Thermal Nature of Mantle Upwellings Below the Ibero‐Western Maghreb Region Inferred From Teleseismic Tomography

Independent models of P wave and S wave velocity anomalies in the mantle derived from seismic tomography help to distinguish thermal signatures from those of partial melt, volatiles, and compositional variations. Here we use seismic data from SW Europe and NW Africa, spanning the region between the Pyrenees and the Canaries, in order to obtain a new S‐SKS relative arrival‐time tomographic model of the upper mantle below Iberia, Western Morocco, and the Canaries. Similar to previous P wave tomographic results, the S wave model provides evidence for (1) subvertical upper‐mantle low‐velocity structures below the Canaries, Atlas Ranges, and Gibraltar Arc, which are interpreted as mantle upwellings fed by a common lower‐mantle source below the Canaries; and (2) two low‐velocity anomalies below the eastern Rif and Betics that we interpret as the result of the interaction between quasi‐toroidal mantle flow induced by the Gibraltar slab and the mantle upwelling behind it. The analysis of teleseismic P wave and S wave arrival‐time residuals and the conversion of the low‐velocity anomalies to temperature variations suggest that the upwellings in the upper mantle below the Canaries, Atlas Ranges, and Gibraltar Arc system may be solely thermal in nature, with temperature excesses in the range ~100–350 °C. Our results also indicate that local partial melting can be present at lithospheric depths, especially below the Atlas Ranges. The locations of thermal mantle upwellings are in good agreement with those of thinned lithosphere, moderate to high heat‐flow measurements, and recent magmatic activity at the surface

A common deep source for upper-mantle upwellings below the Ibero-western Maghreb region from teleseismic P-wave travel-time tomography

Upper-mantle upwellings are often invoked as the cause of Cenozoic volcanism in the Ibero-western Maghreb region. However, their nature, geometry and origin are unclear. This study takes advantage of dense seismic networks, which cover an area extending from the Pyrenees in the north to the Canaries in the south, to provide a new high-resolution P-wave velocity model of the upper-mantle and topmost lower-mantle structure. Our images show three subvertical upper-mantle upwellings below the Canaries, the Atlas Ranges and the Gibraltar Arc, which appear to be rooted beneath the upper-mantle transition zone (MTZ). Two other mantle upwellings beneath the eastern Rif and eastern Betics surround the Gibraltar subduction zone. We propose a new geodynamic model in which narrow upper-mantle upwellings below the Canaries, the Atlas Ranges and the Gibraltar Arc rise from a laterally-propagating layer of material below the MTZ, which in turn is fed by a common deep source below the Canaries. In the Gibraltar region, the deeply rooted upwelling interacts with the Gibraltar slab. Quasi-toroidal flow driven by slab rollback induces the hot mantle material to flow around the slab, creating the two low-velocity anomalies below the eastern Betics and eastern Rif. Our results suggest that the Central Atlantic plume is a likely source of hot mantle material for upper-mantle upwellings in the Ibero-western Maghreb region

The Arraiolos – Portugal – Moderate-Sized 2018 (M = 4.9) earthquake of January 15 and aftershocks: preliminary results

On 15 January 2018 at 11:51 UTC, an earthquake of 4.9 ML occurred in the Northeast of Arraiolos region near Aldeia da Serra village. The hypocentral location, determined by Instituto Português do Mar e da Atmosfera (IPMA), has coordinates 38.79 N, 7.93 W at 11 km depth. The focal mechanisms determined by P-wave first motion polarities and waveforms inversion indicate a dominance of strike-slip events with nodal plans near NS (left lateral) and EW (right lateral) directions. Due to the lower magnitude, the earthquake didn’t cause damage but was widely felt in the Centre and South of Portugal mainland. In the vicinity of the epicentre, at the Aldeia da Serra village, it reached a maximum intensity VI, having been felt with intensity IV/V in the city of Évora, about 20 km from the epicentre. The event was also felt with intensity III in Lisbon at more than 100 km from the epicentre. This event caused alarm in the population that haven’t felt an earthquake for several decades. It also raised the media attention with many reports and interviews on TV and newspapers. The main ear thquake was immediatel y followed by a sequence of aftershocks of which the largest one, with ML=3.1, occurred the 1st February, fifteen days after the main shock and was largely felt by the population in the region of Arraiolos.info:eu-repo/semantics/publishedVersio

Ocean bottom seismic noise : applications for the crust knowledge, interaction ocean-atmosphere and instrumental behaviour

Tese de doutoramento, Ciências Geofísicas e da Geoinformação (Geofisíca), Universidade de Lisboa, Faculdade de Ciências, 2014The seismic observation by long deployment experiments in ocean domain is a great technological challenge, with advancements only having been obtained in the last decade. The number of available instruments is still small and the number of missions successfully carried out remains low. Therefore, unlike land observation, seismic acquisition in the ocean seafloor is in an experimental phase and part of a learning process. In particular, the interaction of the sensor with the water environment has implications and applications that have only recently begun to be explored. In this PhD thesis we intend to use the continuous recording data produced by 24 broad-band (BB) ocean bottom seismometers (OBS), deployed in the Gulf of Cádiz in the context of the NEAREST project (September 2007 – August 2008), to understand the functioning of this type of equipment, the environmental conditions that control the observed noise and the effects of the sensor coupling. Sea floor records have a basic characteristic that makes them different from land records: the sensor’s interaction with the surrounding sea water (e.g. Webb, 1998). Sensors will respond to water movement and pressure variations, which does not happen with land sensors and, in most cases, they are deployed at locations with non-consolidated sediments that dramatically affect the quality of the seismic signal. The future growth of seismic observation at the bottom of the ocean, which is a fundamental complement to land observation, requires that all these mechanisms that affect the seismic records of an OBS are characterized in the best possible way, leading to better equipment design and better conception of observation campaigns, in line with the intended objectives. The continuous acquisition of seismic data by land stations, which is possible today due to the evolution of communications and recording systems, enables us to accumulate an enormous volume of data of which earthquakes are a tiny part, less than 5 % of the recorded data for one day. The remaining 95%, which is rejected in the normal operation of a seismic network, has been receiving attention from researchers in several domains. In particular, when the noise sources are distributed homogeneously (or almost) around a couple of stations it is possible to show that the noise correlation function (NCF) represents the Green function of seismic propagation (in Shapiro and Campillo, 2004, Sabra et al., 2005). This theorem has led to a great number of applications: i) Seismic ambient noise as an efficient tool for seismic tomography (for example: Stehly et al., 2009; Villaseñor et al., 2007, Graça et al., 2013); ii) detection of anomalies in seismic records (e.g., Stehly et al., 2007); iii) variations in superficial structure with time (e.g., Brenguier et al., 2008). Studies of noise sources have increased in the research community, but studies carried out in ocean domains are scarce and joint land-ocean are even more rare. In this thesis, we intend to use the continuous seismic recording, mainly the ambient noise component, to extract information related with: i) shallow crustal structure; ii) conditions of sensor coupling; iii) clock drifts of the acquisition systems; iv) meteorological and oceanographic environmental noise sources. For this purpose it was crucial to implement new methodologies and to adapt current procedures in land records. At the end of these studies we hope to be able to make some recommendations regarding: i) conception and design of BB OBS; ii) interpretation of seismic signals recorded at the ocean bed; iii) planning of future campaigns of long deployment seismic experiments.A observação sísmica de longa duração, em ambiente marinho, representa um grande desafio tecnológico e apenas nas últimas décadas foram obtidos avanços significativos. Os instrumentos disponíveis são limitados e o número de missões com sucesso é ainda reduzido. Ao contrário da observação terrestre, a aquisição sísmica no fundo do oceano encontra-se numa fase experimental e em contínuo processo de aprendizagem. Em particular, a interação do sensor com o ambiente marítimo tem implicações e aplicações que só recentemente começaram a ser exploradas. Nesta tese de doutoramento, pretendemos usar os dados registados em 24 estações sísmicas de banda larga (BB), de fundo oceânico (OBS), implantadas no Golfo de Cádiz, no âmbito do projeto NEAREST (Setembro de 2007- Agosto de 2008), para melhor compreender o funcionamento deste tipo de equipamento, as condições ambientais que controlam o ruído observado e os efeitos de acoplamento do sensor. Os registos no fundo do mar têm uma característica básica que os diferencia dos de terra: a interação do sensor com a água do mar circundante (Webb, 1998). O sensor irá responder ao movimento da água e às variações de pressão, fenómeno que não acontece nos sensores terrestres, e o facto de estar instalado em áreas de sedimentos não consolidados também afeta dramaticamente a qualidade do sinal sísmico. O desenvolvimento da observação sísmica no fundo oceânico, é fundamental para completar a observação em terra e exige que todos os mecanismos que afectam os registos sísmicos de um OBS sejam caracterizados o melhor possível, para aprimorar a concepção dos equipamentos e de uma melhor concepção de campanhas de observação, em função dos objectivos a alcançar. As redes de estações sísmicas terrestres de registo contínuo, permitidas hoje pela evolução das comunicações e dos sistemas de aquisição, permitem acumular um volume enorme de dados, dos quais os eventos sísmicos são apenas uma pequena parte, menos de 5% dos dados registados diariamente. Os remanescentes 95%, os quais eram anteriormente rejeitados, recebem actualmente especial atenção dos investigadores das diferentes áreas da sismologia. Em particular, quando as fontes de ruído, são distribuídas de forma homogénea (ou quase), em redor de um par de estações, é possível demonstrar que a função de correlação do ruído (NCF), representa a “Green function” da propagação sísmica (em Shapiro e Campillo, 2004; Sabra et al., 2005). Este teorema tem levado a um grande número de aplicações : i) Ruído sísmico ambiental como uma ferramenta eficiente para tomografia sísmica (por exemplo: Stehly et al., 2009; Villaseñor et al., 2007; Graça et al., 2013); ii) detecção de sinais anómalos nos registos sísmicos (v.g. Stehly et al., 2007); iii) variações na estrutura de propagação com o tempo (por exemplo, Brenguier et al, 2008). Os estudos sobre fontes de ruído terrestre aumentaram na comunidade científica, mas no domínio oceânico são, ainda, escassos. Nesta tese, pretende-se usar o registo contínuo dos dados sísmicos, principalmente a componente de ruído ambiente, para extrair informações relacionadas com: i) estrutura de crosta superficial e sua reologia; ii) acoplamento do sensor; iii) derivas temporais no relógio dos sistemas de aquisição; iv) características das fontes de ruído ambiental, meteorológico e oceanográfico. Para isso, é crucial implementar novas metodologias e adaptar os procedimentos realizados nos registos terrestres. No final deste estudo, pretende-se formular algumas recomendações sobre: i) concepção e desenho de OBS BB; ii ) a interpretação dos inúmeros sinais sísmicos registados no leito do oceano, iii) o planeamento de futuras campanhas de longa duração

Ocean bottom seismic noise : applications for the crust knowledge, interaction ocean-atmosphere and instrumental behaviour

Tese de doutoramento, Ciências Geofísicas e da Geoinformação (Geofisíca), Universidade de Lisboa, Faculdade de Ciências, 2014The seismic observation by long deployment experiments in ocean domain is a great technological challenge, with advancements only having been obtained in the last decade. The number of available instruments is still small and the number of missions successfully carried out remains low. Therefore, unlike land observation, seismic acquisition in the ocean seafloor is in an experimental phase and part of a learning process. In particular, the interaction of the sensor with the water environment has implications and applications that have only recently begun to be explored. In this PhD thesis we intend to use the continuous recording data produced by 24 broad-band (BB) ocean bottom seismometers (OBS), deployed in the Gulf of Cádiz in the context of the NEAREST project (September 2007 – August 2008), to understand the functioning of this type of equipment, the environmental conditions that control the observed noise and the effects of the sensor coupling. Sea floor records have a basic characteristic that makes them different from land records: the sensor’s interaction with the surrounding sea water (e.g. Webb, 1998). Sensors will respond to water movement and pressure variations, which does not happen with land sensors and, in most cases, they are deployed at locations with non-consolidated sediments that dramatically affect the quality of the seismic signal. The future growth of seismic observation at the bottom of the ocean, which is a fundamental complement to land observation, requires that all these mechanisms that affect the seismic records of an OBS are characterized in the best possible way, leading to better equipment design and better conception of observation campaigns, in line with the intended objectives. The continuous acquisition of seismic data by land stations, which is possible today due to the evolution of communications and recording systems, enables us to accumulate an enormous volume of data of which earthquakes are a tiny part, less than 5 % of the recorded data for one day. The remaining 95%, which is rejected in the normal operation of a seismic network, has been receiving attention from researchers in several domains. In particular, when the noise sources are distributed homogeneously (or almost) around a couple of stations it is possible to show that the noise correlation function (NCF) represents the Green function of seismic propagation (in Shapiro and Campillo, 2004, Sabra et al., 2005). This theorem has led to a great number of applications: i) Seismic ambient noise as an efficient tool for seismic tomography (for example: Stehly et al., 2009; Villaseñor et al., 2007, Graça et al., 2013); ii) detection of anomalies in seismic records (e.g., Stehly et al., 2007); iii) variations in superficial structure with time (e.g., Brenguier et al., 2008). Studies of noise sources have increased in the research community, but studies carried out in ocean domains are scarce and joint land-ocean are even more rare. In this thesis, we intend to use the continuous seismic recording, mainly the ambient noise component, to extract information related with: i) shallow crustal structure; ii) conditions of sensor coupling; iii) clock drifts of the acquisition systems; iv) meteorological and oceanographic environmental noise sources. For this purpose it was crucial to implement new methodologies and to adapt current procedures in land records. At the end of these studies we hope to be able to make some recommendations regarding: i) conception and design of BB OBS; ii) interpretation of seismic signals recorded at the ocean bed; iii) planning of future campaigns of long deployment seismic experiments.A observação sísmica de longa duração, em ambiente marinho, representa um grande desafio tecnológico e apenas nas últimas décadas foram obtidos avanços significativos. Os instrumentos disponíveis são limitados e o número de missões com sucesso é ainda reduzido. Ao contrário da observação terrestre, a aquisição sísmica no fundo do oceano encontra-se numa fase experimental e em contínuo processo de aprendizagem. Em particular, a interação do sensor com o ambiente marítimo tem implicações e aplicações que só recentemente começaram a ser exploradas. Nesta tese de doutoramento, pretendemos usar os dados registados em 24 estações sísmicas de banda larga (BB), de fundo oceânico (OBS), implantadas no Golfo de Cádiz, no âmbito do projeto NEAREST (Setembro de 2007- Agosto de 2008), para melhor compreender o funcionamento deste tipo de equipamento, as condições ambientais que controlam o ruído observado e os efeitos de acoplamento do sensor. Os registos no fundo do mar têm uma característica básica que os diferencia dos de terra: a interação do sensor com a água do mar circundante (Webb, 1998). O sensor irá responder ao movimento da água e às variações de pressão, fenómeno que não acontece nos sensores terrestres, e o facto de estar instalado em áreas de sedimentos não consolidados também afeta dramaticamente a qualidade do sinal sísmico. O desenvolvimento da observação sísmica no fundo oceânico, é fundamental para completar a observação em terra e exige que todos os mecanismos que afectam os registos sísmicos de um OBS sejam caracterizados o melhor possível, para aprimorar a concepção dos equipamentos e de uma melhor concepção de campanhas de observação, em função dos objectivos a alcançar. As redes de estações sísmicas terrestres de registo contínuo, permitidas hoje pela evolução das comunicações e dos sistemas de aquisição, permitem acumular um volume enorme de dados, dos quais os eventos sísmicos são apenas uma pequena parte, menos de 5% dos dados registados diariamente. Os remanescentes 95%, os quais eram anteriormente rejeitados, recebem actualmente especial atenção dos investigadores das diferentes áreas da sismologia. Em particular, quando as fontes de ruído, são distribuídas de forma homogénea (ou quase), em redor de um par de estações, é possível demonstrar que a função de correlação do ruído (NCF), representa a “Green function” da propagação sísmica (em Shapiro e Campillo, 2004; Sabra et al., 2005). Este teorema tem levado a um grande número de aplicações : i) Ruído sísmico ambiental como uma ferramenta eficiente para tomografia sísmica (por exemplo: Stehly et al., 2009; Villaseñor et al., 2007; Graça et al., 2013); ii) detecção de sinais anómalos nos registos sísmicos (v.g. Stehly et al., 2007); iii) variações na estrutura de propagação com o tempo (por exemplo, Brenguier et al, 2008). Os estudos sobre fontes de ruído terrestre aumentaram na comunidade científica, mas no domínio oceânico são, ainda, escassos. Nesta tese, pretende-se usar o registo contínuo dos dados sísmicos, principalmente a componente de ruído ambiente, para extrair informações relacionadas com: i) estrutura de crosta superficial e sua reologia; ii) acoplamento do sensor; iii) derivas temporais no relógio dos sistemas de aquisição; iv) características das fontes de ruído ambiental, meteorológico e oceanográfico. Para isso, é crucial implementar novas metodologias e adaptar os procedimentos realizados nos registos terrestres. No final deste estudo, pretende-se formular algumas recomendações sobre: i) concepção e desenho de OBS BB; ii ) a interpretação dos inúmeros sinais sísmicos registados no leito do oceano, iii) o planeamento de futuras campanhas de longa duração

Ocean bottom seismic noise : applications for the crust knowledge, interaction ocean-atmosphere and instrumental behaviour

Tese de doutoramento, Ciências Geofísicas e da Geoinformação (Geofisíca), Universidade de Lisboa, Faculdade de Ciências, 2014The seismic observation by long deployment experiments in ocean domain is a great technological challenge, with advancements only having been obtained in the last decade. The number of available instruments is still small and the number of missions successfully carried out remains low. Therefore, unlike land observation, seismic acquisition in the ocean seafloor is in an experimental phase and part of a learning process. In particular, the interaction of the sensor with the water environment has implications and applications that have only recently begun to be explored. In this PhD thesis we intend to use the continuous recording data produced by 24 broad-band (BB) ocean bottom seismometers (OBS), deployed in the Gulf of Cádiz in the context of the NEAREST project (September 2007 – August 2008), to understand the functioning of this type of equipment, the environmental conditions that control the observed noise and the effects of the sensor coupling. Sea floor records have a basic characteristic that makes them different from land records: the sensor’s interaction with the surrounding sea water (e.g. Webb, 1998). Sensors will respond to water movement and pressure variations, which does not happen with land sensors and, in most cases, they are deployed at locations with non-consolidated sediments that dramatically affect the quality of the seismic signal. The future growth of seismic observation at the bottom of the ocean, which is a fundamental complement to land observation, requires that all these mechanisms that affect the seismic records of an OBS are characterized in the best possible way, leading to better equipment design and better conception of observation campaigns, in line with the intended objectives. The continuous acquisition of seismic data by land stations, which is possible today due to the evolution of communications and recording systems, enables us to accumulate an enormous volume of data of which earthquakes are a tiny part, less than 5 % of the recorded data for one day. The remaining 95%, which is rejected in the normal operation of a seismic network, has been receiving attention from researchers in several domains. In particular, when the noise sources are distributed homogeneously (or almost) around a couple of stations it is possible to show that the noise correlation function (NCF) represents the Green function of seismic propagation (in Shapiro and Campillo, 2004, Sabra et al., 2005). This theorem has led to a great number of applications: i) Seismic ambient noise as an efficient tool for seismic tomography (for example: Stehly et al., 2009; Villaseñor et al., 2007, Graça et al., 2013); ii) detection of anomalies in seismic records (e.g., Stehly et al., 2007); iii) variations in superficial structure with time (e.g., Brenguier et al., 2008). Studies of noise sources have increased in the research community, but studies carried out in ocean domains are scarce and joint land-ocean are even more rare. In this thesis, we intend to use the continuous seismic recording, mainly the ambient noise component, to extract information related with: i) shallow crustal structure; ii) conditions of sensor coupling; iii) clock drifts of the acquisition systems; iv) meteorological and oceanographic environmental noise sources. For this purpose it was crucial to implement new methodologies and to adapt current procedures in land records. At the end of these studies we hope to be able to make some recommendations regarding: i) conception and design of BB OBS; ii) interpretation of seismic signals recorded at the ocean bed; iii) planning of future campaigns of long deployment seismic experiments.A observação sísmica de longa duração, em ambiente marinho, representa um grande desafio tecnológico e apenas nas últimas décadas foram obtidos avanços significativos. Os instrumentos disponíveis são limitados e o número de missões com sucesso é ainda reduzido. Ao contrário da observação terrestre, a aquisição sísmica no fundo do oceano encontra-se numa fase experimental e em contínuo processo de aprendizagem. Em particular, a interação do sensor com o ambiente marítimo tem implicações e aplicações que só recentemente começaram a ser exploradas. Nesta tese de doutoramento, pretendemos usar os dados registados em 24 estações sísmicas de banda larga (BB), de fundo oceânico (OBS), implantadas no Golfo de Cádiz, no âmbito do projeto NEAREST (Setembro de 2007- Agosto de 2008), para melhor compreender o funcionamento deste tipo de equipamento, as condições ambientais que controlam o ruído observado e os efeitos de acoplamento do sensor. Os registos no fundo do mar têm uma característica básica que os diferencia dos de terra: a interação do sensor com a água do mar circundante (Webb, 1998). O sensor irá responder ao movimento da água e às variações de pressão, fenómeno que não acontece nos sensores terrestres, e o facto de estar instalado em áreas de sedimentos não consolidados também afeta dramaticamente a qualidade do sinal sísmico. O desenvolvimento da observação sísmica no fundo oceânico, é fundamental para completar a observação em terra e exige que todos os mecanismos que afectam os registos sísmicos de um OBS sejam caracterizados o melhor possível, para aprimorar a concepção dos equipamentos e de uma melhor concepção de campanhas de observação, em função dos objectivos a alcançar. As redes de estações sísmicas terrestres de registo contínuo, permitidas hoje pela evolução das comunicações e dos sistemas de aquisição, permitem acumular um volume enorme de dados, dos quais os eventos sísmicos são apenas uma pequena parte, menos de 5% dos dados registados diariamente. Os remanescentes 95%, os quais eram anteriormente rejeitados, recebem actualmente especial atenção dos investigadores das diferentes áreas da sismologia. Em particular, quando as fontes de ruído, são distribuídas de forma homogénea (ou quase), em redor de um par de estações, é possível demonstrar que a função de correlação do ruído (NCF), representa a “Green function” da propagação sísmica (em Shapiro e Campillo, 2004; Sabra et al., 2005). Este teorema tem levado a um grande número de aplicações : i) Ruído sísmico ambiental como uma ferramenta eficiente para tomografia sísmica (por exemplo: Stehly et al., 2009; Villaseñor et al., 2007; Graça et al., 2013); ii) detecção de sinais anómalos nos registos sísmicos (v.g. Stehly et al., 2007); iii) variações na estrutura de propagação com o tempo (por exemplo, Brenguier et al, 2008). Os estudos sobre fontes de ruído terrestre aumentaram na comunidade científica, mas no domínio oceânico são, ainda, escassos. Nesta tese, pretende-se usar o registo contínuo dos dados sísmicos, principalmente a componente de ruído ambiente, para extrair informações relacionadas com: i) estrutura de crosta superficial e sua reologia; ii) acoplamento do sensor; iii) derivas temporais no relógio dos sistemas de aquisição; iv) características das fontes de ruído ambiental, meteorológico e oceanográfico. Para isso, é crucial implementar novas metodologias e adaptar os procedimentos realizados nos registos terrestres. No final deste estudo, pretende-se formular algumas recomendações sobre: i) concepção e desenho de OBS BB; ii ) a interpretação dos inúmeros sinais sísmicos registados no leito do oceano, iii) o planeamento de futuras campanhas de longa duração

Broadband ocean bottom seismometer in the Gulf of Cadiz (offshore SW Iberia and NW of Moroccan margin): Characterization of ambient noise and tomographic model of the crustal structure

In this study, we use the continuous data recorded by 24 broadband ocean bottom seismometers (OBS-BB) deployed in the Gulf of Cadiz, in the framework of the NEAREST project, from September 2007 to July of 2008. Our goals are: i) to understand the instrument and the environmental conditions that control the observed seismic noise; and ii) to obtain reliable broadband surface wave dispersion measurements. The noise sources are investigated through the probability density functions (PDFs) of power spectral density (PSDs), which provides insights on the generation and propagating of seismic noise in the Gulf of Cadiz. We show the results of the Rayleigh wave group velocity tomography performed using ambient seismic noise observed on the 24 broadband OBS and on 7 broadband land stations located in the south of Portugal. The time-series, for the 11 months, were cross-correlated to obtain the empirical Green's functions between all vertical sensors pairs, namely the OBS-vertical, the OBS-hydrophone and the vertical component of the land seismic stations. To improve the signal-to-noise ratio, the individual cross-correlograms were summed using a time-frequency domain phase weighted stack. The stacked cross-correlograms enabled us to compute short-period surface-wave group-velocity measurements for all the interstation paths. We used these measurements to construct maps of Rayleigh-wave group-velocity lateral perturbations, at different periods. Despite the great difference in the crustal structure below the OBS (thin continental or oceanic type) and the land stations (typical continental crust, 30 km thick) we were able to derive high S/N cross-correlations between these different types of sensors. This study was co-sponsored by several projects namely the QuakeLoc-PT (PTDC/GEO-FIQ/3522/2012), AQUAREL (PTDC/CTE-GIX/116819/2010), NEAREST FP6-2005-GLOBAL-4 (OJ 2005 C177/15), WILAS (PTDC/CTE-GIX/097946/2008), and PEST-OE/CTE/LA-0019/2013-2014

Source Levels of 20 Hz Fin Whale Notes Measured as Sound Pressure and Particle Velocity from Ocean-Bottom Seismometers in the North Atlantic

Source level is one factor that determines the effectiveness of animal signal transmissions and their acoustic communication active space. Ocean-bottom seismometers (OBS) are platforms of opportunity to monitor marine species because they record data as pressure fluctuations in the water using a hydrophone and/or as particle velocity of the seabed using a seismometer. This study estimates source levels of 20 Hz fin whale notes recorded simultaneously in these two OBS channels and in two areas of the North Atlantic (Azores and southwest Portugal). It also discusses factors contributing to the variability of the estimates, namely geographical (deployment areas), instrumental (recording channels and sample size), and temporal factors (month of detected notes, inter-note interval, and diving duration). The average source level was 196.9 dB re 1 µPa m for the seismometer (derived from particle velocity measurements) and 186.7 dB re 1 µPa m for the hydrophone. Variability was associated with sample size, instrumental characteristics, acoustic propagation, and month of recordings. Source level estimates were very consistent throughout sequences, and there was no indication of geographical differences. Understanding what causes variation in animal sound source levels provides insights into the function of sounds and helps to assess the potential effects of increasing anthropogenic noise

Source Levels of 20 Hz Fin Whale Notes Measured as Sound Pressure and Particle Velocity from Ocean-Bottom Seismometers in the North Atlantic

Source level is one factor that determines the effectiveness of animal signal transmissions and their acoustic communication active space. Ocean-bottom seismometers (OBS) are platforms of opportunity to monitor marine species because they record data as pressure fluctuations in the water using a hydrophone and/or as particle velocity of the seabed using a seismometer. This study estimates source levels of 20 Hz fin whale notes recorded simultaneously in these two OBS channels and in two areas of the North Atlantic (Azores and southwest Portugal). It also discusses factors contributing to the variability of the estimates, namely geographical (deployment areas), instrumental (recording channels and sample size), and temporal factors (month of detected notes, inter-note interval, and diving duration). The average source level was 196.9 dB re 1 µPa m for the seismometer (derived from particle velocity measurements) and 186.7 dB re 1 µPa m for the hydrophone. Variability was associated with sample size, instrumental characteristics, acoustic propagation, and month of recordings. Source level estimates were very consistent throughout sequences, and there was no indication of geographical differences. Understanding what causes variation in animal sound source levels provides insights into the function of sounds and helps to assess the potential effects of increasing anthropogenic noise.</jats:p

Seasonal changes on tidal-induced noise on long-term OBS deployment

&amp;lt;p&amp;gt;Currently, Ocean Bottom Seismometers (OBS) have sensors comparable to those used on land stations. However, they are exposed to very different conditions that degrade the recordings. One major issue is being directly exposed to global and tidal oceanic currents that, depending on the water velocity flowing around the instrument, can excite parts of the frame or produce drag and lift effects due to vortex shedding. Any of these conditions is detrimental for signal integrity, either from variations of the instrument-to-ground coupling or by introducing unwanted energy that is unrelated with the seismic events the experiment is aimed at.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;The UPFLOW project is aimed at understanding mid-plate, deep upward flow that cannot be explained by plate tectonics, but is critical for continental growth, for returning volatiles to the atmosphere and for producing Earth&amp;amp;#8217;s largest melting events. For this reason, 50 OBS of different types were deployed for a year in the North Atlantic Ocean, including several prototypes aimed at reducing the tidal-induced noise generated by water flowing around the instrument&amp;#039;s frame.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;In this work, we show the seasonal variation of the tidal-induced noise on different instrument types across the Madeira and Seine abyssal plains of the North Atlantic Ocean during neap and spring tides between July 2021 and July 2022. In some instances, where harmonics are detected, individual frame components of the OBS can be identified as a major contributor, paving way to finding mitigation solutions on future deployments.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;This work was funded by the Portuguese Funda&amp;amp;#231;&amp;amp;#227;o para a Ci&amp;amp;#234;ncia e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) &amp;amp;#8211; UIDB/50019/2020.&amp;lt;/p&amp;gt;</jats:p