Gotowa bibliografia na temat „Natural seismicity”

Natural time analysis enables the introduction of an order parameter for seismicity, which is just the variance of natural time χ, κ1=⟨χ2⟩−⟨χ⟩2. During the last years, there has been significant progress in the natural time analysis of seismicity. Milestones in this progress are the identification of clearly distiguishable minima of the fluctuations of the order parameter κ1 of seismicity both in the regional and global scale, the emergence of an interrelation between the time correlations of the earthquake (EQ) magnitude time series and these minima, and the introduction by Turcotte, Rundle and coworkers of EQ nowcasting. Here, we apply all these recent advances in the global seismicity by employing the Global Centroid Moment Tensor (GCMT) catalog. We show that the combination of the above three milestones may provide useful precursory information for the time of occurrence and epicenter location of strong EQs with M≥8.5 in GCMT. This can be achieved with high statistical significance (p-values of the order of 10−5), while the epicentral areas lie within a region covering only 4% of that investigated.

In the Czech Republic, there are seismic territories with little natural seismicity only.On the other hand, the technical seismicity is frequently a pressing issue.Dynamic responses caused by the technical seismicity result in defects − in particular, in cracksin brickworks. This paper discusses possible technical and structural measures which eliminate effects of the technical seismicity and, in turn, reduce risk of damage to the brickworks..

The ability of fluid-generated subsurface stress changes to trigger earthquakes has long been recognized. However, the dramatic rise in the rate of human-induced earthquakes in the past decade has created abundant opportunities to study induced earthquakes and triggering processes. This review briefly summarizes early studies but focuses on results from induced earthquakes during the past 10 years related to fluid injection in petroleum fields. Study of these earthquakes has resulted in insights into physical processes and has identified knowledge gaps and future research directions. Induced earthquakes are challenging to identify using seismological methods, and faults and reefs strongly modulate spatial and temporal patterns of induced seismicity. However, the similarity of induced and natural seismicity provides an effective tool for studying earthquake processes. With continuing development of energy resources, increased interest in carbon sequestration, and construction of large dams, induced seismicity will continue to pose a hazard in coming years.

A lot of work in geosciences has been completed during the last decade on the analysis in the new concept of time, termed natural time, introduced in 2001. The main advances are presented, including, among others, the following: First, the direct experimental verification of the interconnection between a Seismic Electric Signals (SES) activity and seismicity, i.e., the order parameter fluctuations of seismicity exhibit a clearly detectable minimum when an SES activity starts. These two phenomena are also linked closely in space. Second, the identification of the epicentral area and the occurrence time of an impending major earthquake (EQ) by means of the order parameter of seismicity and the entropy change of seismicity under time reversal as well as the extrema of their fluctuations. An indicative example is the M9 Tohoku EQ in Japan on 11 March 2011. Third, to answer the crucial question—when a magnitude 7 class EQ occurs—whether it is a foreshock or a mainshock. This can be answered by means of the key quantities already mentioned, i.e., the order parameter of seismicity and the entropy change of seismicity under time reversal along with their fluctuations. The explanation of the experimental findings identified before major EQs is given in a unified way on the basis of a physical model already proposed in the 1980s.

On 3 March 2021, a strong, shallow earthquake of moment magnitude, Mw6.3, occurred in northern Thessaly (Central Greece). To investigate possible complex correlations in the evolution of seismicity in the broader area of Central Greece before the Mw6.3 event, we apply the methods of multiresolution wavelet analysis (MRWA) and natural time (NT) analysis. The description of seismicity evolution by critical parameters defined by NT analysis, integrated with the results of MRWA as the initiation point for the NT analysis, forms a new framework that may possibly lead to new universal principles that describe the generation processes of strong earthquakes. In the present work, we investigate this new framework in the seismicity prior to the Mw6.3 Thessaly earthquake. Initially, we apply MRWA to the interevent time series of the successive regional earthquakes in order to investigate the approach of the regional seismicity at critical stages and to define the starting point of the natural time domain. Then, we apply the NT analysis, showing that the regional seismicity approached criticality a few days before the occurrence of the Mw6.3 earthquake, when the κ1 natural time parameter reached the critical value of κ1 = 0.070.

Indonesia is located at the confluence of 3 large, active plates that are constantly moving. Therefore, Indonesia is one of the countries that has a high level of seismicity risk. This study aims to classify seismicity data in the Indonesian region based on coordinate data which contains variable data on frequency of occurrence, depth, and strength of seismicity. Seismicity data was obtained from the BMKG official website using data for the period 2018 to 2020. The clustering technique used was the DBSCAN algorithm. This algorithm requires epsilon and MinPts input parameters. The results of the cluster formed will then be validated using silhouette coefficients. Based on the coordinate data, 4 clusters were formed with 4 disturbances. Based on the characteristic data, 3 clusters were formed with 5 disturbances. The silhouette coefficient obtained was 0.35 for coordinate data and 0.39 for characteristic data. This research is useful for increasing the use value of abundant seismicity information and can be used as an effort to mitigate seismicity natural disasters.

A standard approach to quantifying the seismic hazard is the relative intensity (RI) method. It is assumed that the rate of seismicity is constant in time and the rate of occurrence of small earthquakes is extrapolated to large earthquakes using Gutenberg–Richter scaling. We introduce nowcasting to extend RI forecasting to time-dependent seismicity, for example, during an aftershock sequence. Nowcasting uses ‘natural time’; in seismicity natural time is the event count of small earthquakes. The event count for small earthquakes is extrapolated to larger earthquakes using Gutenberg–Richter scaling. We first review the concepts of natural time and nowcasting and then illustrate seismic nowcasting with three examples. We first consider the aftershock sequence of the 2004 Parkfield earthquake on the San Andreas fault in California. Some earthquakes have higher rates of aftershock activity than other earthquakes of the same magnitude. Our approach allows the determination of the rate in real time during the aftershock sequence. We also consider two examples of induced earthquakes. Large injections of waste water from petroleum extraction have generated high rates of induced seismicity in Oklahoma. The extraction of natural gas from the Groningen gas field in The Netherlands has also generated very damaging earthquakes. In order to reduce the seismic activity, rates of injection and withdrawal have been reduced in these two cases. We show how nowcasting can be used to assess the success of these efforts. This article is part of the theme issue ‘Statistical physics of fracture and earthquakes’.

The Hengill area is a complex tectonic and geothermal site, located at the triple junction between the Reykjanes Peninsula (RP), the Western Volcanic Zone (WVZ), and the South Iceland Seismic Zone (SISZ). Geothermal systems in the vicinity of the Hengill volcano are exploited for electrical power and heat production, and today the two largest operating geothermal power plants are located at the Nesjavellir and the Hellisheiði. The region is seismically highly active with several thousand earthquakes located yearly, and whose origin may be either natural or anthropogenic. The thesis focuses on the analysis and investigation of the

Une séquence sismique est un ensemble de tremblements de terre qui se produisent avec une grande proximité spatiale et temporelle. Un essaim de séisme est un type de séquence sismique caractérisé par des tremblements de terre dont la localisation change au cours du temps et qui n’est pas dominé par un seul grand tremblement de terre.Dans cette thèse, j'étudie l'évolution spatio-temporelle d'essaims sismiques naturels et induits dans le but d'identifier les processus physiques qui les produisent et de caractériser les propriétés des systèmes de failles activés. Plus spécifiquement, je me concentre sur trois aspects clés de l'activité sismique : l'évolution temporelle du taux de sismicité, la localisation des séismes et la distribution des magnitudes. L'étude se concentre sur deux essaims sismiques. Le premier, d'origine naturelle, s'est produit dans la vallée de la Maurienne (Alpes françaises) entre 2017 et 2019, tandis que le second a été induit par des opérations de fracturation hydraulique à Preston New Road, au Royaume-Uni, en 2019. Pour assurer une analyse à haute résolution de ces séquences, j'ai d'abord créé des catalogues améliorés qui intègrent de nombreux événements nouvellement détectés ainsi que des magnitudes et des localisations plus précises.La migration des séismes pendant l'essaim de Maurienne suggère que cette séquence a pu être activée par une combinaison de multiples diffusions de fluides à haute pression et d'interactions entre séismes. De plus, la proportion de petits et grands événements (paramètre b de la loi de Gutenberg-Richter) varie dans l'espace, et ce changement peut être lié à la dimension des failles actives.En plus de l'étude de l'activité sismique, j'ai appliqué l'interférométrie du bruit sismique ambiant pour évaluer si les changements de contraintes pendant l'essaim ont créé des variations détectables de la vitesse des ondes sismiques. Les changements de vitesse semblent être principalement influencés par un processus saisonnier probablement lié aux variations de pression interstitielle dues aux précipitations. Cependant, pendant la période principale d'activité sismique, ces changements ont pu être amplifiées par les secousses du sol résultant de l'occurrence prolongée de séismes.La séquence de Preston New Road est caractérisée par un taux de sismicité et une distribution des magnitudes qui évoluent progressivement au fur et à mesure que les fluides sont injectés de manière répétée et que le volume de la zone sismogénique grandit. Ceci suggère que l'activité sismique au cours d'une phase d'injection dépend de l'historique des phases précédentes.Les séquences de Maurienne et de Preston New Road ne sont que deux exemples du phénomène plus large des essaims sismiques et induits. Néanmoins, ces deux séquences illustrent le fait que les essaims induits et naturels peuvent présenter des propriétés similaires dans leur évolution spatio-temporelle, comme la migration des séismes et la dépendance de la valeur de b à l'échelle du système de failles. Cela souligne la possibilité d'appliquer les connaissances acquises lors de l'étude d'un type d'essaim pour améliorer notre compréhension de l'autre
A seismic sequence is a cluster of earthquakes that occur in close spatial and temporal proximity. One type of seismic sequence is a seismic swarm, which is typically characterized by earthquakes whose location changes over time and by the absence of a single, dominant, large earthquake.In this thesis, I investigate the spatio-temporal evolution of natural and induced seismic swarms with the aim of identifying the physical processes that drive them and characterizing the properties of the activated fault systems. More specifically, I focus on three key aspects of the seismic activity: temporal evolution of the seismicity rate, earthquake location and frequency-magnitude distribution. The study focuses on two seismic swarms. The first one, of natural origin, occurred in the Maurienne valley (French Alps) between 2017 and 2019, while the second one was induced by hydraulic fracturing operations at Preston New Road, UK, in 2019. To ensure a high-resolution analysis of these sequences, I first created improved catalogs, which incorporate newly detected events and more accurate magnitudes and hypocenter locations.The migration of earthquakes during the Maurienne swarm suggests that this sequence may have been triggered by a combination of multiple pulses of high-pressure fluids and earthquake-to-earthquake interactions. Additionally, the proportion of small and large events (i.e., the b-value of the Gutenberg-Richter law) varies in space, and this change may be linked to the size of the active fault systems.In addition to the study of the seismic activity, I applied ambient noise interferometry to assess if stress changes during the Maurienne swarm produced detectable variations in seismic wave velocity. The velocity changes appear to be primarily influenced by a seasonal process possibly related to pore pressure variations due to rainfall. However, during the main period of seismic activity, such changes may also be accentuated by the continuous ground shaking resulting from the prolonged occurrence of earthquakes.The Preston New Road sequence is characterized by a seismicity rate and a frequency-magnitude distribution that gradually evolve as fluids are repeatedly injected and the seismogenic volume expands in size. This suggests that the seismic activity during an injection stage depends on the injection history of past stages.The Maurienne and Preston New Road sequences are just two examples of the broader phenomenon of seismic and induced swarms. Nonetheless, these two sequences illustrate that induced and natural swarms can exhibit similar patterns in their spatio-temporal evolution, such as earthquake migration and the dependence of the b-value on the scale of the fault system. This emphasizes the potential of applying the knowledge gained from studying one type of swarm to improve our understanding of the other

Allo scopo di determinare il regime di sforzo in Sicilia orientale, sono stati analizzati gli eventi sismici (Md superiore a 2.0) registrati nel periodo Agosto 2001 - Dicembre 2008. I settori delle Isole Eolie, della Sicilia nord-orientale, dell'Etna e della Sicilia sud-orientale sono stati esaminati separatamente. La distribuzione ipocentrale di circa 464 terremoti, si concentra in corrispondenza dei principali lineamenti tettonici, sia nella porzione interna che in mare. Per ottenere soluzioni focali piu' affidabili e' stata eseguita l'analisi di polarizzazione delle onde di taglio, per gli eventi di buona qualita', al fine di invertire congiuntamente le polarita' dei primi impulsi delle onde P e le direzioni di polarizzazione delle fasi S. Nella regione di studio, sono state ricavate differenti tipologie di soluzioni di piani di faglia. In Sicilia sud-orientale e nelle Isole Eolie si ha una predominanza di meccanismi di strike-slip e di tipo normale, mentre nell' area dello Stretto di Messina e in Sicilia nord-orientale prevalgono soluzioni di dip-slip di tipo normale. Una consistente orientazione NNW-SSE degli assi P e' stata ottenuta per l' Avampaese Ibleo, sia nella parte interna che nel Mar Ionio, e per il settore nord-occidentale dell' Etna. Per gli altri settori non e' stata individuata alcuna orientazione preferenziale degli assi P. Al fine di determinare il campo di sforzo, circa 250 meccanismi focali e diversi sottovolumi, basati sulla distribuzione della sismicita' e sul metodo del misfit cumulativo, sono stati invertiti utilizzando l' algoritmo di Gephart e Forsyth (1984). I risultati ottenuti per lo Stretto di Messina e la Sicilia nord-orientale, evidenziano un regime di tipo distensivo, con sigma1 prossimo alla verticale e sigma3 orizzontale, orientato circa WNW-ESE. Un campo di stress di tipo compressivo, con sigma1 che varia da NNW-SSE a N-S, e' stato ricavato nelle Isole Eolie, in Sicilia sud-orientale e nel settore nord-occidentale dell' Etna. Tale direzione del tensore dello sforzo, e' consistente con l' orientazione del campo di stress a scala regionale.
Seismic activity (Md higher than 2.0), recorded from August 2001 to December 2008, has been analyzed in order to estimate the seismic stress orientation in eastern Sicily and surrounding areas. The sub-areas of Aeolian Islands, northeastern Sicily, Mt. Etna volcano and southeastern Sicily have been separately analysed. The distribution of about 464 earthquakes highlighted the main tectonic features both inland and offshore. For the best located events, a shear wave polarization analysis has been performed with the aim to obtain more reliable focal mechanisms by combining P-wave polarities with S-wave polarizations. Different fault plane solution categories are present in the investigated region. A predominance of strike-slip and normal faults characterize the southeastern Sicily and Aeolian Islands sectors, whereas a prevalence of normal dip-slip solutions was observed in Messina Strait and northeastern Sicily. The P-axes analysis showed a significant NNW-SSE orientation both in the Hyblean Foreland, inland and offshore, and in the northwestern slope of Mt. Etna. No preferential orientation was observed in other sectors. About 250 focal mechanisms and various subsamples, based on the pattern of seismicity distributions and cumulative misfit method, were subsequently inverted by using the algorithm of Gephart and Forsyth (1984), for the best fitting stress tensor. The results show an extensional domain, characterized by a nearly vertical sigma1 and a horizontal WNW-ESE trending sigma3 axis in Messina Strait and northeastern Sicily. A compressional stress regime, with a sigma1 oriented from NNW-SSE to N-S was estimated in Aeolian Islands, southeastern Sicily and in the northwestern side of Mt. Etna, respectively, which is consistent with the regional stress field.

Dans mon projet, j'ai utilisé les gaz nobles (He, Ne, Ar, Kr et Xe) pour étudier les processus naturels se déroulant dans différents contextes géodynamiques (c.-à-d. subduction, collision continentale, rifting), montrant ainsi comment l'utilisation des gaz rares est fondamentale pour contraindre l'origine des substances volatiles et comment ils permettent une évaluation qualitative et quantitative des processus (interaction eau-gaz-roche) qui se produisent pendant la remontée des fluides de l'intérieur de la Terre vers l'atmosphère. Les résultats de mon projet peuvent être résumés en cinq thèmes principaux : 1) Aperçu de l’histoire du dégazage du manteau terrestre à partir d'analyses de haute précision des gaz rares du gaz magmatique ; 2) Systématique des gaz nobles et des isotopes du carbone sur le volcan Ciomadul, apparemment inactif (Roumanie): Preuve du dégazage volcanique ; 3) Fluides dérivés du manteau dans le bassin sédimentaire de Java oriental, Indonésie ; 4) Dégazage des volatiles du manteau dans un régime tectonique de compression hors du volcanisme: rôle de la délamination continentale ; 5) Dégazage continental de l'hélium dans un contexte tectonique actif (nord de l'Italie) : le rôle de la sismicité
In my project, I used the nobles gases (He, Ne, Ar, Kr and Xe) to investigate natural processes occurring in different geodynamical contexts (i.e., subduction, continental collision, rifting), showing how the use of the noble gases is fundamental to constrain the origin of volatiles, and to investigate the Earth interior. Furthermore, I also used these volatiles to recognize the processes (water-gas-rock interaction) that occur during the fluids up rise from the Earth’s interior to the atmosphere and quantitatively constrain the extents of these processes. The results of my project are summarized in five main topics: 1) Insights into the degassing history of Earth’s mantle from high precision noble gas analysis of magmatic gas 2) Noble Gas and Carbon Isotope Systematics at the Seemingly Inactive Ciomadul Volcano (Romania): Evidence for Volcanic Degassing 3) Mantle‐Derived Fluids in the East Java Sedimentary Basin, Indonesia 4) Outgassing of Mantle Volatiles in Compressional Tectonic Regime Away From Volcanism: The Role of Continental Delamination 5) Continental degassing of helium in an active tectonic setting (northern Italy): the role of seismicity

abstract: The dynamic Earth involves feedbacks between the solid crust and both natural and anthropogenic fluid flows. Fluid-rock interactions drive many Earth phenomena, including volcanic unrest, seismic activities, and hydrological responses. Mitigating the hazards associated with these activities requires fundamental understanding of the underlying physical processes. Therefore, geophysical monitoring in combination with modeling provides valuable tools, suitable for hazard mitigation and risk management efforts. Magmatic activities and induced seismicity linked to fluid injection are two natural and anthropogenic processes discussed in this dissertation. Successful forecasting of the timing, style, and intensity of a volcanic eruption is made possible by improved understanding of the volcano life cycle as well as building quantitative models incorporating the processes that govern rock melting, melt ascending, magma storage, eruption initiation, and interaction between magma and surrounding host rocks at different spatial extent and time scale. One key part of such models is the shallow magma chamber, which is generally directly linked to volcano’s eruptive behaviors. However, its actual shape, size, and temporal evolution are often not entirely known. To address this issue, I use space-based geodetic data with high spatiotemporal resolution to measure surface deformation at Kilauea volcano. The obtained maps of InSAR (Interferometric Synthetic Aperture Radar) deformation time series are exploited with two novel modeling schemes to investigate Kilauea’s shallow magmatic system. Both models can explain the same observation, leading to a new compartment model of magma chamber. Such models significantly advance the understanding of the physical processes associated with Kilauea’s summit plumbing system with potential applications for volcanoes around the world. The unprecedented increase in the number of earthquakes in the Central and Eastern United States since 2008 is attributed to massive deep subsurface injection of saltwater. The elevated chance of moderate-large damaging earthquakes stemming from increased seismicity rate causes broad societal concerns among industry, regulators, and the public. Thus, quantifying the time-dependent seismic hazard associated with the fluid injection is of great importance. To this end, I investigate the large-scale seismic, hydrogeologic, and injection data in northern Texas for period of 2007-2015 and in northern-central Oklahoma for period of 1995-2017. An effective induced earthquake forecasting model is developed, considering a complex relationship between injection operations and consequent seismicity. I find that the timing and magnitude of regional induced earthquakes are fully controlled by the process of fluid diffusion in a poroelastic medium and thus can be successfully forecasted. The obtained time-dependent seismic hazard model is spatiotemporally heterogeneous and decreasing injection rates does not immediately reduce the probability of an earthquake. The presented framework can be used for operational induced earthquake forecasting. Information about the associated fundamental processes, inducing conditions, and probabilistic seismic hazards has broad benefits to the society.
Dissertation/Thesis
Doctoral Dissertation Geological Sciences 2018

The primary objective of this study is to improve understanding of the mechanics of hydraulic fracturing in naturally-fractured reservoirs. The study focuses on enhancing the interpretation of hydraulic fracture-induced microseismic data using an S-wave Gaussian-beam method and numerical modeling techniques for interpretation. The S-wave Gaussian-beam method was comprehensively calibrated by synthetic and real data sets with different recording networks, and this showed the potential to retrieve additional microseismic data from hydraulic fracturing with linear receiver arrays. This approach could enhance current practice because a large number of induced events in these environments have very strong S-waves with P-wave amplitudes similar, or less than, background noise levels. The numerical study using the distinct element methods PFC2D and PFC3D was used to validate the understanding of the hydraulic fracturing mechanisms induced in laboratory and field fluid treatments in naturally-fractured reservoirs. This was achieved through direct comparison with the results of the geometry of hydraulic fractures and seismic source information (locations, magnitudes, and mechanisms) from both laboratory experiments and field observations. A suite of numerical models with fully-dynamic and hydro-mechanical coupling has been used to examine in detail the interaction between natural and induced fractures with the variations of the differential stresses and the orientations of the pre-fractures, and the relationship between the fluid front, the fracture tip, and the induced seismicity. The numerical results qualitatively agreed with the laboratory and field observations of the geometry of hydraulic fractures, confirmed the possible mechanics of new fracture development and their interactions with natural fractures, and illustrated the possible relationship between the fluid front and the fracture tip. The validated model could therefore help track the potential extent of induced fracturing in naturally-fractured reservoirs and the extent to which it can be detected by a microseismic monitoring array in order to assess the effectiveness of a hydraulic fracturing project.

Tese de mestrado em Ciências Geofísicas, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2016
O golfo de Cádis está localizado no sudoeste da Península Ibérica, na fronteira entre as placas tectónicas Euroasiática e Africana, onde a convergência é acomodada de forma difusa por uma rede de falhas activas. Esta região sismogénica é conhecida pela sua capacidade de gerar sismos de elevada magnitude, como o de 28 de Fevereiro de 1969 (MS = 7:9) ou o grande terremoto de Lisboa de 1 de Novembro de 1755 (MW = 8:7). O objectivo deste trabalho é compreender melhor a distribuição da sismicidade e falhas que estão actualmente activas na zona. Para estudar a sismicidade utilizámos seis meses de dados, obtidos a partir de uma rede temporária de vinte e quatro Ocean Bottom Seismometers (OBS), colocados a sudoeste da Península Ibérica, entre 20 de Janeiro e 21 de Julho de 2010, no âmbito do projecto internacional TOPO-MED. Adicionalmente, de forma a complementar estes dados, a melhorar geometria da rede e consequentemente minimizar os erros de localização hipocentral, usámos dados de quarenta e nove estações em terra. Estas estações fazem parte de redes permanentes e temporárias, localizadas a sul de Portugal, sudoeste de Espanha e noroeste de Marrocos. A região a ser aqui objecto de estudo foi limitada a uma área rectangular de 4:5◦ X 2:0◦, de 11:0◦O a 6:5◦O e de 35:0◦N a 37:0◦N, e ao tempo de operação dos OBS. Fizemos uma análise da completude e identificação das componentes sísmicas destas estações, de onde concluímos que, à excepção de uma estação que dispunha apenas do hidrofone, OBS 35, todas as outras tinham dados disponíveis para a análise. Para reduzir o tempo de computação dos processos de detecção e localização, tanto os dados de OBS como os de terra foram nesta primeira fase de análise, decimados para 50 Hz. Como estamos a analisar seis meses de dados contínuos de setenta e três estações, detectar as fases sísmicas de forma manual seria um processo muito dispendioso em termos de tempo. Decidimos por isso adoptar uma abordagem automatizada. Utilizámos o algoritmo Lassie (Heimann et al., 2015), que permite a detecção de chegadas coincidentes com energia sísmica às estações, e baseia-se nas funções características das formas de onda. Este método foi utilizado porque os resultados que fornece podem ser directamente usados num programa de localização, como o LOKI (Grigoli et al., 2013), sem passar pela tarefa de picar e identificar as chegadas sísmicas. O catálogo fornecido pelo Lassie com informações sobre o tempo a que os eventos foram detectados, as suas coordenadas e o valor de amplitude que as detecções obtiveram, tem de ser analisado manualmente para rejeitar potenciais falsos alarmes que tenham sido registados. Estes falsos alarmes podem ser acontecimentos locais, problemas no sensor, mudança de dia ou falta de dados. Todos estes têm uma assinatura particular e de elevada amplitude, que ao distinguir-se tanto do resto do ruído, são considerados como sismos. O algoritmo Lassie tem como parâmetros de entrada a grelha espacial, a largura de banda do filtro a aplicar aos dados, a velocidade aparente das ondas que se movem nesta grelha, e o valor limite acima do qual se pode considerar ter-se detectado um sismo. Este último parâmetro é o mais difícil de determinar e não é imediato. Para saber qual o que melhor serve os dados usados é necessário fazer um teste prévio. Fizemos esta pré-análise a uma amostra de teste de 10% dos dados, o que corresponde a uma análise de dezanove dias, escolhidos aleatoriamente entre o período de 2010-05-07 e 2010-05-25. Após analisar manualmente todas as detecções do catálogo que caem dentro da região de estudo com o Snuffler (Heimann, 2012), assumimos que o valor de 110.0 de amplitude é um bom compromisso entre detectar um número considerável de sismos, e evitar a detecção de demasiados falsos alarmes. Este valor foi aplicado aos seis meses de dados e todas as detecções do catálogo que caíram dentro da zona de estudo foram novamente inspeccionadas manualmente com o Snuffler. Foram detectados 153 sismos dentro da área de interesse. Para o processo de localização destes sismos, utilizámos dois programas, o LOKI e o NonLinLoc (Lomax et al., 2000). Antes de utilizar o LOKI, fizemos uma pré-análise às formas de onda, para retirar dos eventos as formas de onda que apresentassem uma razão sinal-ruído muito baixa. Neste processo também ajustámos as janelas temporais à duração do sismo, para evitar que o LOKI localizasse ruído coerente. Com esta análise foi possível perceber que tanto as estações de OBS, como as da rede sísmica XB, não iriam contribuir de forma significativa para a localização dos sismos, visto apresentarem uma razão sinal-ruído baixa. O LOKI tem a vantagem de não necessitar de um catálogo de fases, o que permite trabalhar bem com formas de onda cujas chegadas P e/ou S sejam emergentes. O programa utilizou uma grelha espacial de 520 km X 370 km X 80 km a partir do ponto 11:0◦O, 35:0◦N e fez uso da razão entre a short-timeaverage e a long-time-average (STA/LTA), por forma a construir uma matriz de coerência a partir da qual as localizações podem ser inferidas. Este programa foi utilizado com dois modelos de velocidades diferentes, um 1D (Geissler et al., 2010) e outro 3D (Arroucau and Custódio, 2015). Mostra-se que a utilização do modelo de velocidades 3D permite obter hipocentros mais robustos. Por inspecção visual, o modelo 3D apresenta maiores valores de coerência no ponto que é considerado uma localização, e os pontos na vizinhança deste têm valores de coerência muito menores. Para mais, para cada evento calculámos a área que era ocupada por 95% do valor da coerência máxima. Somámos todas as áreas e o modelo 1D apresenta uma área de 66809 km2, enquanto que o modelo 3D tem apenas 43851 km2, valor bastante inferior. O LOKI tem a desvantagem de ser susceptível a localizar ruído coerente, e como também não apresenta os erros de localização, como RMS, GAP ou comprimento do semi-eixo maior da elipse, decidimos utilizar outro método, como o NonLinLoc. O NonLinLoc é um conjunto de programas (Vel2Grid, Grid2Time e NLLoc) que utiliza técnicas não lineares para fazer a localização dos hipocentros. O NLLoc necessita de um catálogo de fases picadas e identificadas, que foi construído previamente com o Snuffler. Para a localização utilizámos o modelo de velocidades 3D e o algoritmo de Oct-Tree Importance Sampling. Este algoritmo baseia-se no cálculo da função de densidade de probabilidade, em células que vão sendo sucessivamente divididas em oito partes iguais. Conseguimos localizar 61 sismos dentro da área de estudo, com RMS ≤ 1:0 segundos, comprimento do semi-eixo maior da elipse ≤ 10 km, e GAP ≤ 220◦. A sismicidade está localizada até cerca de 60 km de profundidade. Os resultados obtidos foram comparados com as localizações reportadas por Grevemeyer et al. (2016) e pelo IPMA, obtidas para o mesmo período de tempo. Em ambos os casos, mostrámos que com a utilização de dados de OBS e um modelo de velocidades 3D, consegue-se localizar um maior número de sismos de forma mais robusta. Analisámos também os eventos do catálogo sísmico para estimar magnitudes. Começámos por remover a resposta instrumental das formas de onda, o que implica saber os pólos, zeros, sensibilidade e ganho dos sensores. Como esta informação não estava disponível para os instrumentos dos OBS e algumas estações em terra, só foi possível remover a resposta de 33 das 73 estações. Recorremos à fórmula de Carrilho and Vales (2009) e concluimos que o intervalo de magnitudes varia entre 0.0 e 4.3. Os sismos de maior magnitude (≥ 3:0) podem ser encontrados até cerca de 40 km de profundidade, enquanto que sismos de magnitude menor, entre 2.0 e 3.0, podem ser encontrados até cerca de 60 km. Estudámos ainda a similaridade entre formas de onda para determinar se existem aglomerados de eventos. Estes terão de apresentar os mesmos mecanismos focais, os mesmos efeitos de sítio e o mesmo padrão de radiação, indicativo de terem sido gerados na mesma estrutura tectónica. Para poderem apresentar o mesmo efeito de sítio, as ondas sísmicas têm de percorrer caminhos semelhantes, como tal, a correlação é feita apenas para os eventos registados na mesma estação. Utilizámos bibliotecas do Python/Obspy (Beyreuther et al., 2010) para analisar as componentes Z de cada evento, em torno da chegada da onda P, 0.6 segundos antes desta e 1.0 segundo depois. Embora com poucos eventos, apenas 61, conseguimos encontrar dois aglomerados, ambos com 4 eventos com coeficientes de correlação maiores ou iguais a 0.8. Um destes aglomerados encontra-se perto de falhas já mapeadas, enquanto que o outro encontra-se numa zona sem qualquer falha mapeada. Para se conseguir inferir alguma conclusão, é necessário repetir a mesma análise mas para um período de tempo mais alargado.
The gulf of Cadiz is a seismogenic region located southwest of the Iberia Peninsula, in the Eurasian-African plate boundary, where plate convergence is accommodated in a diffuse way along a complex network of faults. This region is known for its ability to cause earthquakes of large magnitude, such as the great Lisbon earthquake of November 1, 1755. With the aim of better understanding faults that are currently active in the region, we studied the seismicity in a particular area of 4:5◦ X 2:0◦, from 11:0◦W to 6:5◦W and from 35:0◦N to 37:0◦N, in the gulf of Cadiz, from January, 20 to July, 21 of 2010. We focused on using data from 24 OBS, deployed within this region and at this time under the TOPO-MED project (Grevemeyer, 2011). In order to improve the geometry of the network we further used 49 land stations, from temporary and permanent networks, located in south of Portugal, southwest of Spain and northwest of Morocco. We used a new methodology to study the seismicity of this region. The detection of earthquakes of low magnitude was performed using the algorithm Lassie (Heimann et al., 2015). We further used a 3D velocity model (Arroucau and Custódio, 2015) to locate the 61 earthquakes detected within the study area. Our locations are compared with others obtained by studies made for the same period, by Grevemeyer et al. (2016) and by Portuguese Sea and Atmosphere Institute (IPMA), both using 1D velocity models. We show that hypocentres resulted from the 3D velocity model analysis are better constrained. We also estimated local magnitudes, which range between 0.0 and 4.3, and searched for clusters of events that belong to the same tectonic structure. We were able to find two clusters, with 4 events each and cross-correlation coefficients higher than or equal to 0.8. While one of these is located near faults, the other cluster is located in an area far away from the mapped faults.

Tese de doutoramento, Geologia (Geodinâmica Interna), Universidade de Lisboa, Faculdade de Ciências, 2017
The Gulf of Cadiz offshore SW Iberia is an area linked with episodic destructive seismic and tsunamigenic events, such as the M~8.8, 1st November 1755 Lisbon earthquake among others. The association of active faults to this kind of high magnitude event has been extensively studied specially due to the contribution of several international projects for more than two decades. However, the meaning of the persistent small to intermediate magnitude seismicity recognized in this region is still particularly not fully understood. This is, at least, related to the lack of an accurate hypocenter location of these events resulting from an asymmetrical geographical distribution of the permanent seismic network. One of the main purposes of the NEAREST project (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system GOCE, contract n. 037110) was the identification and characterization of seismogenic and tsunamigenic structures in the Gulf of Cadiz area, source region of the Lisbon 1755 earthquake and tsunami. To address this problem 24 broadband Ocean Bottom Seismometers (OBS) and a seafloor multiparametric station GEOSTAR (Geophysical and Oceanographic Station for Abyssal Research) acquired between August 2007 and July 2008 passive seismic data in this region. The results delivered a detailed record of the local seismicity, revealing 3 main clusters of earthquakes, two of them coinciding with the location of the 3 larger instrumental earthquakes in the area: i) the 28th February 1969 (Mw~8.0); ii) the 12th February 2007 (Mw=6.0) and iii) the 17th December 2009 (Mw=5.5). Focal mechanisms show a mixed pattern, mostly strike-slip and reverse dip-slip with a very few normal mechanisms. The results show that of the recorded events are located in the mantle (at depths between 30 and 60 km). This implies the existence of tectonically active structures located much deeper than the ones mapped by Multichannel seismic reflection. A thorough analysis shows that the seismicity clusters are offset with respect to the upper crustal active thrusts. The wide-range solutions of focal mechanisms also imply that the related source processes are complex. This can reflect the interaction of different active geological features, such as faults and rheological boundaries. To understand these new results in the context of the seismotectonics of the Gulf of Cadiz a review of some available geophysical data (reflection and refraction seismic profiles interpretation) in this area is presented as well as novel work on seismic reflection profile IAM GB1 across a rheologic boundary and seismicity cluster. Our study shows that the seismicity clusters are located at faults intersections mapped at the seafloor and shallow crust, suggesting that the crustal tectonic faults are replicated in the lithospheric mantle. These fault interferences are associated with boundaries of lithospheric domains prone to localize stress and seismic strain. Active crustal faults are either locked or move through slow aseismic slip. Frictional slip in crustal faults is probably limited to high magnitude earthquakes. Serpentinization probably induces tectonic decoupling limiting micro-seismicity to depths below the serpentinized layer. It is expected that during highmagnitude events seismic rupture is favored by weakening mechanisms and propagates upwards through the serpentinized layer up to the surface. The results obtained in this work improve our knowledge about the local seismicity and related active faults in the Gulf of Cadiz area, giving a new contribution to access to the seismic hazard in the Nubia-Iberia plate boundary in the Northeast Atlantic Region.
O Golfo de Cádis é uma região com uma sismicidade moderada embora se conheça, tanto no registo histórico como instrumental, eventos de elevada magnitude. O sismo de 1 de Novembro de 1755 é um exemplo paradigmático com uma magnitude estimada de 8.8 e um tsunami associado com Mt = 8.5. Já o sismo de 28 de Fevereiro de 1969, é o mais importante registado instrumentalmente, teve uma Ms de 7.9, ao qual esteve associado um pequeno tsunami. Mais recentemente, salientam-se os sismos de 12 de Fevereiro de 2007, com Mw=6.0 e o de 17 de Dezembro de 2009, com Mw =5.5 (EMSCEuropean-Mediterranean Seismological Centre). No entanto, a sismicidade nesta região é descrita como de magnitude baixa a intermédia, com uma distribuição em profundidade acima dos 60 km. Correlacionar esta sismicidade com potenciais estruturas sismogénicas no Golfo de Cádis constituiu um dos objectivos do projecto NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system GOCE, contract n. 037110). Neste contexto, foram necessárias uma caracterização e localização mais precisas dos eventos sísmicos ocorridos nesta região, até agora limitadas pelos constrangimentos inerentes à distribuição geográfica das estações permanentes terrestres. Por isso, foi desenvolvida uma campanha de aquisição de dados contínuos utilizando uma rede de sismómetros de fundo do mar. A rede sísmica NEAREST operou de modo contínuo num período de 11 meses, entre Agosto de 2007 Julho de 2008, integrando 24 sismómetros de fundo do mar (OBS) e uma estação multiparamétrica- GEOSTAR. Durante as campanhas de colocação e recuperação dos instrumentos, as manipulações dos OBS e GEOSTAR estiveram a cargo do Alfred Wegener Institute for Polarand Marine Research e do Istituto Nazionale di Geofisica e Vulcanologia – INGV, respectivamente. Os OBS foram construídos pela K.U.M. Umwelt- und Meerestechnik Kiel GmbH, Germany e incorporavam sismómetros de banda larga Güralp CMG-40T e um hidrofone. A GEOSTAR é um observatório que integra diversos equipamentos para a recolha de dados geofísicos e oceanográficos em contínuo. Nesta estação estão incluídos um sismómetro de banda larga com 3 componentes e um hidrofone usados nesta campanha. As estações terrestres estão a cargo do Instituto Português do Mar e da Atmosfera (IPMA) e Instituto Dom Luiz (IDL) correspondendo a sismómetros de banda larga também com 3 componentes. Os registos nestas estações foram apenas utilizados para constranger as soluções dos mecanismos focais. Em trabalhos futuros, prevê-se a sua inclusão na localização dos eventos identificados pela rede NEAREST. Durante o período de aquisição foram registados na rede terrestre, para a área delimitada pela rede NEAREST, cerca de 270 sismos locais. Durante o período de funcionamento da rede NEAREST foram identificados cerca de 750 eventos observados em mais de 3 estações. Deste total 590 sismos estavam localizados na área da rede NEAREST. A localização hipocentral foi testada usando diferentes metodologias e modelos de velocidades: a) inversão conjunta das posições hipocentrais e correcções de estações; b) o método das diferenças duplas e c) a inversão conjunta do modelo de velocidades-localizações hipocentrais e correcções de estações. O catálogo final inclui 443 eventos identificados em mais de 6 estações e localizados na área da rede NEAREST. De um modo geral, a maioria dos hipocentros estão localizados a mais de 30 km de profundidade, portanto no manto. As magnitudes locais variam entre 1.2 e 4.8. As localizações epicentrais e hipocentrais baseadas na rede NEAREST divergem das soluções conhecidas para a rede terrestre (providenciadas pelo IPMA), estando deslocadas para SW e sendo mais profundas. A diferença de profundidade pode atingir os 40 km. A campanha do projecto NEAREST permitiu a identificação de uma grande quantidade de eventos não detectada pela rede terrestre. Esta campanha permitiu ainda uma redefinição da distribuição da sismicidade na região, até então considerada difusa. Destes resultados foi possível reconhecer 3 enxames de sismicidade, dois destes coincidentes com 3 dos maiores eventos observados no registo instrumental. Tanto os sismos de 28 de Fevereiro de 1969 (Mw~8.0) como 12 de Fevereiro de 2007 (Mw=6.0) na proximidade da Falha da Ferradura e 17 de Dezembro de 2009 (ML=6.0) na região do canhão de São de Vicente. Os mecanismos focais do catálogo NEAREST são consistentes com estes eventos bem como com soluções de tensores de momento publicadas para esta região. No enxame do canhão de São Vicente é onde estão localizados a maioria dos eventos. Os hipocentros encontram-se a profundidades entre os 20 e os 55 km. A distribuição dos epicentros apresenta um alinhamento ≈ NE-SW ao longo do canhão de São Vicente e prolongando-se para o limite NE da Falha da Ferradura. Os mecanismos focais dominantes são de desligamento e oblíquos, combinando movimento de desligamento com uma menor contribuição de movimento inverso. Foram ainda registados raros eventos em falha normal. A compressão máxima é aproximadamente sub-paralela ao SHmax, com uma direcção ≈NW-SE. Os epicentros localizados no enxame a SW da Falha da Ferradura, tem um alinhamento aproximadamente NW-SE, sub-paralelo à direcção de SHmax regional. Neste enxame os hipocentros são mais profundos localizando-se entre os 30 e os 55km. Os mecanismos focais são na sua maioria de desligamento puro existindo alguns eventos em falha inversa e também raras soluções em falha normal. Importa salientar que as soluções de desligamento apresentam frequentemente um plano subparalelo à orientação das falhas de desligamento SWIM (≈WNW-ESE a E-W). A compressão máxima é aproximadamente NW-SE e NNW-SSE, a W e E do enxame de sismicidade, respectivamente. As direcções de SHmax são mais uma vez coincidentes com a direção de compressão máxima. No enxame do Banco do Gorringe maioria dos sismos estão localizados no bordo SW deste relevo submarino, sub-paralelos à falha do Gorringe. Os eventos são menos profundos quando comparados com os outros dois enxames, na sua maioria acima dos 40 km. Os mecanismos focais são na sua maioria de desligamento e em falha inversa. Também neste enxame foram registados alguns sismos em falha normal. A direcção de compressão máxima e o SHmax são NNW-SSE. O facto de estes eventos se localizarem predominantemente no manto constitui um dos principais resultados deste trabalho. Neste contexto, tendo em consideração a profundidade dos eventos sísmicos, a correlação da sismicidade com as estruturas sismogénicas na região do Golfo de Cádis é particularmente complexa. Esta comparação foi desenvolvida com base nos dados de sísmica de reflexão e refração disponíveis. Do nosso estudo resulta que a sismicidade parece estar concentrada em zonas de interferência de falhas localizadas no manto subcrustal litosférico. Estas deverão ser uma replicação do padrão observado a níveis crustais e parecem ser coincidentes com transições entre diferentes domínios litosféricos. Estas zonas de interferência de falhas deverão ser áreas favoráveis à acumulação de tensões e deformação sísmica. As falhas activas crustais deverão estar ou bloqueadas ou movimentar-se de modo assísmico. A movimentação sísmica pode estar associada apenas a sismos de maior magnitude. A existência de níveis serpentinizados no Golfo de Cádis é suportada por dados de sísmica refracção e furos de sondagens profundas. Estes podem funcionar como planos de descolamento para as grandes falhas inversas, acomodando a movimentação asísmica e impedindo a micro-sismicidade de se propagar aos níveis crustais. Durante os sismos de elevada magnitude estes níveis serpentinizados deverão funcionar como zona enfraquecida, de baixo atrito, favorecendo a propagação da ruptura sísmica até à superfície. Os resultados obtidos neste trabalho melhoram o nosso conhecimento sobre a sismicidade e a sua relação com as falhas activas na região do limite de placas litosféricas no Golfo de Cádis, contribuindo para o estudo do risco sísmico associada a sismos devastadores.

The development of unconventional hydrocarbon resources in the Norman Wells region of the Central Mackenzie Valley, Northwest Territories, has been explored by the energy industry. In early 2014, Conoco-Philips Canada conducted two multi-stage test operations of hydraulic fracturing (HF) in the region. In this study, we combine seismic data from the Canadian National Seismograph Network, four new stations established by the Northwest Territories Geoscience Office in collaboration with Natural Resources Canada in the Norman Wells region, and a local dense array installed by Conoco-Philips Canada to study the seismicity distribution during the pre-HF, HF and post-HF periods. We have identified and located 130 earthquakes within 100 km of the geographic centre of the local seismic network near Norman Wells for the pre-HF period (11 September 2013 - 7 February 2014). In comparison, 231 events are located during the HF period (8 February 2014 - 10 March 2014), and for the two post-HF periods, 11 March 2014 - 31 July 2014 and 27 February 2015 - 31 December 2015, we have catalogued 255 and 138 events, respectively. Source parameters and detailed phase pickings of each earthquake are given in the Appendices.

Near-to-surface geothermal energy with heat pumps is state of the art and is already widespread in Switzerland. In the future energy system, medium-deep to deep geothermal energy (1 to 6 kilometres) will, in addition, play an important role. To the forefront is the supply of heat for buildings and industrial processes. This form of geothermal energy utilisation requires a highly permeable underground area that allows a fluid – usually water – to absorb the naturally existing rock heat and then transport it to the surface. Sedimentary rocks are usually permeable by nature, whereas for granites and gneisses permeability must be artificially induced by injecting water. The heat gained in this way increases in line with the drilling depth: at a depth of 1 kilometre, the underground temperature is approximately 40°C, while at a depth of 3 kilometres it is around 100°C. To drive a steam turbine for the production of electricity, temperatures of over 100°C are required. As this requires greater depths of 3 to 6 kilometres, the risk of seismicity induced by the drilling also increases. Underground zones are also suitable for storing heat and gases, such as hydrogen or methane, and for the definitive storage of CO2. For this purpose, such zones need to fulfil similar requirements to those applicable to heat generation. In addition, however, a dense top layer is required above the reservoir so that the gas cannot escape. The joint project “Hydropower and geo-energy” of the NRP “Energy” focused on the question of where suitable ground layers can be found in Switzerland that optimally meet the requirements for the various uses. A second research priority concerned measures to reduce seismicity induced by deep drilling and the resulting damage to buildings. Models and simulations were also developed which contribute to a better understanding of the underground processes involved in the development and use of geothermal resources. In summary, the research results show that there are good conditions in Switzerland for the use of medium-deep geothermal energy (1 to 3 kilometres) – both for the building stock and for industrial processes. There are also grounds for optimism concerning the seasonal storage of heat and gases. In contrast, the potential for the definitive storage of CO2 in relevant quantities is rather limited. With respect to electricity production using deep geothermal energy (> 3 kilometres), the extent to which there is potential to exploit the underground economically is still not absolutely certain. In this regard, industrially operated demonstration plants are urgently needed in order to boost acceptance among the population and investors.