiental entre pares de estações sismográficas, técnica conhecida com Ambient Noise Tomography (ANT). Os mapas de velocidade de grupo obtidos correspondem bem com as principais províncias geológicas já conhecidas na América do Sul: baixas velocidades sob os Andes e bacias sedimentares e altas velocidades sob regiões cratônicas. As anomalias de alta velocidade encontradas para o manto superior, concordam com trabalhos prévios que confirmam a presença de um núcleo cratônico sob a bacia. Contudo, não é possível afirmar se tal núcleo está intacto ou não, principalmente devido resultados anteriores que mostram evidências da existência de um núcleo dividido por zonas de sutura. No entanto, a tomografia de ondas de superfície não possui resolução para confirmar este modelo. Como as etapas do processamento de dados são bem definidas e independentes, à medida que novas estações forem instaladas com o avanço do projeto BRASIS, novos caminhos serão adicionadas à base inicial, aumentando a resolução e confiabilidade dos resultados futuros. / Conventional seismic tomography is known to have poor resolution in regions of low seismicity, therefore, studies carried out in South America did not mapped clearly smaller areas of the continent, for example the region of the Parana basin. To describe the of the crustal structure of the Parana basin, in addition to data obtained by analysis of Andean earthquakes, we used dispersion curves from ambient noise correlation between pairs of seismographic stations, a technique known by Ambient Noise Tomography (ANT). The obtained group velocity maps correspond well with the main geological provinces already known in South America: low velocities under the Andes and sedimentary basins and high velocities in cratonic regions. The high velocity anomalies encountered in the upper mantle, agree with previous studies that confirm the presence of a nucleus in the cratonic basin. However, it is not possible to say whether such a nucleus is intact or not, mainly because previous results showing evidence of the existence of a nucleus divided by suture zones. Nevertheless, surface wave tomography has no resolution to confirm this model. The steps of data processing of this work are well defined and independent, thus, as new stations will being deployed with the advance of BRASIS Project, new paths will be added to the database, increasing the resolution and reliability of the future results.

A tomografia sísmica convencional apresenta baixa resolução em regiões de baixa sismicidade, como consequência, estudos prévios realizados na América do Sul não mapearam com clareza áreas menores do continente, como a região da bacia do Paraná. Para descrever a estrutura da crosta na região da bacia do Paraná, além de dados obtidos através da análise de terremotos andinos, foram utilizadas curvas de dispersão provenientes da correlação do ruído ambiental entre pares de estações sismográficas, técnica conhecida com Ambient Noise Tomography (ANT). Os mapas de velocidade de grupo obtidos correspondem bem com as principais províncias geológicas já conhecidas na América do Sul: baixas velocidades sob os Andes e bacias sedimentares e altas velocidades sob regiões cratônicas. As anomalias de alta velocidade encontradas para o manto superior, concordam com trabalhos prévios que confirmam a presença de um núcleo cratônico sob a bacia. Contudo, não é possível afirmar se tal núcleo está intacto ou não, principalmente devido resultados anteriores que mostram evidências da existência de um núcleo dividido por zonas de sutura. No entanto, a tomografia de ondas de superfície não possui resolução para confirmar este modelo. Como as etapas do processamento de dados são bem definidas e independentes, à medida que novas estações forem instaladas com o avanço do projeto BRASIS, novos caminhos serão adicionadas à base inicial, aumentando a resolução e confiabilidade dos resultados futuros. / Conventional seismic tomography is known to have poor resolution in regions of low seismicity, therefore, studies carried out in South America did not mapped clearly smaller areas of the continent, for example the region of the Parana basin. To describe the of the crustal structure of the Parana basin, in addition to data obtained by analysis of Andean earthquakes, we used dispersion curves from ambient noise correlation between pairs of seismographic stations, a technique known by Ambient Noise Tomography (ANT). The obtained group velocity maps correspond well with the main geological provinces already known in South America: low velocities under the Andes and sedimentary basins and high velocities in cratonic regions. The high velocity anomalies encountered in the upper mantle, agree with previous studies that confirm the presence of a nucleus in the cratonic basin. However, it is not possible to say whether such a nucleus is intact or not, mainly because previous results showing evidence of the existence of a nucleus divided by suture zones. Nevertheless, surface wave tomography has no resolution to confirm this model. The steps of data processing of this work are well defined and independent, thus, as new stations will being deployed with the advance of BRASIS Project, new paths will be added to the database, increasing the resolution and reliability of the future results.

El coneixement del subsòl sempre ha estat objecte d’interès per diversos motius. La disponibilitat de recursos naturals (minerals, aigua, hidrocarburs...), l’obra civil, el vulcanisme o la sismicitat són factors que condicionen i modifiquen el nostre ús del sòl i per tant és convenient conèixer-los. En aquest sentit, la tomografia sísmica representa una tècnica de prospecció indirecta i no destructiva molt adient per a gran quantitat d’estudis. Mitjançant aquesta metodologia es poden obtenir les velocitats que caracteritzen els materials presents a la zona d’estudi. Aquest fet ja permet, en molts casos, correlacionar aquest paràmetre (la velocitat) amb la composició del terreny. A més a més, els models de velocitats que s’obtenen són de molta utilitat per l’aplicació d’altres tècniques de processat de dades sísmiques com les correccions estàtiques o les migracions en fondària. En aquest treball s’ha aplicat la tomografia sísmica a diversos casos amb característiques molt diferenciades. Ja se sap que la tomografia sísmica és una metodologia resolutiva sempre i quan els contrastos de velocitats en el medi siguin importants. D’entre aquests contrastos, en virtut del principi de Fermat, les anomalies d’alta velocitat són les que es poden reproduir i caracteritzar amb més fiabilitat. En canvi, les anomalies negatives constitueixen en molts casos una limitació per a l’aplicació d’aquesta tècnica. En dos dels estudis que es presenten en aquest treball, s’ha utilitzat la tomografia sísmica per detectar i caracteritzar plutons granítics, i obtenir informació sobre les seves propietats a escala superficial (fins poc més de mil metres de fondària). Els resultats mostren una bona identificació dels cossos granítics, a més a més les variacions de velocitat en l’interior d’aquests cossos s´on atribuïbles a canvis de propietats de la roca. En altres dos estudis s’ha utilitzat la tomografia sísmica per intentar delimitar colades basàltiques (altes velocitats) situades entre sèries sedimentàries de velocitat inferior en una conca marina. La sedimentació posterior als episodis de vulcanisme es pot caracteritzar raonablement, així com el límit superior de la colada basàltica. No succeeix el mateix amb la base de la colada i molt menys amb els sediments subbasàltics. En aquest cas el millor model tomogràfic obtingut s’ha utilitzat per implementar una migració pre-stack en fondària, fet que ha millorat sensiblement la imatge obtinguda utilitzant un processat de sísmica de reflexió convencional. En el darrer estudi que es presenta, la tomografia sísmica ha proporcionat un model detallat de velocitats per generar dades sintètiques. Aquestes dades al comparar-les amb les dades reals, han permès inferir certes propietats de l’escorça inferior i de la discontinuïtat escorça-mantell. En alguns d’aquests treballs la sísmica de reflexió també ha tingut un paper important, com a pas previ a un estudi més detallat, o bé com a complement i aportació d’informació addicional sobre les estructures implicades en l’estudi. En la majoria dels casos estudiats s’han dut a terme simulacions sintètiques amb diferents propòsits: verificar software de processat, fer tests sobre la idoneïtat dels diferents dispositius experimentals o bé verificar hipòtesis sobre la naturalesa dels materials que formen el subsòl. Encara que la tomografia sísmica té certes limitacions, en general, s’ha mostrat com una tècnica robusta i resolutiva aplicada a diferents contextos geològics i a diferents escales. / The internal structure of the subsoil has been a topic of major interest for several reasons. The availability of natural resources (water, oil, minerals..), seismicity or volcanism condition the use of the soil, therefore knowing its properties is mandatory. Synthetic simulations and forward modelling have been intensively used in the present work to compare tomographic softwares, to test a new pre-stack depth migration code and to test several acquisition geometries. Synthetic data provide a fast (and cheap) way to study many hypothetical situations and experimental conditions and to predict results. Seismic tomography is an indirect and non-destructive prospection technique that can be used in many studies. By means of this methodology, velocities that feature materials of the study area can be obtained. In most of cases, this parameter (velocity) can be correlated with the subsurface composition. Furthermore, velocity models obtained are very useful for the application of other seismic reflection techniques such as static corrections or depth migrations. In this work, seismic tomography has been used for a wide range of situations. Theoretical considerations (Snell’s law) suggest that low-velocity anomalies are under-sampled while high-velocity zones were well resolved using this methodology. For the optimal case (positive anomalies), seismic tomography has been successful in detecting granitic plutons and basalt layers because of the contrast of acoustic properties between these bodies and the sorrounding rocks. The performance of seismic tomography has been tested in less favourable situations such as the presence of subterranean cavities (extreme low-velocity anomalies), fracturation zones or subbasalt sediments. In some of these cases, tomographic models are not conclusive and require the use of other methodologies in order to solve the problem. Despite these limitations, seismic tomography can be considered as a robust technique to obtain velocity models in different geological contexts and at different scales.

On May 29th 2008 two earthquakes with moment magnitude of Mw ~6 occurred in the southwestern part of Iceland. The second earthquake struck within only seconds after the first, on a fault ~5 km west from the first fault. The aftershock sequence was recorded by 14 seismic stations during the subsequent 34 days. The recorded earthquakes were detected and located with a Coalesence Microseismic Mapping (CMM) technique. The output data from this program has been used as basis for the tomography algorithm PStomo_eq, which simultaneously inverts for both P- and S-wave velocities and relocates the events. Within the study area of 46×36 km the three-dimensional velocity structure has, successfully but not conclusively, been modeled to depths of ~10 km. The Vp/Vs ratio varies from 1.74 to 1.82 within the study area. The velocity increases with depth starting from 2 km where the P-wave velocity is 4.6 km/s and the S-wave velocity is 2.7 km/s, at a depth of 10 km the P-wave velocity is 6.9 km/s and S-wave velocity is 4.0 km/s. In the horizontal slices a high velocity area is seen in the northwestern part of model. This is interpreted to be caused by a magma body rising up from below and lithifying at high pressure. From cross-sections a large low velocity zone is seen in the western part of model area concentrated above the seismicity. The low velocity anomaly is found between depths of 2 km to 4 km, stretching from 21.5° to 21.2° W. It is interpreted to be caused by high porosity within the area. The depth to the brittle crust is increasing from the western part of the model towards the eastern part, right in the middle of the model it abruptly decreases again. The depth to the base of the brittle crust is increasing from 7 km in west to 9 km in the middle of model. / Den 29:e maj 2008 inträffade två jordbävningar med magnitud Mw ~6 på sydvästra Island. Den första jordbävningen följdes tätt av en andra jordbävning på en förkastning ~5 km väster om den första. Påföljande efterskalvssekvens registrerades av 14 seismiska stationer under 34 dagar efter huvudskalven. De registrerade skalven har detekterats och lokaliserats med en Coalesence Microseismic Mapping (CMM) teknik. Utdata från detta program har använts som grund för tomografin som genomförts med PStomo_eq, en algoritm som inverterar oberoende för både P- och S-vågs hastigheter och samtidigt omlokaliserar eventen. Inom det undersökta området på 46×36 km har en tredimensionell hastighetsmodell, om än inte slutgiltigt, modellerats för djup ned till 10 km. Vp/Vs kvoten varierar mellan 1.74 och 1.82 inom studieområdet. Hastigheterna ökar med ökande djup, på ett djup av 2 km är P-vågs hastigheten 4.6 km/s och S-vågs hastigheten 2.7 km/s och vid 10 km är P-vågs hastigheten 6.9 km/s och S-vågs hastigheten 4.0 km/s. I den nordvästra delen av modellen återfinns en höghastighetszon. Denna tolkas vara orsakad av en magma kropp som stigit och kristalliserat under högt tryck. De vertikala tvärsnitten visar en låghastighetsanomali i västra delen av modellen, koncentrerat ovan seismiciteten. Denna anomali sträcker sig från ett djup på 2 km ned till 4 km, från 21.5° till 21.2° V. Den tolkas vara orsakad av en hög grad av porositet. Djupet för den bräckliga jordskorpan ökar från väster till öster i modellen, för att i mitten abrupt minska igen. Basen av den bräckliga skorpan ökar från 7 km i väst till 9 km i mitten av modellen.

Ocean-bottom seismometers (OBS) allow wider angle recording and therefore, they have the potential to significantly enhance imaging of deep subsurface structures. Currently, conventional OBS data analysis still uses first arrival traveltime tomography and prestack Kirchhoff depth migration method. However, using first arrival traveltimes to build a velocity model has its limitations. In the Taiwan region, subduction and collision cause very complex subsurface structures and generate extensive basalt-like anomalies. Since the velocity beneath basalt-like anomalies is lower than that of high velocity anomalies, no first-arrival refractions for the target areas occur. Thus, conventional traveltime tomography is not accurate and amplitude constrained traveltime tomography can be dangerous. Here, a new first-arrival stochastic tomography method for automatic background velocity estimation is proposed. Our method uses the local beam semblance of each common-shot or common-receiver gathers instead of first-arrival picking. Both the ray parameter and traveltime information are utilized. The use of Very Fast Simulated Annealing (VFSA) method also allows for easier implementation of the uncertainty analysis. Synthetic and real data benchmark tests demonstrate that this new method is robust, efficient, and accurate. In addition, migrated images of low-fold data or data with limited observation geometry like OBS are often corrupted by migration aliasing. Incorporation of prestack instantaneous-slowness information into the imaging condition can significantly reduce migration artifacts and noise and improve the image quality in areas of poor illumination. Here I combine slowness information with Gaussian beam depth migration and implement a new slowness driven Gaussian beam prestack depth migration. The prestack instantaneous slowness information, denoted by ray parameter gathers p(x,t), is extracted from the original OBS or shot gathers using local slant stacking and subsequent localsemblance analysis. In migration, we propagate both the seismic energy and the principal instantaneous slowness information backward. At a specific image location, the beam summation is localized in the resolution-dependent Fresnel zone where the instantaneousslowness-information-related weights are used to control the beams. The effectiveness of the new method is illustrated using two synthetic data examples: a simple model and a more realistic complicated sub-basalt model. / text