Session SM3.2

[Programme]

SM3.2 | Advancing Ocean Bottom Seismology and Geophysics: from novel instrumentation and data processing to new scientific discoveries

Advancing Ocean Bottom Seismology and Geophysics: from novel instrumentation and data processing to new scientific discoveries

Convener: Maria TsekhmistrenkoECSECS | Co-conveners: Ana Ferreira, Takehi Isse, Roberto Cabieces, Afonso LoureiroECSECS

Orals

| Mon, 28 Apr, 08:30–12:20 (CEST)

Room K2

Posters on site

| Attendance Mon, 28 Apr, 16:15–18:00 (CEST) | Display Mon, 28 Apr, 14:00–18:00

Hall X1

The oceans cover about 71% of the Earth's surface. Yet, our understanding of the oceanic crust and mantle primarily relies on seismic data from continents or islands. Ocean-bottom seismometers (OBS) are powerful tools for revealing the intricate details of the Earth's sub-oceanic interior, but large-scale OBS deployments remain challenging due to technical, logistical, and financial hurdles.

Over the past two decades, various OBS arrays and other passive ocean-bottom geophysical instruments, such as geodetic and magnetotelluric sensors, have been deployed globally. This led to fascinating new discoveries worldwide, from exciting earthquake phenomena (e.g., slow slip events) to new constraints on subduction processes, mantle plumes, ridges, transform faults, thermal heterogeneity and volatile cycling. However, despite technological advances and improved data processing techniques, significant challenges persist. For example, the sharing and application of best practices for OBS deployments, data preprocessing and formatting is still limited. Many processed data sets are not released for years (if at all), limiting the long-term impact and sustainability of data from these expensive, often publicly-funded projects.

We invite contributions from the global ocean-bottom geophysics community to share their knowledge, experiences and scientific findings. We welcome contributions on all aspects from instrumentation development, experiment design, data processing and analysis (e.g., software, machine learning tools), to new scientific results (e.g., tomography, receiver functions, ambient noise studies, earthquake source analysis, active source imaging, etc). We encourage contributions in all relevant areas, such as from seafloor environmental sensors (e.g., using submarine fibre cables), magnetotellurics, geodesy, ocean acoustics, oceanography and marine biology.

Orals: Mon, 28 Apr, 08:30–12:30 | Room K2

Chairpersons: Maria Tsekhmistrenko, Takehi Isse, Roberto Cabieces

08:30–08:35

5-minute convener introduction

08:35–08:45

EGU25-17183

ECS

On-site presentation

Microearthquakes and Seismicity in the Marmara Sea: An Analysis Using OBS Data

Tuğçe Ergün, Nurcan Meral Özel, Yojiro Yamamoto, Narumi Takahashi, Ahmet Anıl Dindar, Remzi Polat, Fatih Turhan, Uğur Mustafa Teoman, Yoshiyuki Kaneda, and Kato Aitaro

The Sea of Marmara, located along the North Anatolian Fault (NAF) system, represents a geologically significant region where the fault’s bifurcation into northern and southern branches plays a key role in shaping the region's tectonic framework. The NAF, acting as a primary tectonic driver, has not only shaped the basin's morphology but also controlledits seismicity, making the region a key area for studying active tectonic processes and earthquake hazards. The İzmit Gulf, located at the easternmost part of the Marmara Sea, is of particular tectonic significance as it marks the location where the NAF splits into its northern and southern branches. This bifurcation creates a structurally complex and seismically active environment characterized by fault interactions and the transfer of stress, offering a natural laboratory for investigating the mechanisms of fault segmentation, branching, and seismic activity in this tectonically intricate region. These insights are essential for understanding the broader dynamics of the NAF system and for assessing seismic risks in northwestern Turkey.
Given the seismic risk and tectonic complexity of the Marmara Sea region, in September 2023, eight Ocean Bottom Seismometers (OBS) equipped with 4.5 Hz geophones were deployed across the Sea of Marmara, including the İzmit Gulf, at depths ranging from 145 to 1269 meters. These instruments were operated for 10 months, recording seismic data with a high sampling rate of 100 samples per second (sps). The data were manually analyzed through visual inspection to pick P- and S-wave arrivals enabling precise calculation of earthquake locations, depths, and magnitudes. Preliminary analysis of 3 month data we identified 45 micro-earthquakes that were not present in land-based seismic catalogs, demonstrating the enhanced detection capability of the OBS array. The OBS network achieved a minimum detectable earthquake magnitude of 0.3, significantly improving the resolution of seismic monitoring in the region. These micro-earthquakes were primarily clustered along the northern and southern branches of the NAF with distinct waveform characteristics suggesting localized fault activity and varying focal depths.
The findings also revealed variations in seismic clustering patterns between the northern and southern fault branches. Seismic activity along the northern branch was observed to have distinct waveform characteristics and shallower focal depths compared to the southern branch, indicating differences in fault behavior and stress accumulation processes. These insights into fault dynamics underline the importance of high-resolution OBS data in characterizing microseismic events and understanding fault interactions within this tectonically complex region. In future stages of the study, AI-based automatic modules will be utilized to process the same dataset and their performance will be compared with manual analysis to evaluate their relative advantages. This approach is expected to streamline data processing and improve the accuracy and efficiency of microseismic event detection.
The findings demonstrate the critical importance of OBS technology in advancing our understanding of the tectonic and seismic behavior of the Marmara Sea region. Continuous, real-time monitoring of the Marmara Sea is essential for capturing microseismic events and detecting early signs of larger seismic activity.

How to cite: Ergün, T., Özel, N. M., Yamamoto, Y., Takahashi, N., Dindar, A. A., Polat, R., Turhan, F., Teoman, U. M., Kaneda, Y., and Aitaro, K.: Microearthquakes and Seismicity in the Marmara Sea: An Analysis Using OBS Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17183, https://doi.org/10.5194/egusphere-egu25-17183, 2025.

08:45–08:55

EGU25-9380

On-site presentation

DeepDAS: An Earthquake Phase Picker for Submarine Distributed Acoustic Sensing Data

Han Xiao, Frederik Tilmann, Martijn van den Ende, Diane Rivet, Afonso Loureiro, Takeshi Tsuji, and Arantza Ugalde

Given the scarcity of seismometers in marine environments, traditional seismology has limited effectiveness for early earthquake warning in oceanic regions. Submarine Distributed Acoustic Sensing (DAS) systems offer a promising alternative for seismic monitoring in these areas. The EU-INFRATECH funded SUBMERSE project will establish continuous monitoring of several oceanic telecom cables for landing sites in Portugal, Greece, and Svalbard. However, the existing machine learning models trained on land-based DAS data do not perform well with submarine DAS due to differences in noise characteristics, deployment conditions, and environmental factors.

This study presents a machine learning approach tailored specifically for submarine DAS data to enable automated seismic event detection and P and S wave identification. Leveraging DeepLab v3, a neural network architecture optimized for semantic segmentation, we developed a specialized model to handle the unique challenges of submarine DAS data. Our model was trained and validated on a dataset comprising nearly 92 million manually and semi-automatically labeled seismic records from multiple international submarine sites, providing a robust basis for accurate seismic detection. We compared the performance of DeepDAS and PhaseNet DAS in picking seismic P and S waves from submarine DAS data. Our findings suggest that DeepDAS (F1 score 0.89) outperforms PhaseNet DAS (F1 score 0.53,) in those datasets. This result is understandable, as PhaseNet DAS was originally trained on DAS seismic data from land-based DAS.

Beyond developing the model, we generated a comprehensive submarine DAS earthquake dataset with manually picked P and S arrivals. This dataset includes 6,326 submarine seismic events (magnitudes ranging from -2 to 5, depths from 0 to 200 km) and spans diverse deployment scenarios with varying cable lengths, configurations, and channel spacings. Recognizing the importance of open collaboration and reproducibility, we plan to open-source this dataset. We aim to establish it as a benchmark dataset for submarine DAS research, enabling broader adoption and facilitating advancements in the field.

How to cite: Xiao, H., Tilmann, F., van den Ende, M., Rivet, D., Loureiro, A., Tsuji, T., and Ugalde, A.: DeepDAS: An Earthquake Phase Picker for Submarine Distributed Acoustic Sensing Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9380, https://doi.org/10.5194/egusphere-egu25-9380, 2025.

08:55–09:05

EGU25-10475

On-site presentation

The BRUIT-FM Seafloor Noise Reduction Challenge

Wayne Crawford, Stephan Ker, Simon Rebeyrol, Mohamad-Amin Aminian, Guihem Barruol, Laurent Duval, and Eleonore Stutzmann

09:05–09:15

EGU25-14795

ECS

On-site presentation

Enhancing Ocean Bottom Seismometer Data: Diffusion Model Applications for Noise Reduction in Marine Surveys Near Noto Peninsula, Japan

Jun Su, Ryoichiro Agata, Gou Fujie, and Yasuyuki Nakamura

Seismic profiles obtained from ocean bottom seismometers (OBS) during seismic surveys are crucial for understanding subsurface structures. However, these profiles often contain records that are affected by varying levels of noise due to factors such as weather conditions, ocean currents, and background seismicity, which complicates interpretation. One common approach to mitigate this uncertainty is diversity stacking, which involves retrieving seismic records with the same source-receiver pairs multiple times to filter out noise. Unfortunately, this technique can increase the costs of marine seismic surveys and limit data availability.

In this study, we utilize OBS data collected from a seismic survey near the Noto Peninsula, Japan, conducted between August 30 and September 6, 2024. Airgun shots were fired at 200-meter intervals, repeated five times along a 100-kilometer survey line, and recorded by 40 OBS with 2-kilometer spacing. We trained a machine learning model to reduce the noise in seismic profiles from each shot, using profiles processed through diversity stacking as a reference. Specifically, we applied a denoising diffusion probabilistic model (DDPM) based on the methodology outlined by Durall et al. (2023). This model, which has recently demonstrated efficacy as an image generator, takes a list of words, sentences, or images as input and iteratively refines the result towards the desired output using a neural network. While Durall et al. (2023) trained their model solely on simulated seismograms as target images, our approach leveraged diversity stacking and incorporated real-world waveforms as training data for the first time.

An example profile from the test set indicates that the trained model effectively addresses both random background noise and extreme noise present in certain traces, successfully reducing noise levels in profiles from a singular shot to be comparable to those achieved through diversity stacking. These results suggest that by enhancing OBS data with the DDPM, it is possible to obtain a clearer seismic structure of the deeper subsurface and a broader range of data with fewer airgun shots.

How to cite: Su, J., Agata, R., Fujie, G., and Nakamura, Y.: Enhancing Ocean Bottom Seismometer Data: Diffusion Model Applications for Noise Reduction in Marine Surveys Near Noto Peninsula, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14795, https://doi.org/10.5194/egusphere-egu25-14795, 2025.

09:15–09:25

EGU25-14566

On-site presentation

Multicomponent Seismic Data Antialiasing Interpolation and Its Application for Sparse OBN Data Processing

Jianjun Gao and Fan Li

During the acquisition of seismic data offshore, due to the complex seabed terrain, ocean currents, and the costs of instruments, the multicomponent seismic data recorded by OBS or OBN are usually sparsely and irregularly sampled. Sparse sampling and irregular missing seismic traces generate severe spatial aliasing, which disturbs subsequent seismic migration processing. Furthermore, the data collected by both OBS and OBN are multicomponent in nature. Regarding the reconstruction of multicomponent data, the current methods are mainly scalar-based, treating the multicomponent data as several independent components and interpolating each component separately. These component-wise approaches ignore the internal mutual relationships among the different components, thereby damaging the vector-field nature of the seismic elastic wavefield. Given this, we propose a vector Project onto Convex Sets (POCS) reconstruction method based on the complexified quaternion Fourier transform, which achieves joint vector reconstruction of the three-component (3C) OBN data. This proposed method not only reconstructs data for the three components with their respective missing patterns, but also preserves the vector polarization characteristics of the subsurface particles.

For sparse 3C OBN sampling data, we propose a new vector anti-aliasing POCS interpolation method based on a dip angle scanning strategy. There are two innovative points for this method: Firstly, we utilize the first L maximum values of the negative second derivative of the dip scanning energy spectrum to pinpoint the position of effective wave dips, enhancing the accuracy of dip identification. Secondly, we adopt a 2D Gaussian tapered window function instead of the tradational 2D rectangular tapered window function to mitigate the Gibbs oscillation phenomenon and suppress the energy tailing effect at the edges of recovered seismic events. Finally, several sparse OBN field data reconstruction test results demonstrate the effectiveness of the proposed anti-aliasing vector POCS reconstruction method.

How to cite: Gao, J. and Li, F.: Multicomponent Seismic Data Antialiasing Interpolation and Its Application for Sparse OBN Data Processing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14566, https://doi.org/10.5194/egusphere-egu25-14566, 2025.

09:25–09:35

EGU25-1653

ECS

On-site presentation

A comprehensive empirical study of T-wave properties

Pierre-Yves Raumer, Sara Bazin, Dorian Cazau, Jean-Arthur Olive, Romain Safran, and Jean-Yves Royer

The Sound Fixing and Ranging (SOFAR) channel facilitates the propagation of underwater acoustic waves over vast distances with minimal attenuation. Exploiting this property, various passive hydroacoustic networks have been deployed globally to monitor signals trapped within the SOFAR channel. One notable application involves T-waves (or "Tertiary waves"), which are acoustic waves generated from the conversion of seismic waves at the ocean-bottom interface and propagating in the SOFAR. T-waves offer significant advantages in seismic monitoring. For instance, hydrophones positioned strategically across the ocean often provide better earthquake detection coverage compared to terrestrial seismic stations, especially for low-magnitude events occurring remotely from land-based instruments, such as along mid-ocean ridges. The ability to detect earthquakes with magnitudes as low as ~3.0 (lower magnitude of completeness) enables us to focus on microseismicity that might otherwise go unnoticed.

Over the past few decades, numerous studies have used T-waves for monitoring seismic events. The detection of the same event at several hydrophone stations enables the time and origin of earthquakes to be estimated by least-squares inversion. Although these analyses have proved very useful, one wonders whether T-waves can tell us more about earthquakes. For example, some studies have established empirical relations between T-wave source level and earthquake magnitude to estimate seismic moment rates in remote parts of the ocean. Such relations, however, typically rely on very limited data sets (e.g., a few hundred events). Modeling work also suggests that additional T-wave parameters, such as amplitude and duration of the wave, or its frequency content, might reflect parameters such as rupture length or seafloor roughness. However, these hypotheses require thorough empirical validation, which has been hampered by the lack of comprehensive data sets.

To address this issue, we leverage a new detection tool called TiSSNet to automate the picking process of T-waves generated by events from the ISC ocean-wide catalog of teleseismic events. This provides a dataset of approximately 10,000 events, enabling extensive empirical comparisons focused on magnitude and source level parameters.

How to cite: Raumer, P.-Y., Bazin, S., Cazau, D., Olive, J.-A., Safran, R., and Royer, J.-Y.: A comprehensive empirical study of T-wave properties, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1653, https://doi.org/10.5194/egusphere-egu25-1653, 2025.

09:35–09:45

EGU25-3152

ECS

Virtual presentation

Improvements of GNSS-A seafloor geodetic observation for the crustal deformation monitoring around Japan

Shun-ichi Watanabe, Koya Nagae, Tadashi Ishikawa, Yuto Nakamura, and Yusuke Yokota

The Japan Coast Guard conducts the GNSS-A seafloor geodetic observation to monitor the crustal deformation on the seafloor in the subduction zones along the Japanese islands. We detected seafloor motions that tied to the geophysical processes in the Japan Trench, the Sagami Trough, and the Nankai Trough (e.g., Sato et al., 2011 Science; Watanabe et al., 2015 EPS; Yokota et al., 2016 Nature; Yokota and Ishikawa, 2020 Sci. Adv.; Watanabe et al., 2021 EPS). Recently, we are developing the notable methods to improve the positioning accuracy, for example, with the implementation of full-Bayes approach and the model selection method (as an open-source program GARPOS-MCMC: Watanabe et al., 2023 J. Geod.; Watanabe et al., under review), and the bias reduction scheme for instrumental characteristics (Acoustic Ambiguity Reduction (AAR) method: Yokota et al., 2024 EPS). The former enables us to evaluate the sound speed model itself statistically. Using this method, density structure of the seawater can also be reproduced, as GNSS-A oceanography (e.g., Yokota et al., 2024 GJI). For the latter, applying the AAR method to the GNSS-A data, the vertical component of site displacement was significantly improved. In the presentation, we will review the recent improvements and show the results of seafloor crustal deformation pattern.

How to cite: Watanabe, S., Nagae, K., Ishikawa, T., Nakamura, Y., and Yokota, Y.: Improvements of GNSS-A seafloor geodetic observation for the crustal deformation monitoring around Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3152, https://doi.org/10.5194/egusphere-egu25-3152, 2025.

Coffee break

Chairpersons: Ana Ferreira, Afonso Loureiro, Maria Tsekhmistrenko

10:45–11:15

EGU25-9217

solicited

On-site presentation

Past, present, and future controlled source Ocean Bottom Seismometer (OBS) surveys for exploring the oceanic transform discontinuities in the Atlantic Ocean

Milena Marjanovic

About 70% of the Earth’s surface is covered by the ocean, hiding some of the fundamental processes behind the inner workings of our planet. Over a century ago, in the unknown seafloor panorama, the maps of Marie Tharp and colleagues revealed the presence of mid-ocean ridges, wrapping around the globe along which the plates have been forming at rates varying from slow to fast. Even in these early bathymetry maps, it was clear that the ridges are not continuous features but interrupted by the transform faults along which the plates slide past each other, leaving the record in the seafloor that can be followed on the flanks for hundreds of millions of years. The lithospheric structure of these tectonic discontinuities has remained one of the big unknowns since the early days of deep-sea exploration.

One way to scan the subsurface of the oceanic crust is by using a controlled source to produce seismic waves that propagate below the seafloor, which are finally captured by Ocean Bottom Seismometers (OBS) laid on its surface. In the 1980/90s, such surveys focused on exploring transform discontinuities offsetting the Mid-Atlantic Ridge: Kane, Vema, Oceanographer, Charlie Gibbs, and Tydeman Transform Faults (TF). Although limited in number of instruments, these studies established the common view that transform faults in slow-slipping environments are typically represented by significantly thinner than average oceanic crust (~3km vs. 6-7km), comprised of a mafic layer.

In the past several years, a wealth of modern OBS data has been acquired, providing new insights into the morphotectonic characteristics of the transforms. Here, I focus on the main findings from the data collected in the equatorial Atlantic¹. First, along the profile crossing the Romanche TF, the most extended tectonic structure on Earth, close to normal depth to Moho (~5 km below seafloor), is found, proposing that the crustal structure along the TF strike can vary significantly and, therefore influence seismogenic behavior along the transform plate boundary, which is poorly understood. In addition, lower velocities in the upper mantle suggest extensive serpentinization and water infiltration down to ~16km. In contrast to the transform fault domains, their fossilized trace consistently shows crustal thicknesses close to the average igneous crust, reported in legacy and modern data (Chain and St. Paul). This intriguing observation is explained by the mechanism of lateral dike propagation, supported by the presence of globally observed J-shaped structures in the seafloor bathymetry. A global compilation of bathymetry data further supports this view, proposing a new framework to be established behind the formation of oceanic crust at the ridge transform intersection. In fact, little is known about the formation of oceanic crust in slow-spreading environments globally. To shed light on this aspect, new dedicated OBS surveys are necessary. One such collaborative project that will employ active and passive seismic, in concert with interdisciplinary data sampling, is in preparation for the Mohn’s Ridge² in the Arctic and will be presented in more detail during the talk.

¹ & ² contributions from the ILAB-SPARC and MoKA-Pot teams, respectively.

How to cite: Marjanovic, M.: Past, present, and future controlled source Ocean Bottom Seismometer (OBS) surveys for exploring the oceanic transform discontinuities in the Atlantic Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9217, https://doi.org/10.5194/egusphere-egu25-9217, 2025.

11:15–11:25

EGU25-5242

ECS

On-site presentation

Acoustic Noise Recorded by MERMAID Floats and its Relation to Ocean WaveClimate in the Mediterranean

Frederik J. Simons, Thomas A. Lee, and Lucia Gualtieri

11:25–11:35

EGU25-18304

On-site presentation

Temporal Variation of Oceanographic Parameters Observed by DAS Strain Data in the Atlantic

David Schlaphorst, Luís Matias, Afonso Loureiro, Athanasia Papapostolou, Susana Custódio, Carlos Corela, Álvaro Peliz, Susana Gonçalves, and Rui Caldeira

Distributed Acoustic Sensing (DAS) recordings close to the coast are influenced by pressure signals from land- and seaward ocean surface gravity waves. The amplitude and period of the signal can be interpreted as a proxy for the sea state. Measurements along the cable at larger water depths show secondary microseisms related to the sea state away from the shore. The significant wave height and ocean currents along the cable can be evaluated continuously during the experiment, resembling a dense sampling array of closely spaced buoys. However, to provide useful results, the measurements have to be calibrated with existing buoy or wave model data.

In October 2023, the GeoLab dark fibre off Madeira Island in the Atlantic was fitted with a DAS interrogator under a project by ARDITI and the Oceanic Observatory of Madeira. As a pilot site, the experiment is linked to the SUBMERSE project that is trying to establish continuous DAS monitoring along fibre-optic cables at multiple locations around Europe.

We use one recording in 2023 (7 days) and one in 2024 (5 days) to show changes in the DAS data close to the shore where the water depth is small, and temporal variations of the secondary microseisms further along the cable. The f-k spectra of different time intervals show the effects of varying sea states on the dispersion curves between land- and seaward waves. Tides, significant wave height and current measurements from buoy measurements match the amplitudes and shapes of the dispersion curves of the measured data.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (DOI: 10.54499/UIDB/50019/2020), UIDP/50019/2020 (DOI: 10.54499/UIDP/50019/2020) and LA/P/0068/2020 (DOI: 10.54499/LA/P/0068/2020), and by EC project SUBMERSE, HORIZON-INFRA-2022-TECH-01-101095055.

How to cite: Schlaphorst, D., Matias, L., Loureiro, A., Papapostolou, A., Custódio, S., Corela, C., Peliz, Á., Gonçalves, S., and Caldeira, R.: Temporal Variation of Oceanographic Parameters Observed by DAS Strain Data in the Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18304, https://doi.org/10.5194/egusphere-egu25-18304, 2025.

11:35–11:45

EGU25-18309

On-site presentation

From the Earth to the whale: Opportunistic use of ocean-bottom seismometers to track fin whales

Andreia Pereira, Rose Hilmo, Tiago A. Marques, William Wilcock, David K. Mellinger, Danielle V. Harris, and Luis Matias

Monitoring baleen whales often involves significant costs, making the use of opportunistic data from instruments deployed for other purposes an attractive and cost-effective alternative. However, such data typically require adjustments to standard analytical approaches. The CORTADO project (Combining global OBS and CTBTO recordings to estimate abundance and density of fin and blue whales) uses data from two types of bottom-deployed sensors to develop and implement methods for tracking and estimating the density of fin (Balaenoptera physalus) and blue (B. musculus) whales. While Ocean Bottom Seismometers (OBS) and hydroacoustic data from the Comprehensive Test Ban Treaty Organization (CTBTO) have proven to be valuable in prior studies, tools for routine application of these datasets remain limited. The primary objective of CORTADO is to develop a set of software tools and training resources to facilitate the analysis of large, historical datasets of baleen whales. This presentation focuses on phase 1 of CORTADO, showcasing the workflow and comparing two single-station ranging techniques for fin whales: the particle velocity method and the multipath method. Using OBS datasets from six deployment areas—three in the Pacific Ocean (Marianas Trench, Hawaii, and Oregon OOI) and three in the North Atlantic Ocean (ENAM, Azores, and Gulf of Cadiz)—ranges to 20-Hz fin whale calls were estimated and averaged into 1-minute bins. Differences between the two ranging methods were further assessed with a Generalized Additive Mixed Model (GAMM) to account for the different tracks and areas. Results indicated that the multipath method achieved larger range estimates, exceeding 15 km, while the particle velocity method was limited by a site-specific validity range influenced by OBS depth and propagation properties. The multipath method performed best in sedimented areas with identifiable multipath arrivals, while the particle velocity method was more effective in deeper, softly sedimented regions. Both methods showed challenges for complex bathymetry, complex calling behaviour, and chorusing. On average, multipath estimates were 557 m greater than particle velocity estimates, but variability was highly dependent on site and track conditions, with sediment type being a key factor. These findings provide critical insights into the performance of single-station ranging techniques and their application, contributing to the broader usability of opportunistic datasets for marine conservation. This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020) and UIDB/00006/2020 (https://doi.org/10.54499/UIDB/00006/2020). It is also supported by US Navy Living Marine Resources program award N39430-21-C-2208 and N00014-21-1-2564, and EC project SUBMERSE project HORIZON-INFRA-2022-TECH-01-101095055.

How to cite: Pereira, A., Hilmo, R., A. Marques, T., Wilcock, W., K. Mellinger, D., V. Harris, D., and Matias, L.: From the Earth to the whale: Opportunistic use of ocean-bottom seismometers to track fin whales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18309, https://doi.org/10.5194/egusphere-egu25-18309, 2025.

11:45–11:55

EGU25-13148

On-site presentation

What are the structure and temperature of the mantle transition zone in the North Atlantic? Insights from the SEA-SEIS ocean-bottom seismometer network

Thomas Merry, Sergei Lebedev, Sanne Cottaar, Janneke de Laat, Raffaele Bonadio, Maria Tsekhmistrenko, and David Stalling and the The SEA-SEIS Team

The existence, location and structure of the Iceland plume in the deep mantle are debated, in part due to the gap in seismic data sampling in the North Atlantic Ocean. To address this, in 2018-2020, a network of 18 ocean bottom seismometers (OBS) was deployed at 1–4 km depths on the North Atlantic seafloor by the project SEA-SEIS (Structure, Evolution and Seismicity of the Irish Offshore). Fourteen seismometers were recovered, of which 12 provided high-quality 3-component data. For these, we have obtained robust orientations of the horizontal components using Rayleigh-wave polarization.

To better understand the structure of the Iceland plume through the mantle transition zone, we aim to infer lateral temperature variations using Ps receiver functions. These receiver functions record P-to-S wave conversions from upgoing teleseismic waves beneath seismic stations, allowing us to estimate the depths of the main mantle transition zone discontinuities (the ‘410’ and ‘660’), which are temperature-dependent.

Reverberations of seismic waves within the water and sediment columns local to the seismometers, in addition to high levels of noise generated by ocean currents and waves, make calculation of receiver functions challenging. We test a variety of methods to produce useable receiver functions from the data. We present these, alongside receiver functions calculated for nearby onshore seismic stations, and latest tomography models, to make inferences on the temperature of the mantle transition zone.

How to cite: Merry, T., Lebedev, S., Cottaar, S., de Laat, J., Bonadio, R., Tsekhmistrenko, M., and Stalling, D. and the The SEA-SEIS Team: What are the structure and temperature of the mantle transition zone in the North Atlantic? Insights from the SEA-SEIS ocean-bottom seismometer network, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13148, https://doi.org/10.5194/egusphere-egu25-13148, 2025.

11:55–12:05

EGU25-5257

On-site presentation

Rayleigh-wave Ambient Noise Analysis for the OHANA Experiment in the Northeast Pacific

Gabi Laske, Grace Atkisson, John A. Collins, and Donna K. Blackman

The 2022-2023 OHANA OBS deployment in the northeast Pacific ocean provides a rich dataset for comprehensive seismic studies to explore the crust, lithosphere and asthenosphere in a 600~km wide region west of the Moonless Mountains. The study area covers mainly 40-to-50 Myr old Pacific lithosphere. A fundamental question to be addressed is whether this particular area has the signature of a typical oceanic lithosphere that has a normal plate cooling history. Alternatively, we seek evidence for a previously proposed reheating process, e.g. resulting from small-scale shallow-mantle convection.

Continuous 4-component data (broadband ground motion and pressure) were recovered at 24 sites. In a top-down approach, we start with the assembly and analysis of ambient-noise cross-correlation functions (CCFs) of the vertical components, between 5 and 35 s. The CCFs contain prominent waveforms from overtones that can help improve resolution as a function of depth.

We present the analysis of path-averaged dispersion curves for the fundamental mode. Forward modeling and the inversion of the average dispersion across the OHANA network both indicate normal oceanic crust over a fairly typical mature oceanic lithosphere though shear velocities in the upper lithosphere are a few percent lower than is expected for a 50-Myr old lithosphere. Velocities in the mid-to-lower lithosphere may be 2-3% higher than expected but resolution degrades with increasing depth. We observe significant and internally consistent azimuthal anisotropy in both the fundamental mode as well as the first overtone. We juxtapose this analysis to an earthquake-based analysis that reaches deeper into the lower lithosphere and upper asthenosphere.

How to cite: Laske, G., Atkisson, G., Collins, J. A., and Blackman, D. K.: Rayleigh-wave Ambient Noise Analysis for the OHANA Experiment in the Northeast Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5257, https://doi.org/10.5194/egusphere-egu25-5257, 2025.

12:05–12:20

AtlanticArray + How to build an OBS community in Europe (fast)?

Posters on site: Mon, 28 Apr, 16:15–18:00 | Hall X1

Display time: Mon, 28 Apr, 14:00–18:00

Chairpersons: Afonso Loureiro, Maria Tsekhmistrenko, Roberto Cabieces

OBS

X1.98

EGU25-12130

Ambient Noise Tomography from the UPFLOW Seafloor Array in the Atlantic

Roberto Cabieces, Ana M.G. Ferreira, and Graça Silveira

We use 14 months of continuous recordings (June 2021 - August 2022) from the UPFLOW seafloor passive seismic array experiment. The experiment included 50 broadband ocean bottom seismometers (OBS) deployed in the Azores-Madeira-Canary Islands region, with an average interstation distance of ~130 km. The availability of both OBS and land stations makes this experiment ideal for determining the best approach to extract reliable empirical Green's functions (EGFs) and construct 3D shear-wave velocity models. The measured dispersion curves are used in a Bayesian inversion to obtain a series of 1D shear wave velocity models, which are then interpolated to construct a 3D model of the region. We present the EGFs and dispersion measurements obtained using different techniques, ranging from distinct cross-correlation methods to different stacking procedures. We also show preliminary phase and group velocity maps and a preliminary 3D shear-wave Earth velocity model. The 3D upper lithosphere images are compared with the main geological structures of the Azores and Madeira oceanic region. Moreover, we compare our new images with previous global and regional tomography models and discuss the differences introduced by the UPFLOW data.

How to cite: Cabieces, R., Ferreira, A. M. G., and Silveira, G.: Ambient Noise Tomography from the UPFLOW Seafloor Array in the Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12130, https://doi.org/10.5194/egusphere-egu25-12130, 2025.

X1.99

EGU25-3507

ECS

Accelerating global tomography inversions with multiple land and offshore datasets

Mohammad Veisi, Ana MG Ferreira, and Sung-Joon Chang

Understanding mantle circulation and its current thermo-chemical state requires integrating information about S and P wave velocities, as well as their anisotropic variations within the mantle. Seismic tomography provides a method for mapping seismic observations—such as surface wave group velocities and body wave travel times—onto mantle properties, including S and P wave velocities and their anisotropic variations.

Projecting seismic datasets onto mantle properties (e.g., S and P velocities and their anisotropic variations) through spherical harmonics and radial spline basis functions typically involves finding the eigenvalues and eigenvectors of a symmetric, dense, positive-definite matrix. The size of this matrix depends on the maximum degree of the spherical harmonics, the number of radial splines, and the parameters included in the tomography. For instance, when considering both P and S wave velocities, along with their anisotropic variations, the eigenvalue-eigenvector problem becomes computationally very demanding. Such computations require efficient parallel processing schemes, especially on high-performance computing (HPC) clusters.

We present a new library designed for inverting Gram matrices, optimized to utilize an arbitrary number of cores on a single machine or an HPC cluster. This library can also be employed to compute the eigenvalues and eigenvectors of any symmetric positive-definite matrix, making it ideal for solving seismic tomography problems with a large number of unknown parameters. We carry out extensive synthetic inversion tests combining land and offshore surface and body wave synthetic data and quantify the resolution improvements due to offshore data. Additionally, we present preliminary results towards a new SPGlobe-rani model, which integrates ~43,000,000 surface wave and ~600,000 body wave travel time global measurements, expanded into spherical harmonic basis functions up to degree 35 and with 21 radial splines, and with crustal corrections applied.

How to cite: Veisi, M., Ferreira, A. M., and Chang, S.-J.: Accelerating global tomography inversions with multiple land and offshore datasets, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3507, https://doi.org/10.5194/egusphere-egu25-3507, 2025.

X1.100

EGU25-9821

Towards a global seismic model of attenuation combining on- and offshore data

Ana MG Ferreira and William Sturgeon

We present our new 2-D and 3-D global models of Rayleigh wave and shear attenuation (SGLOBE-Q2D and SGLOBE-Q3D, respectively), including uncertainties. We use a dataset of ~10 million fundamental and higher mode (up to 4th overtone) Rayleigh wave amplitude measurements with wave periods T~38-275 s. The amplitude measurements are corrected for source effects as well as for the influence of along-path and local receiver elastic structure. Extensive synthetic inversion tests are carried out to guide the model parameterisation used. This enables us to expand our fundamental mode Rayleigh wave attenuation maps up to spherical harmonic degree 20, which is higher than in recent global attenuation studies. We observe strong low attenuation beneath all major global cratons, including a clear separation between the Congo and Kalahari cratons in South Africa, and between the East European and Siberian cratons at T~40-100 s. We also observe low attenuation perturbations beneath the continents and high attenuation anomalies beneath the oceans. We compare the observed upper mantle variations in attenuation beneath different oceans as a function of plate age and speed, and contrast them with variations in shear wave speed and anisotropy. In addition, we carry out synthetic inversion tests combining land and offshore surface wave synthetic data considering the source-receiver paths recorded by the UPFLOW ocean bottom seismology experiment in the Atlantic to quantify the potential resolution improvements thanks to offshore data.

How to cite: Ferreira, A. M. and Sturgeon, W.: Towards a global seismic model of attenuation combining on- and offshore data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9821, https://doi.org/10.5194/egusphere-egu25-9821, 2025.

X1.101

EGU25-4359

Waveform anisotropy tomography of the Azores-Canary-Madeira mantle region using UPFLOW OBS data

Katrina Harris, Michael Witek, Ana Ferreira, Sung-Joon Chang, Graça Silveira, and Miguel Miranda

The Azores-Canary-Madeira region has been a focal point of scientific investigation regarding the presence and characteristics of mantle plumes, but so far seismological studies of the region's mantle structure have been limited due to sparse seismic coverage.
Global tomography models exhibit considerable discrepancies concerning the presence and morphology of a lateral connection between the Azores and Canary plume systems in the mantle. Furthermore, few seismic studies have examined potential interactions between the region's lithosphere, plumes and the adjacent Mid-Atlantic Ridge. Related highly relevant topics include characterising the age-dependent variability in oceanic lithosphere thickness and illuminating lithospheric and sub-lithospheric seismic structure. To address these issues, it is crucial to not only image isotropic seismic wave speeds but also map seismic anisotropy, as it can provide unique constraints on lithospheric thickness, fabric, and ultimately mantle flow. We build a new radially anisotropic tomographic model of the Azores-Canary-Madeira region using the partitioned waveform inversion method. Ocean bottom seismometer data from the UPFLOW array are combined with seismic data from nearby land stations. Automated waveform fitting of surface waves between ~ 300 and ~16 s period is used to retrieve path average model perturbations. These perturbations are subsequently used to invert for 3-D radially anisotropic mantle structure with a regularized least squares inversion. Mantle flow is interpreted from the retrieved patterns in isotropic velocity and radial anisotropy, while depth-dependent trends in these parameters provide insights into the region's oceanic lithosphere thickness and on potential plume-ridge and plume-lithosphere interactions.

How to cite: Harris, K., Witek, M., Ferreira, A., Chang, S.-J., Silveira, G., and Miranda, M.: Waveform anisotropy tomography of the Azores-Canary-Madeira mantle region using UPFLOW OBS data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4359, https://doi.org/10.5194/egusphere-egu25-4359, 2025.

X1.102

EGU25-14141

ECS

S-wave velocity uppermost mantle structure around beneath the Central Mariana subduction zone inferred from ambient noise tomography

Tae-shin Kim and Sung-Joon Chang

Understanding the structure of the subduction zones is crucial for unraveling water transport mechanisms to the mantle, as the subducting slab serves as the primary channel for water entering the deep Earth. The Central Mariana subduction zone, where some of the oldest oceanic crust is subducting, is characterized by serpentinization in the fore arc evidenced by the presence of serpentinite mud volcanoes, arc volcanism driven by slab-released volatiles, and the formation of new oceanic lithosphere at back-arc spreading centers. In this study, we estimated fundamental-mode Rayleigh-wave group- and phase-velocity dispersion curves for periods from 3 to 35 s, as well as first overtone Rayleigh-wave group- and phase-velocity dispersion curves for periods from 4 to 7 s and from 5 to 13 s, using continuous seismic data from 32 ocean-bottom seismometers and 20 island stations. Additionally, group and phase velocities between asynchronous station pairs were determined using the C3 method. By jointly inverting the multimode Rayleigh-wave dispersion curves, we calculated an S-wave velocity model with resolutions down to 100 km depth, using a 3D reference model based on Crust1.0 and ak135 incorporating modified Moho depth and topography from seismic refraction data. Our results reveal low-velocity anomalies along the slab down to ~40 km depth, indicative of serpentinization, as well as beneath the volcanic arc (60-90 km depth) and the back-arc spreading center (10-30 km depth). Notably, a connection between the low-velocity anomalies beneath the arc and the back-arc spreading center is also observed.

How to cite: Kim, T. and Chang, S.-J.: S-wave velocity uppermost mantle structure around beneath the Central Mariana subduction zone inferred from ambient noise tomography , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14141, https://doi.org/10.5194/egusphere-egu25-14141, 2025.

X1.103

EGU25-17345

ECS

Intraplate deformation and mantle shear zones in the Gorda plate driven by the northward migration of the Mendocino triple junction

Yu Ren, Dietrich Lange, and Ingo Grevemeyer

Since 28 Ma, the North American, Pacific, and Gorda plates have been coupled, with the Mendocino triple junction (MTJ) migrating northward and exerting significant influence on coastal California and the northern San Andreas Fault system. While deformation within the continental domain has been well-documented, the dynamics of the oceanic lithosphere remain poorly understood. The subducting Gorda plate, constituting the southern block of the Juan de Fuca plate, is known as a nonrigidly deforming zone bounded by the Gorda ridge to the west, the Cascadia deformation front to the east, and the Mendocino transform fault to the south. However, its northern boundary—delineating the limit of deformation—remains contentious due to insufficient offshore seismic evidence. Furthermore, earthquake locations within the Gorda plate, derived primarily from land-based seismic networks, are inherently biased, further constraining insights into deformation patterns and styles in the offshore region. Enhanced offshore seismic observations are essential to resolving these uncertainties and better understanding the plate's geodynamic behavior driven by the northward migration of the MTJ.

We utilized ocean-bottom seismometer (OBS) networks (network codes 7D, X9, Z5, OO) deployed as part of the Cascadia Initiative to investigate offshore micro-seismicity in Cascadia between July 2012 and October 2015. We firstly evaluated the performance of multiple deep-learning pickers, including EQTransformer, PhaseNet, and PickBlue. Among these, the OBS picker, PickBlue, demonstrated superior event detection performance when applied to OBS data compared to pickers trained on onshore datasets. Using PickBlue, we further derived 2,253,059 P-phases and 1,405,180 S-phases with confidence values exceeding 0.6, enabling us to locate 14,057 local earthquakes offshore Cascadia. The significant enhancement in the detection of offshore seismic events allowed us to reveal more detailed patterns of seismic activity, particularly in the Gorda plate and its surrounding boundaries, which had been largely absent in earlier studies. We observed a high density of intraplate micro-seismicity, underscoring the complex tectonic interactions within the Gorda plate. The spatial distribution of micro-events delineates the northern limit of deformation, which aligns precisely with pseudofault traces between 42°N and 42.5°N. These micro-earthquakes are well-focused and closely correspond with spreading fabrics and magnetic anomalies, providing seismic evidence for the reactivation of spreading-related faults responsible for much of the internal nonrigid deformation of the Gorda plate. Additionally, a spatial trend of increasingly deeper seismic events is evident with greater distance from the Gorda ridge and closer proximity to the Cascadia deformation front. This pattern reflects the combined influence of north-south compression from the Pacific plate and plate bending prior to subduction. Notably, clusters of deep micro-earthquakes reaching depths of up to 40 km are observed along the southern boundary, the Mendocino transform fault. These findings offer the first seismological evidence for the development of shear zones in the uppermost mantle of the Gorda plate, likely resulting from active asthenospheric flow influenced by the northward migration of the MTJ and advection due to the larger Pacific plate.

How to cite: Ren, Y., Lange, D., and Grevemeyer, I.: Intraplate deformation and mantle shear zones in the Gorda plate driven by the northward migration of the Mendocino triple junction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17345, https://doi.org/10.5194/egusphere-egu25-17345, 2025.

X1.104

EGU25-14298

First-order Control Factors for Ocean-bottom Ambient Seismology Interferometric Observations

(withdrawn)

Jeffrey Shragge, Adesh Pandey, and Aaron J. Girard

X1.105

EGU25-3535

Investigation of high-frequency (> 1Hz) ambient noise in seismic data recorded in very shallow, near-shore settings

Florian Schmid and Arne Schwenk

X1.106

EGU25-13853

ECS

Ultra-low-frequency Ambient Ocean-Bottom Nodal Seismology

(withdrawn)

Aaron Girard and Jeffrey Shragge

X1.107

EGU25-17064

ECS

Identification and application of PmP multiples from onshore-offshore seismic surveys in Eastern Guangdong

Genggeng Wen, Zuoyong Lv, Xiuwei Ye, and Yanxin Zhang

Onshore-offshore deep seismic exploration is a significant method for studying the crustal structure and geological tectonics in the land-sea transition zone. Special conditions, such as large offsets and differences in geologic structures, allow for the observation of seismic multiples. Multiples are reflections from subsurface interfaces that include additional information about subsurface structures, which can improve imaging resolution. In particular, it is able to make up for the limitation of ray coverage induced by unilateral source excitation in onshore-offshore deep seismic exploration. In this work, a systematic analysis of seismic multiples associated with the reflection phase of the Moho interface (PmP) is carried out using the seismic data from the L1-NW03 survey line of an onshore-offshore seismic experiment conducted in eastern Guangdong in 2021. First, based on the features that multiples have comparable frequency characteristics to PmP (4–6 Hz) and that their travel times grow as offset increases, it is suggested that the multiples are secondary PmP phases and that the reflection occurs on the sedimentary basement interface. Then, using theoretical raypath simulation, it is verified that the multiples' reflection interfaces are the seafloor interface and the sedimentary basement interface (multiples is named PmP2PsP). Finally, a RAYINVR forward P-wave velocity model is derived from all reflection and refraction seismic phases. The VMONTECARLO method is used to assess the constraint accuracy of the sedimentary basement interface based on the forward model. According to the results, using the PmP2PsP seismic phase reduced the interface depth errors from ±0.21–1.16 km to ±0.11–0.58 km. The imaging resolution of the shallow crust and sedimentary layer was also considerably improved. In addition, a time-depth conversion formula for the sedimentary basement interface was obtained by fitting multi-channel reflection seismic data and the data of time difference between PmP2PsP and PmP.

How to cite: Wen, G., Lv, Z., Ye, X., and Zhang, Y.: Identification and application of PmP multiples from onshore-offshore seismic surveys in Eastern Guangdong, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17064, https://doi.org/10.5194/egusphere-egu25-17064, 2025.

X1.108

EGU25-1119

ECS

The Effectiveness of Automatic Seismic Phase Picking and Detection Capabilities of Deep Learning Methods for Local On- and Offshore Seismic Data: The Case of the Terceira Rift, Azores, Portugal

Paulino Cristovao Feitio, Susana Custodio, Ana Ferreira, Stephen Hicks, and Daud Jamal

High-quality earthquake catalogues for seismic hazard and tectonic assessment of a region, primarily require high-accuracy and high-precision hypocentral locations, along with a low completeness magnitude. The Terceira Rift is an active structure which accommodates a slow transtensional deformation of about 5 mm/yr, induced by eastward differential displacement between the Nubian and Eurasian plates. Due to its active and intense seismicity and volcanism, the Terceira Rift constitutes a natural laboratory to investigate active rifting processes. In our research, novel data from the UPFLOW project, encompassing 49 Ocean Bottom Seismometers, covering the Azores-Madeira-Canary Islands region, will be combined with existing land stations, to analyse the seismicity of the Terceira Rift. The detection capabilities of the existing traditional land seismic network have demonstrated weaknesses in detecting smaller events, highlighting its limitations in precise event location when classical analysis methods are applied. It is expected that well distributed network, and the use of Machine Learning methods, will provide us with the possibility to detect events of smaller magnitude with high-accuracy and high-precision. In this study, we tested the detection and phase picking capabilities of the deep learning phase picker EQTransformer for the land network alone and compared its performance with a manually analysed catalogue from the same network. Data used consist of ten days of continuous waveform from IPMA stations network, between October 10 and 20, 2021. Waveform pre-processing included the removal of instrument response, detrending, applying maximum taper of 1%, and high-pass filter at 2 Hz. For pick classification we used the cut-off threshold of 0.20 and 0.15 for P and S phases, respectively. Although there are some outliers for both P and S pick probabilities, we found that the median probability is approximately ~90% for P phase, and ~70% for S phase. Time differences between the catalogue pick-time and EQTransformer pick-time range approximately between -0.5 and +0.5 seconds for P phase and -1.0 and +1.0 seconds for S phase, denoting a high picking precision of EQTransformer. Within the time window analysed, deep learning methods detected more events than those in manually analysed catalogue. We also present initial results of our analysis using both deep learning networks EQTransformer and PickBlue applied to ocean bottom seismology recordings from the UPFLOW passive array deployed in the Azores-Madeira-Canaries region between June 2021 and August 2022.

How to cite: Feitio, P. C., Custodio, S., Ferreira, A., Hicks, S., and Jamal, D.: The Effectiveness of Automatic Seismic Phase Picking and Detection Capabilities of Deep Learning Methods for Local On- and Offshore Seismic Data: The Case of the Terceira Rift, Azores, Portugal, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1119, https://doi.org/10.5194/egusphere-egu25-1119, 2025.

X1.109

EGU25-20002

Deep ocean current regime as inferred from OBS noise offshore SW Iberia

Carlos Corela, Luis Matias, Afonso Loureiro, and Wolfram Geissler

From September 2007 to August 2008, an ocean bottom seismometer (OBS) experiment took place offshore of Cape S. Vincent and in the Gulf of Cádiz (Geissler et al., 2010), within the framework of the EU-funded project NEAREST (Integrated observations from NEAR shore sourcES of Tsunamis: towards an early warning system). On board the Italian research vessel URANIA, 24 LOBSTER (OBS) from the German instrument pool for amphibian seismology (DEPAS) were deployed at water depths ranging from 1990m to 5100m.

OBSs are usually deployed for seismological investigations, but these objectives are impaired by noise resulting from the ocean environment. The deep ocean, where OBSs are generally deployed, was considered until the 1980s a relatively low-energy and quiescent depositional environment where deep water masses flow as relatively slow-moving tabular bodies and deposition is episodically interrupted by down-slope gravity-driven processes. Since the 1990s, it has been demonstrated that deep-water masses can exhibit relatively high speed and play a dominant depositional role in certain areas. “Bottom current” refers to deep water capable of eroding, transporting and depositing sediments along the seafloor.

The permanent low-frequency geostrophic flow regime around the Atlantic Iberian margin has several water masses flowing at different depths in the same or opposite directions. Two main water masses in SW Iberia have been identified for the deep ocean. The Lower Deep Water (LDW) is composed mainly of Antarctic Bottom Water (AABW) and flows regionally below 4000m depth across the abyssal plains. The second is the North Atlantic Deep Water (NADW), which flows in various directions between 1400–4000m depth. Oceanic gateways are essential in controlling water-mass exchange between the abyssal plains and bottom current speed flow and pathways. The deep-water currents capable of eroding, transporting and depositing sediments along the seafloor exhibit relatively high speed and play a dominant depositional role in certain areas when interacting with local seafloor irregularities like seamounts, scarps, ridges, etc.

We focus on OBS-recorded noise analysis in two frequency bands, the 1-10Hz (harmonic tremors) and the long-period (10s-60s) bands, in the seismometer’s horizontal Y and vertical Z components. We see a robust seismometer response to deep ocean currents modulated by tides during the flood and ebb tides through spring and neap tides, which impact the permanent low-frequency flow from AABW and NADW.

This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020). The NEAREST project was funded by EC (GOCE, contract 037110).

How to cite: Corela, C., Matias, L., Loureiro, A., and Geissler, W.: Deep ocean current regime as inferred from OBS noise offshore SW Iberia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20002, https://doi.org/10.5194/egusphere-egu25-20002, 2025.

X1.110

EGU25-9067

A MERMAID Lander seismo-acoustic float records numerous seismic events in the Ligurian Sea

Sébastien Bonnieux, Frédéric Rocca, Franck Hieramente, Olivier Philippe, Yann Hello, and Karin Sigloch

A MERMAID float was deployed in the Mediterranean Sea offshore Nice between July and September 2024. It dove to a depth of 2,200 meters to record earthquake time series using its hydrophone. MERMAID floats have been developed as part of a collaboration between GEOAZUR research lab and their manufacturing partner OSEAN since 2014.

An anchoring system, called guiderope, allows the float to rest on the seabed at depth up to 4000 meters, preventing its drift in deep sea currents. In two and a half months, the float surfaced three times and changed position by only 10 km, mainly due to suface drift for transmitting data. Without an anchoring system, the drift would have been in excess of 100 km, as for standard MERMAID floats, which have been operating for example in the southern Pacific since 2018.

The onboard signal acquisition and processing system automatically detected 11 teleseismic P-waves from earthquakes of magnitudes between 6.0 and 7.4, as well as one T wave from a nearby earthquake in the Mediterranean of magnitude 4.1. These recordings were transmitted by Iridium satellite each time the float surfaced.

Manual analysis of the continuous times series recording after recovery of the float, too voluminous to transmit by satellite during the mission, revealed numerous additional events. Namely 17 teleseismic P-waves from earthquakes of magnitudes 4.5 to 7.9 and 57 T-waves from the Mediterranean basin from earthquakes of magnitudes 1.6 to 4.5, as well as 24 T waves not identifiable in the catalogs. Numerous other signals related to maritime navigation and weather conditions were also recorded.

How to cite: Bonnieux, S., Rocca, F., Hieramente, F., Philippe, O., Hello, Y., and Sigloch, K.: A MERMAID Lander seismo-acoustic float records numerous seismic events in the Ligurian Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9067, https://doi.org/10.5194/egusphere-egu25-9067, 2025.

Fibre-optic sensing

X1.111

EGU25-1124

ECS

Decoding Seismic Signals with Seafloor Optical Fiber Strainmeters (SOFS)

Saiful Islam Apu, Noel Jackson, Mark Zumberge, and William Hatfield

Episodic tremor and slip (ETS) events, commonly recorded by onshore GNSS stations in the Cascadia region, typically occur where the plate interface is approximately 30-40 km deep. Notable research, such as that by Bartlow (2020), has suggested the extension of these phenomena offshore, potentially increasing the risk of triggering significant earthquakes. The challenge in detecting offshore ETS lies in the ineffectiveness of GNSS technology on the seafloor. Previous efforts using seafloor optical fiber strainmeters (SOFS) have provided continuous measurements over extended periods (e.g., Zumberge et al., 2018) with an uncertainty level of about 30nε during ETS activity. However, these studies did not report any detectable strain transients, indicating no major short-term stress or slip changes in the monitored segments of the subduction zone. Building on these initial findings, we have recently deployed advanced SOFS to collect yearlong strain data from offshore areas in the Cascadia Subduction Zone, marking a pioneering step in monitoring these regions. This innovative method aims to capture the elusive signals of offshore tremors, small-scale earthquakes, and slow slip events (SSEs). We hypothesized that the updated instruments also record low-frequency seismic earthquakes (LFE, VLFE). We have applied temperature and tidal corrections, harmonic analysis, and conventional filtering and smoothing techniques to enhance data quality. In addition to the continuous strain data, our instruments detected uncatalogued events and several seismic activities, including a significant M7.6 earthquake near Mexico’s Pacific Coast on September 19, 2022, attributed to shallow thrust faulting. Concurrently, an onshore ETS event was observed in borehole strainmeters from mid-September to October 2022. Besides combining the onshore data, we have incorporated advanced time series analysis techniques, such as matrix profiling, motif, and discord discovery into our analytical processes. These novel algorithms have significantly enhanced our ability to identify patterns and anomalies within complex and noisy strain data from seismic events.

How to cite: Islam Apu, S., Jackson, N., Zumberge, M., and Hatfield, W.: Decoding Seismic Signals with Seafloor Optical Fiber Strainmeters (SOFS) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1124, https://doi.org/10.5194/egusphere-egu25-1124, 2025.

X1.112

EGU25-7079

Mapping Mesoscale Eddies and Internal Tides Using Distributed Acoustic Sensing in Madeira Island

Afonso Loureiro, David Schlaphorst, Susana Gonçalves, Luís Matias, Carlos Corela, Álvaro Peliz, and Rui Caldeira

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.

How to cite: Loureiro, A., Schlaphorst, D., Gonçalves, S., Matias, L., Corela, C., Peliz, Á., and Caldeira, R.: Mapping Mesoscale Eddies and Internal Tides Using Distributed Acoustic Sensing in Madeira Island, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7079, https://doi.org/10.5194/egusphere-egu25-7079, 2025.

Instruments and pools

X1.113

EGU25-3511

Evolving challenges, innovations, and opportunities in Ocean Bottom Sensing

Marián Jusko, Ted Somerville, Geoff Bainbridge, and Michael Laporte

X1.114

EGU25-18156

Güralp Ocean Bottom Monitoring Solutions: Autonomous Nodes, Cabled Observatories and SMART

Federica Restelli, Phil Hill, Neil Watkiss, Sally Mohr, Antoaneta Kerkenyakova, and Jamie Calver

Autonomous free-fall OBS units allow users flexibility in deployment and the ability to redeploy in different locations. The Güralp Aquarius functions at any angle without using a gimbal system, and can wirelessly transmit SOH and seismic data to the surface via an integrated acoustic modem. These features allow researchers to monitor and transmit data packets without offshore cabling, reducing logistical challenges whilst maintaining some degree of real-time data transmission. This broad functionality and connectivity has made the Aquarius well-suited for OBS pool use, such as with the National Facility for Seismic Imaging in Canada.

Alternatively, cabled solutions provide access to high-resolution data in real time via a physical link to on-shore infrastructure. As an example, the Güralp Orcus provides a complete underwater seismic station with an observatory-grade seismometer and a strong-motion accelerometer in a single package. The slimline Guralp Maris also provides a more versatile solution, using the same omnidirectional sensor as the Aquarius and can be installed either on the seabed or in a narrow-diameter subsea borehole. Both systems are deployed globally as part of multi-disciplinary observatories such as the Neptune array operated by Ocean Networks Canada.

SMART cables show great potential for increasing the number of cabled ocean observatory deployments in the future with substantially reduced deployment costs. Combining several applications into a single system, including seismology, oceanography and telecommunications, large scale monitoring networks can be created cost-effectively by combining logistical and fundraising efforts from multiple industries. Güralp is leading the way with a wet demonstration SMART Cable system in the Ionian Sea in collaboration with Instituto Nazionale Di Geofisica e Vulcanologia (INGV) which has proven to be the first practical demonstration of this technology. There are plans for additional projects in the future by leveraging new low-volume and low-power iterations of Güralp sensors and data acquisition modules.

How to cite: Restelli, F., Hill, P., Watkiss, N., Mohr, S., Kerkenyakova, A., and Calver, J.: Güralp Ocean Bottom Monitoring Solutions: Autonomous Nodes, Cabled Observatories and SMART, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18156, https://doi.org/10.5194/egusphere-egu25-18156, 2025.

X1.115

EGU25-8805

MUG-OBS and Halios – versatile platforms for long-term geophysical deployments on the seafloor

Yann Hello, Charles Rebour, Olivier Philippe, Karin Sigloch, Sébastien Bonnieux, and Aude Lavayssière

MUG-OBS is an autonomous, free-fall system for multi-parameter observations on the seafloor, equipped with numerous geophysical and oceanographic sensors. It is designed for depths up to 6,000 meters and to withstand trawling. All components, such as polyethylene and titanium, are non-corrosive, and buoyancy is ensured by syntactic foam. MUG-OBS has an autonomy of 48 months when equipped with a compact Trillium 120s ocean-bottom seismometer, a triaxial accelerometer, absolute and differential pressure sensors, CTD, and a hydrophone. The seismometer is shielded in a central well in the main structure to protect it from the convection of ocean currents, and is decoupled from the main frame. Acoustic communications to and from the sea surface allow for all functionalities to be controlled during deployment, key acquisition parameters to be modified, and an on-demand health report to be obtained on each visit.

Data can be retrieved during a deployment via six messenger shuttles, released to rise to the surface by acoustic command while recording on the seafloor continues. This makes MUG-OBS an ideal platform for long-term, autonomous seafloor observations close to coasts with seismic hazards, where the data shuttles can be retrieved on day trips with a small ship. Thus, the prototype has been operating for 8 years in the Mediterranean Sea, 40 km offshore Nice. As station MUG01.FR of the French national broadband network (redeployed for the third time in 11/2024), its data are freely accessible via through Epos-France (formerly RESIF). https://seismology.resif.fr/browse-by-station/#/FR/MUG01

Alternatively, if no ship is available to physically recover a surfaced shuttle, it can instead transmit data to its owner via satellite link. Its integrated Iridium modem can first transmit a catalog of seismic events. Communicating back, the user can then ask the shuttle to transmit more voluminous seismogram time series. This option corresponds to long-term deployment needs in the open ocean without easy ship access, improving the feasibility and carbon footprint of such missions.

Only the main MUG-OBS platform needs to be recovered after four years. Once at the surface, shuttles and MUG-OBS transmits its GPS position via a VHF system to the nearby ship to guide its approach. For the eventuality of an untimely ascent, for example caused by trawling, MUG-OBS is also equipped with an Iridium modem to transmit its position and to facilitate the organization of its recovery. The drift of MUG-OBS’ master clock on the seafloor is determined each time a shuttle surfaces and synchronizes to a GNSS signal, which ensures precise data timing constraints over the entire mission.

MUG-OBS was developed jointly by Géoazur research lab and its manufacturing partner OSEAN, who are commercializing the instrument. Its features and innovations have more recently been implemented in the smaller “Halios” broadband OBS, which has an autonomy of 20 months. In Halios, the shuttles have been replaced by an acoustic modem for parsimonious data retrieval from the surface.

How to cite: Hello, Y., Rebour, C., Philippe, O., Sigloch, K., Bonnieux, S., and Lavayssière, A.: MUG-OBS and Halios – versatile platforms for long-term geophysical deployments on the seafloor, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8805, https://doi.org/10.5194/egusphere-egu25-8805, 2025.

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EGU25-17248

iMARL - A Marine Research Laboratory for Geosystems

Louise Collins, Christopher Bean, David Craig, and Martin Möllhoff