This work presents an experimental framework for offshore seismic monitoring that combines Distributed Acoustic Sensing (DAS) with ocean-bottom seismometers (OBS). The study was conducted in the Azores region – Faial, where an HDAS interrogator prototype was connected to dark fiber submarine fiber-optic cable, complemented by the installation of two Ocean Bottom Seismometers (OBS) for calibration and validation of DAS technology. The main objective is to demonstrate that seismic observations obtained by DAS from seafloor cables can provide essential information similar to OBS and particularly in areas where land-based monitoring stations are limited.

The Azores Plateau (Portugal), in the North Atlantic Ocean, was formed by the complex geodynamic setting of the tectonic triple junction between the American, Eurasian and Nubian plates. The Azores islands are situated along the Terceira Rift, the third branch of the triple junction. The obliquity of this segment of the Eurasian-African boundary results in differential rates of seafloor expansion, as well as intense volcanic and seismic activities. Between 2019 and 2021, a dense temporary seismic deployment in Terceira Island allowed a significant increase in detection capability and spatial coverage. From the collected dataset, we obtained the first local earthquake tomography 3D model of the island's crust. We also analyzed shear wave splitting and focal mechanisms to obtain information on crustal stress. Two major features were imaged, beneath the main volcanic structures of the island: Santa B & aacute;rbara volcano to the west, and the couple Guilherme Moniz-Pico Alto volcanoes in central Terceira. They both exhibit high Vp/Vs values and a roughly North-South segmentation, characterised by distinct Vp values and seismic activity. Seismicity also tends to be concentrated along the borders of these features, which are interpreted as corresponding to the plumbing system or magmatic chamber that feeds the volcanoes. A very shallow high Vp area, coincident with a volcanic cone alignment, marks the inland surface expression of the Terceira Rift, the Vp/Vs depth pattern suggests that the main feeder is Santa B & aacute;rbara volcano. Focal mechanisms and shear wave splitting point to a radial stress pattern around Santa B & aacute;rbara volcano, but a more complex pattern in the Guilherme Moniz-Pico Alto volcanoes. Although some fluctuations occur, the orientation of the fast polarisation direction and the SHmax tend to mark a SW-NE direction, similar to 90 degrees to the regional orientation, indicating the transition in the Terceira Rift orientation from NW-SE to the WNW-ESE.

Distributed acoustic sensing (DAS) deployed in submarine telecom cables is bound to revolutionize the earthquake monitoring routine currently performed by seismic network operators, particularly in domains where most of seismic activity is originated offshore. In this work we analyze one year DAS data recorded on a submarine telecommunication cable joining the Islands of Faial and Flores in the Azores, an area where seismic crisis of volcano-tectonic origin are frequent. We explore whether spatially decimated channels can be added to the operation routine without major changes. During the DAS operation the land network recorded 368 local and regional earthquakes. The best of these events were jointly analyzed and earthquake parameters were compared between original locations and adding DAS picks. Despite the larger picking uncertainty on DAS channels, adding P and S or S-wave picks only from DAS data showed a considerable improvement on earthquake parameters, reducing the error ellipse area and the focal depth uncertainty. These results demonstrate that in domains where offshore seismicity is a concern and submarine telecom cables are available, integrating DAS channels into the routine operation of seismic networks considerably improve the accuracy earthquake parameter estimation. A procedure to accomplish these results is presented.

Ocean-bottom seismometers (OBSs) are used increasingly often to track baleen whale signals, employing single-station ranging techniques such as the three-component (3C) method. By using the orientation of ground motion from OBS components, the 3C method provides robust range estimates of direct-path signals within a validity range that relates to instrument depth. Consequently, the method requires a classification process to determine whether a signal falls within the validity range. Fin whale tracks, composed of 20-Hz notes from six locations, were used to develop and evaluate three classification models: decision trees (DTs), generalized additive models, and neural networks. Models were trained using different data combinations and incorporated a comprehensive set of variables related to channel amplitude, signal quality, polarization, and estimated signal angles. The DT achieved the highest performance, reaching an accuracy of 0.94 on the test data. Key variables for predicting the validity of the 3C ranges included the difference between observed horizontal-to-vertical amplitude ratios and its theoretical value, polarization metrics, and the amplitude of one horizontally oriented OBS component (Y-channel). The resulting framework contributes to improving the utility of seismic data for biological studies, which are critical for marine mammal monitoring and conservation strategies.

Mesoscale eddies are large, swirling anomalies of temperature and salinity, found almost everywhere in the ocean, extending from the surface to its deepest layers. They are generated by the meandering of major ocean currents, water flow past islands and interactions with rough seafloor, or even wind-driven. Typically forming and dissipating within a month, mesoscale eddies are routinely tracked at the surface using satellite data, but their vertical structure and subsurface dynamics remain less frequently studied.Similarly, internal tides, which are vertical oscillations of stratified density layers in the ocean, represent another dynamic subsurface process influenced by ocean currents and seafloor topography. While traditionally studied using moorings or ship-based measurements, their role in ocean mixing processes highlights the need for advanced techniques to better observe these phenomena.Distributed Acoustic Sensing (DAS) allows for extremely high spatial and temporal resolution measurements of strain along fibre optic cables. Rayleigh-based methods are sensitive to strain, temperature and pressure, but isolating these effects often requires supplementary sensors.From the analysis of temperature-induced variations in strain measurements, we show how DAS can be used for the mapping of internal tides as they interact with the island slope.We can also track the diffusion and dissipation of a mesoscale eddy in the deep basin south of Madeira Island.The displacement of the eddy at the seafloor is consistent with average surface velocities of eddies observed via satellite in this region. Tidal control of the eddy track is also apparent.Unlike satellite data, which primarily capture surface expressions of eddies, DAS provides a unique perspective by tracking these features from the seafloor in unprecedented resolution in both space and time. This capability allows for detailed observations of their vertical structure and interaction with the deep ocean, opening new pathways for studying previously inaccessible submesoscale and mesoscale ocean dynamics.This work was supported by ARDITI-Agencia Regional para o Desenvolvimento da lnvestigação, Tecnologia e lnovação, and was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) - UID/50019/2025 and LA/P/068/2020, by the MODAS project 2022.02359.PTDC, and by EC project SUBMERSE project HORIZON-INFRA-2022-TECH-01-101095055.

T phases are acoustic waves that propagate in the low velocity zone of the oceanic sound channel that acts as a waveguide, the SOFAR channel. They are generated by earthquakes through the conversion of seismic energy at the solid-liquid interface, but the exact processes involved are still under debate.Due to their low attenuation and slow propagation velocity, these arrivals are especially useful for the detection and characterisation of small earthquakes in marine basins, as they can improve the location of the event while their waveforms can yield information on source rupture.In October 2023, a Distributed Acoustic Sensing (DAS) interrogator was installed on the GeoLab dark fibre in the Atlantic, starting at the Praia Formosa CLS, in Madeira Island, Portugal. The instrumentation of this cable is part of a project by ARDITI and the Oceanic Observatory of Madeira where oceanographic data recorded by buoys and autonomous vessels are combined with DAS data to obtain a global view of the underwater environment of Madeira Island in all its physical, chemical and biological aspects, including the characterisation of regional seismicity. This initiative is also linked to the SUBMERSE project, as the Madeira cable is a pilot site to establish continuous DAS monitoring along many more submarine fibre-optic cables.On October 27th, a near-source (

Distributed Acoustic Sensing allows extremely dense acquisition geometries of strain data. The GeoLab fibre, installed on the southern coast of Madeira Island, extends for ∼56 km and is exclusively dedicated to research. This fibre provides an invaluable resource for continuous data collection and analysis. Between October 26th and November 3rd , 2023, ∼7 TB of data were collected at 500 Hz with gauge length of 10.2 m at each 5.1 m. This report highlights the versatility of the GeoLab fibre and the present dataset for seismological, oceanographic, and biological research with a non-exhaustive collection of examples in various scientific domains, such as local and teleseismic events, calls from different species of baleen whales, waves and their interaction with the shoreline, ocean-bottom currents and temperatures, and anthropogenic noise sources.

Regional and local tsunami sources are a cliché of scientific disaggregation. From the physical perspective, despite emerging studies on cascading hazard and risk, hazard characterization often sees the tsunami as an individual event without addressing the effects of the primary hazard (typically a high-magnitude earthquake) that triggered the tsunami. Moreover, tsunami effects are partitioned into single processes: hydraulic effects or induced effects, such as debris transport, which is a representative approach often assumed when treating complex phenomena. From a technical perspective, describing cascading hazards and translating them into a composite loading pattern for natural and built environments is challenging, and the difficulty increases exponentially when fluid-soil-interactions are considered. From a modeling perspective, physical and numerical simulations are employed to complement scarce databases of extreme tsunami events. However, the level of modeling sophistication deemed necessary to reproduce such complex phenomena is elevated and there are uncertainties associated with natural phenomena and their modelling, ranging from the genesis of the tsunami to structural and community response. The number and influencing potential of uncertainties pose an extraordinary concern when developing mitigation measures. From a risk management perspective, cascading natural and anthropogenic hazards constitutes a challenge for combining safety requirements with financial, social, and ecological concerns. Risk management can benefit from strengthening the ties between natural hazards and engineering practitioners, linking science and industry, and promoting dialogue between risk analysts and policy-makers. Ultimately, risk management requires heterogeneous data and information from real and synthetic origins. Yet, the quality of data used for risk management may often depend on the computational resources (in terms of performance, energy, and storage capacity) needed to simulate complex multi-scale and multi-physics phenomena, as well as to analyze large data sets. For example, the quality of the numerical solutions is often dependent on the amount of data used to calibrate the models and the runtime of the models needs to be aligned with time constraints (ex.: faster than real time tsunami simulations for early warning systems). The North American platform Hazus is capable of producing risk maps. In the European risk assessment, there is a lack of integration and interaction of results from GEM and SERA, and TSUMAPS-NEAM projects, intended to develop seismic and tsunami hazard studies, respectively. The computational modeling aids in the advancement of scientific knowledge by aggregating the numerous factors involved and their translation to tsunami risk management policies. A global trend in geosciences and engineering is to develop sophisticated numerical schemes and to build computational facilities that can solve them, thereby aiming to reduce uncertainty levels and preparing the scientific (r)-evolution for the so-called Exascale Era. The present work aims to gather multidisciplinary perspectives on a discussion about: 1) challenges to overcome on tsunami risk management, such as sophistication of earthquake and tsunami numerical schemes; 2) uncertainty-awareness and future needs to develop unanimous and systematic measures to reduce uncertainties associated with geophysical and engineering processes; 3) pros and cons of using HPC resources towards safety and operational performance levels; and 4) applicability to critical infrastructures.

Sub-mesoscale and mesoscale (i.e., 1–10 km and 10–200 km, respectively) ocean processes are highly relevant for the understanding of global circulation, mixing of water masses and energy exchange between ocean layers. However, the processes happening at these scales are hard to be characterized using direct measurements of temperature and salinity. Direct measurements are obtained from vertical probes and/or autonomous vehicles, which, despite their high vertical resolution, are sparsely located in space and therefore unable to capture spatial details at these scales. Seismic oceanography (SO) data have been successfully used to imaging and characterize the ocean at these spatial scales. These data represent indirect measurements of the ocean temperature and salinity along kilometric transects with high horizontal resolution (i.e., a near-synaptic view of the system under investigation), but lower vertical resolution when compared with direct observations. Despite its complex oceanographic setting, the Madeira Abyssal Plain is still largely uncharacterized due to the lack of direct observations. We show for the first time a comprehensive processing, modelling and interpretation of three 2-D seismic oceanography sections from this region. The data show coherent seismic reflection in space, depth and time and shed light into this oceanographic setting with an unprecedent horizontal resolution. The SO modelling and interpretation are combined with existing direct measurements and a quantitative method to correlate thermohaline staircases interpreted from conductivity-temperature-depth casts and seismic reflections is proposed. The results show the relatively stable presence of thermohaline staircases in simultaneously time and space between 1200 and 2000 m of water depth and their spatial variability and contribute to the generalization of SO in physical oceanography.

In the last few years, a number of technologies to use fiber optic cables as sensing devices have been established, among them Distributed Acoustic Sensing (DAS) and State-of-Polarisation (SoP). The potential of these technologies for monitoring a range of Earth System parameters in submarine cables has been demonstrated through several pilot experiments, but full integration with telecommunication infrastructure has not yet been achieved. The SUBMERSE (SUBMarinE cables for ReSearch and Exploration) project links Research and Education Networks (RENs), universities, research institutes and industry to establish multi-method monitoring along submarine optical telecommunication cables at several key oceanic cable routes branching off from Sines in Portugal, Madeira, Svalbard and in the Ionian Sea, and in addition the Transatlantic cable between Fortaleza and Sines. Those pilot sites should serve as a blueprint for establishing continuous monitoring services along many more cables. The project comprises technical developments for integrating DAS and SoP measurements, for establishing differential SoP measurements between repeaters and for operating DAS in a co-existence mode, i.e., in fibers also carrying telecommunications traffic. Furthermore, a range of geoscientific and marine biology use cases are included, which seek to establish code/services for monitoring earthquakes and tsunamis, tracking whales, measuring the sea state and other Earth System variables. The data collected by SUBMERSE will be distributed according to FAIR principles through established community-specific distribution channels such as EIDA for seismological data, with exceptions for security sensitive time periods, spatial or frequency ranges. The presentation will present some example data and methodological developments in the context of this project. Furthermore, an outlook on the seismological real-time and archive products will be provided.

Animal songs can change within and between populations as the result of different evolutionary processes. When these processes include cultural transmission, the social learning of information or behaviours from conspecifics, songs can undergo rapid evolutions because cultural novelties can emerge more frequently than genetic mutations. Understanding these song variations over large temporal and spatial scales can provide insights into the patterns, drivers and limits of song evolution that can ultimately inform on the species’ capacity to adapt to rapidly changing acoustic environments. Here, we analysed changes in fin whale (Balaenoptera physalus) songs recorded over two decades across the central and eastern North Atlantic Ocean. We document a rapid replacement of song INIs (inter-note intervals) over just four singing seasons, that co-occurred with hybrid songs (with both INIs), and a clear geographic gradient in the occurrence of different song INIs during the transition period. We also found gradual changes in INIs and note frequencies over more than a decade with fin whales adopting song changes. These results provide evidence of vocal learning in fin whales and reveal patterns of song evolution that raise questions on the limits of song variation in this species.

Optimized deep learning (DL)-based workflows can improve the efficiency and accuracy of earthquake detection and location processes. This article introduces a six-step automated event detection, phase association, and earthquake location workflow, which integrates the state-of-the-art pair-input DL (PIDL) model and waveform migration location methods [integrated PIDL and MIL (IPIML)]. Applying IPIML on an 18-month dataset of Ghana Digital Seismic Network (GHSDN) recorded from 2012 to 2014, a catalog with 461 events is automatically obtained. Compared to other DL catalogs obtained using EQTransformer (EQT) and Siamese EQT (S-EQT), the seismic event clusters in the IPIML catalog focus more on tectonically active regions or known seismogenic source areas and show a consistent depth distribution. The compiled catalog is $6.3\times $ larger than the reported catalog obtained by applying EQT with the default settings, indicating the importance of optimization and hyperparameter tuning when applying DL models. As a result, a previously unknown seismogenic fault with a clear spatial trend has been identified using the new IPIML catalog, which provides more insights into the fault activities and seismic hazards in the region. The IPIML codes and datasets are available at the GitHub repository https://github.com/SigProSeismology/IPIML.git , contributing to the geoscience community.

Ocean-bottom seismometers (OBSs) are usually deployed for seismological investigations, but these objectives are impaired by noise resulting from the ocean environment. We split the OBS-recorded seismic noise into three bands: short periods, microseisms and long periods, also known as tilt noise. We show that bottom currents control the first and third bands, but these are not always a function of the tidal forcing. Instead, we suggest that the ocean bottom has a flow regime resulting from two possible contributions: the permanent low-frequency bottom current and the tidal current. The recorded noise displays the balance between these currents along the entire tidal cycle, between neap and spring tides. In the short-period noise band, the ocean current generates harmonic tremors corrupting seismic dataset records. We show that, in the investigated cases, the harmonic tremors result from the interaction between the ocean current and mechanical elements of the OBS that are not essential during the sea bottom recording and thus have no geological origin. The data from a new broadband OBS type, designed and built at Instituto Dom Luiz (IDL – University of Lisbon)/Centre of Engineering and Product Development (CEIIA), hiding non-essential components from the current flow, show how utmost harmonic noise can be eliminated.

The Ghana Digital Seismic Network (GHDSN) data, with six broadband sensors, operating in southern Ghana for two years (2012-2014). The recorded dataset is processed for simultaneous event detection and phase picking by a Deep Learning (DL) model, the EQTransformer tool. Here, the detected earthquakes consisting of supporting data, waveforms (including P and S arrival phases), and earthquake bulletin are presented. The bulletin includes the 559 arrival times (292 P and 267 S phases) and waveforms of the 73 local earthquakes in SEISAN format. The supporting data encompasses the preliminary crustal velocity models obtained from the joint inversion analysis of the detected hypocentral parameters. These parameters comprised of a 6- layer model of the crustal velocity (Vp and Vp/Vs ratio), incident time sequence, and statistical analysis of the detected earthquakes and hypocentral parameters analyzed and relocated by the updated crustal velocity and graphic representation of them a 3D live figure enlighting the seismogenic depth of the region. This dataset has a unique appeal for earth science specialists to analyze and reprocess the detected waveforms and characterize the seismogenic sources and active faults in Ghana. The metadata and waveforms have been deposited at the Mendeley Data repository [1].

SUMMARY In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (< 14 s) and teleseismic surface waves at long periods (> 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate.

Seismic oceanography as remote sensing of the ocean structure by multichannel reflection seismic method can provide high-resolution images enabling the study of fine-scale ocean processes along large distances.The seismic acoustic response depends on differences in ocean temperature and salinity, and the resulting seismic images track the interfaces between those thermohaline layers both laterally and in depth. The structural interpretation of observed seismic reflections provides valuable oceanographic insights to understand mixing processes and phenomena occurring at different water column depths.Three parallel 2D multichannel seismic reflection profiles acquired by the Portuguese Task Force for the Extension of the Continental Shelf in the Madeira Abyssal Plain (MAP), profiles covering 300km and ~100km apart from each other, dating from 2006, were processed to enhance the amplitudes of the water column (Azevedo, L. et al., 2021) and analyzed jointly with conductivity-temperature-depth probes (CTDs) from 2002 and 2005 acquired by Poseidon research vessel.The structure of the water column in this area is characterized by the intrusion of Mediterranean Outflow Waters (MOW), warmer and salty water mass expressing between the 500 and 1500 m depth, and overlaying Subarctic Intermediate Water where temperature and salinity decrease in depth. Due to the differences in temperature and salinity gradients, the MAP region is auspicious for developing double diffusion, specifically thermohaline staircases (van der Boog, C. et al., 2021). Double diffusion is shown to influence the efficiency of vertical mixing of the different water masses; it affects the vertical transport of nutrients, temperature, and salt and contributes to ocean circulation, which is intrinsically connected to the control of the earth’s climate. Nevertheless, it is still lacking information.We detected the thermohaline staircases expression in temperature and salinity profiles plotted as a function of depth, noticing that the interfaces of mixing followed by layers of well-mixed temperature and salinity are well defined as a step structure and were validated as double diffusion by calculating the Turner angle and Density Ratio at those depths.Simultaneously, the seismic profiles are characterized by continuous sub-horizontal reflections between the ~1200 to 2000 meters of depth. By correlating the CTD profiles with the seismic images, it is noticeable that the staircases on the vertical profiles correspond to the reflections on the seismic at the expected depths and are covering almost the entirety of seismic profiles.Since those reflections are present in the three parallel seismic profiles, we use them to predict the lateral continuity of the step-like structures and build models of the incidence of double-diffusive thermohaline staircases in the region, contributing to the knowledge of those processes' extension and expression in the Madeira Abyssal plain.References:van der Boog, C. G., Dijkstra, H. A., Pietrzak, J. D., & Katsman, C. A. (2021). Double-diffusive mixing makes a small contribution to the global ocean circulation. Communications Earth & Environment, 2(1), 1-9.Azevedo, L., Matias, L., Turco, F., Tromm, R., & Peliz, Á. (2021). Geostatistical seismic inversion for temperature and salinity in the Madeira Abyssal Plain. Frontiers in Marine Science, 8, 685007.

&lt;p&gt;We present the analysis of the local and regional seismicity recorded in 2019-2021 by a temporary seismic network installed on Terceira Island. This new seismic dataset allow us to study the induced seismicity caused by fluid extraction in a geothermal powerplant and to image the subsurface seismic structure with local earthquake tomography.&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;p&gt;From the distribution of the seismicity, it is possible to highlight two regions with a high number of events amongst other areas. The first one, located in the central part of the island, is associated with the volcanoes of Pico Alto and Guilherme Moniz, with&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160; &lt;/span&gt;low magnitude earthquakes, and hypocentre&#039;s depths less than 10 km. The geothermal power plant is located in the transition between these two volcanic systems. We identify a cluster of earthquakes in the neighbourhood of the geothermal power plant at depths ranging from 1 to 3 km, consistent with the induction by the powerplant operation. The second seismicity region is located on the island&#039;s western sector, at the Santa B&amp;#225;rbara volcanic system. There, the seismicity pattern is more complex, mainly by the occurrence of both tectonic and seismo-volcanic earthquakes. &lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;p&gt;Local earthquake tomography allows imaging of the crust from the surface to the upper-middle crust, up to 8 km depth. The Santa B&amp;#225;rbara and Pico Alto volcanoes are characterized by low Vp and high Vp/Vs anomalies, stronger in the first and typically related to active volcanoes. In the transition between the two volcanoes, we observe shallow strong Vp and very low Vp/Vs anomalies typical of geothermal fields. On the other hand, the Guilherme Moniz volcano exhibits high Vp anomaly and normal Vp/Vs values.&lt;/p&gt; &lt;p&gt;The eastern sector of the Terceira is characterized by low seismicity and, consequently, low tomographic resolution.&lt;span class=&quot;Apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;p&gt;This work is a contribution to projects GEMMA (PTDC/CTA-GEO/2083/2021) and RESTLESS (PTDC/CTA-GEF/6674/2020), and it was also supported by the Portuguese Funda&amp;#231;&amp;#227;o para a Ci&amp;#234;ncia e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) &amp;#8211; UIDB/50019/2020-IDL, UIDB/04683/2020 - ICT and UIDP/04683/2020 - ICT&lt;/p&gt;

We present the characterization of regional seismicity in Ghana by processing the Ghana Digital Seismic Network (GHDSN) data set recorded between September 2012 and April 2014, implementing deep learning (DL). Local earthquakes are detected in this dataset using EQTransformer, a DL model with a hierarchical attentive mechanism (HAM) for simultaneous earthquake detection and P- and S-phase picking. A Conservative Strategy (CS) is devised to detect the missing phases and to associate the detected phases to circumvent the false-negative issue of EQTrans-former processing low signal-to-noise ratio (SNR) seismograms. We performed a joint inversion by grid search in 1D velocity model space and simultaneous inversion for the hypocentral parameters, incorporating 559 detected arrival times (292 P and 267 S phases). The results obtained by velocity inversion contain thicknesses of 1, 13, 8, 13, and 10 km, from the surface to a depth of 45 km, with Vp = 5.9, 6.1, 6.3, 6.5, 6.9, and 7.2 km/s, respectively. The updated velocities for the first and last layers are 6% and 26% and the Vp/Vs (=1.70) is 3.03% higher than the previously reported values. A total number of 73 earthquakes with a local magnitude of 2.5 < Ml < 3.9 are located, comprising four main clusters of events, showing a high correlation with the mapped faults zones. The hypocentral depth distribution is mainly in the range of 7-15 km, confined to the upper crust in the region. No specific seismic activity in the eastern branch of Coastal Boundary Fault (CBF) and the continuation of Romanche Fracture Zone (RFZ) in the study period was observed, casting further doubt on the activity of this branch and the hypothesis of stress transfer from RFZ to southern Ghana. The results reinforce the intraplate nature of the tectonic activities in the region. Finally, an updated seismic catalog up to April 2022 is presented for Ghana by incorporating all reported catalogs and combining the newly detected events.

Abstract. Ocean bottom seismometers (OBS) are usually deployed for seismological investigations but these objectives are impaired by noise resulting from ocean environment. We split the OBS recorded seismic noise into three domains, short-period, microseisms and long-period, also known as tilt-noise. We show that the first and third domains are controlled by bottom currents but these are not always a function of the tidal forcing. Instead we suggest that the ocean bottom has a flow regime resulting from two possible contributions, the permanent low frequency bottom current and the tidal current. The recorded noise displays the balance between these two currents along the full tidal cycle, between neap and spring tides. In the short-period noise band the ocean current generates harmonic tremors that corrupt the dataset records. We show that, in the analyzed cases, the harmonic tremors result from the interaction between the ocean current and mechanical elements of the OBS that are not essential for sea bottom recording and thus have no geological origin. The data from a new Broadband OBS type, designed and built at Instituto Dom Luiz (University of Lisbon)/CEIIA, hiding no essential components from current flow, shows how utmost of the harmonic noise can be eliminated.