Deep upward mantle flow is key to bring volatiles to the atmosphere and to produce Earth’s largest melting events extending through hundreds of thousands of kilometres, which coincide with major extinctions and changes in the geodynamo. Without knowing upward flow, we cannot understand global mantle flow and directly link the Earth’s interior with the surface. Yet, while downward flow (at subductions zones) is well constrained by seismology, plume-like mantle upwellings that connect the deepest mantle to the surface are poorly understood. The goal of the UPFLOW project (https://upflow-eu.github.io/) is to develop new high-resolution seismic imaging approaches along with new data collection, and to use them to constrain upward flow in unprecedented detail. We conducted a large amphibian experiment in the Azores-Madeira-Canary Islands region, which is a unique natural laboratory with multiple upwellings that are poorly understood in general. UPFLOW deployed 50 and recovered 49 ocean bottom seismometers (OBSs) in a ~1,000×2,000 km² area in the Azores-Madeira-Canary Islands region starting in July 2021 for ~13 months, with an average station spacing of ~150-200 km. These data will be combined with land data from over 30 seismic stations in nearby islands. The seismic deployment and recovery involved institutions from five different countries: Portugal (IPMA, IDL, Univ. of Lisbon, ISEL), Ireland (DIAS), UK (UCL), Spain (ROA) and Germany (Potsdam University, GFZ, GEOMAR, AWI). In addition to its scientific component, the experiment also had a substantial science outreach component, with participation from children from primary schools in the UK, Germany, Portugal, Spain and Ireland, facilitated by UCL’s GeoBus project. We discuss some of the activities conducted, such as the children’s naming of the OBSs, their drawings on how they imagined the OBSs, their stories on what may have happened to the OBSs during the experiment and zoom sessions including live calls to the ship. Names chosen included Neptune, Triton, Jelly, Caesar and Thor. The deployment (28 days offshore) and recovery (35 days offshore) of the OBS instruments was carried out during two dedicated expeditions with IPMA’s research vessel NI Mário Ruivo with the strong support of its skilled and highly motivated crew. 32 OBSs were loaned from the DEPAS international pool of instruments maintained by the Alfred Wegener Institute (Bremerhaven), Germany, while other institutions borrowed additional instruments (7 from DIAS, 4 from IDL, 3 from ROA, 4 from GEOMAR). Most of the instruments have three-component wideband seismic sensors and hydrophones, but three different designs of OBS frames were used. Initial data analysis shows high-quality data, notably a substantial decrease in noise levels in the vertical component long-period data (T>~30s). We show illustrative recordings of teleseismic events, a local seismic swarm, and long-period seismic signals as well as of non-seismic signals such as whales vocalisations and ships’ noise. We discuss the lessons learned from our international collaborative expedition, as well as possible future directions. 

How to cite: Ferreira, A. M. and Miranda, M. and the UPFLOW team: The UPFLOW experiment: peeking from the sea floor to the deep mantle with a ~1,500 km aperture array of 49 ocean bottom seismometers in the mid-Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10019, https://doi.org/10.5194/egusphere-egu23-10019, 2023.

Currently, Ocean Bottom Seismometers (OBS) have sensors comparable to those used on land stations. However, they are exposed to very different conditions that degrade the recordings. One major issue is being directly exposed to global and tidal oceanic currents that, depending on the water velocity flowing around the instrument, can excite parts of the frame or produce drag and lift effects due to vortex shedding. Any of these conditions is detrimental for signal integrity, either from variations of the instrument-to-ground coupling or by introducing unwanted energy that is unrelated with the seismic events the experiment is aimed at.

The UPFLOW project is aimed at understanding mid-plate, deep upward flow that cannot be explained by plate tectonics, but is critical for continental growth, for returning volatiles to the atmosphere and for producing Earth’s largest melting events. For this reason, 50 OBS of different types were deployed for a year in the North Atlantic Ocean, including several prototypes aimed at reducing the tidal-induced noise generated by water flowing around the instrument's frame.

In this work, we show the seasonal variation of the tidal-induced noise on different instrument types across the Madeira and Seine abyssal plains of the North Atlantic Ocean during neap and spring tides between July 2021 and July 2022. In some instances, where harmonics are detected, individual frame components of the OBS can be identified as a major contributor, paving way to finding mitigation solutions on future deployments.

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.

How to cite: Loureiro, A., Corela, C., Tsekhmistrenko, M., Miranda, M., and Ferreira, A. and the UPFLOW Team: Seasonal changes on tidal-induced noise on long-term OBS deployment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5928, https://doi.org/10.5194/egusphere-egu23-5928, 2023.

We present preliminary results from the UPFLOW (Upward mantle flow from novel seismic observations; https://upflow-eu.github.io/) project and associated large-scale amphibian experiment. The UPFLOW project, funded by the European Research Council (2021-2026), led a passive seismology large-scale experiment in the Azores-Madeira-Canary region starting in July 2021 and ending in September 2022. 

We recovered 49 (out of 50) OBSs deployed in a ~1,000×2,000 km² area and with an average station spacing of ~150-200 km. Most instruments have three-component wideband seismic sensors (Trillium compact OBS version with a corner period of 120 s) and broadband hydrophones (type HTI-01 and HTI-04-PCA/ULF) with a corner period of typically 100 seconds. Three different designs of OBS frames were used LOBSTER, NAMMU and DUNE. We achieved data recovery rates of ~90-100% in 37 stations and ~50% in 6 stations, with only 7 stations being faulty, highly problematic or lost. 

We present various illustrative examples of seismic waveforms, spectrograms and data quality analysis from the recovered OBS data. Systematic probability power spectra density (PPSD) plots for all stations and over the whole deployment show a marked improvement in the quality of the long-period data (T>~30s) compared to previous experiments. 

We analyse the data performance as a function of the three models of OBS, recorder and seismometer types. Additionally, we present initial results from: (i) noise correlation analysis to quantify the clock skew during the deployment; (ii) estimation of horizontal component rotation angles from teleseismic data (body and surface waves); and, (iii) tilt and compliance analysis. 

Finally, we present an initial set of ~6000 multi-frequency body-wave travel time measurements generated with the UPFLOW OBS data and show a glimpse of preliminary body wave tomographic models built with this new dataset.

How to cite: Tsekhmistrenko, M., Diaz, R., Harris, K., Tilmann, F., Krüger, F., Ferreira, A., and Miranda, M. and the UPFLOW Team: UPLFOW ocean bottom seismometers data: preliminary performance report, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8108, https://doi.org/10.5194/egusphere-egu23-8108, 2023.

Pusan National University deployed sixteen ocean bottom seismometers (OBSs) in the eastern offshore of the southern Korean peninsula. The primary purpose of the OBS network is monitoring earthquakes in the eastern offshore to investigate potential fault systems in the offshore region which was formidable using limited apertures by land-based observations. A seismic network's performance highly depends on each site's background noise level. We analyze the nature of the ambient noise and site response for ocean bottom seismometers (OBSs) deployed in the 2021-2022 period. The power spectral densities (PSDs) of the OBSs exhibited dis-similar features from those of land-based stations; most temporary broadband seismic stations on land showed relatively lower background noise levels. In the meanwhile, OBSs showed higher background noise levels. We report the nature of ambient noise at various channels, water depths, spatial locations, temporal variations, extreme weather conditions, and their potential causes. In general, horizontal components are noisier than vertical components. For longer periods, horizontal components are larger by ~45 dB. The probability density functions (PDFs) of OBSs show that the noise level is within the range of McNamara’s model (2004) for higher frequencies (3.5~50 Hz) although they are still high. When examining long periods (> 20 s), the noise level is higher than what would be given by McNamara’s model. Although we do not observe diurnal or weekly variations in OBS, as expected, we observe varying degrees of seasonal variations in OBSs. Apparently, water depth is the most important factor in deciding noise levels and their seasonal variations. At shallow-depth OBSs, we observe a strong correlation between noise levels and wave heights estimated by the Korea Meteorological Administration (KMA). The ambient noise is down to -130 dB for the band from 5 to 15 Hz, which provides the best signal-to-noise ratio for local microearthquakes. We also present the horizontal-to-vertical spectral ratio (HVSR) of the ambient noise recorded by OBSs. They present significant amplifications at lower frequencies, which indicates large amplification by combined effects due to lower density and lower wave velocity at shallow sediments, and greater depths to major impedance contrast. We confirm that modeling HVSR of noise data recorded by a three-component OBS offers a fast and inexpensive method for site investigation in deep water with the potential of in situ seafloor sediment characterization.

How to cite: Sohn, Y. J., Kim, K.-H., Kang, S. Y., Kim, D., and Lee, Y.-C.: Seismic noise characteristics of PNU OBS network in 2021-2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5838, https://doi.org/10.5194/egusphere-egu23-5838, 2023.

The ocean covers two-thirds of the Earth's surface, making it difficult to study the structure of the Earth in these areas.  The ocean, however, provides a pressure signal that can be used to study the oceanic crust, by measuring the seafloor deformation under this pressure signal.  We use continuous signals of seismic ground velocity combined with differential pressure at the sea floor recorded by ocean bottom stations (OBS).  By combining pressure and displacement data of the OBS, we can calculate the compliance function from the pressure signal of infra-gravity waves in the frequency band (0.003-0.03 Hz) . The displacement depends on the seismic properties of the earth's crustal layers, primarily its shear modulus.
The compliance function is very sensitive to the regions with a low shear velocity which makes it an excellent tool for investigating the earth crust for these regions.

To calculate the compliance function, the data must be preprocessed in several steps including glitch and earthquake removal, tilt correction, etc.
To remove earthquakes we used earthquake catalogs for large events and a recursive STA/LTA algorithm for local events. We use a comb
filter to remove hourly glitches  produced by the internal OBS system . 
Tilt noise is caused by deep ocean currents,  we minimize it on the vertical record by rotating the 3-component seismic data to an angle that minimize the variance of the vertical record, then by removing noise coherent with the horizontal components.

In this study, we used the French OBS of the Rhum-Rum experiment (Barruol and Sigloch, EOS, 2013) near La Reunion Island for 13 months in 2012, then calculated the compliance function. The crustal shear velocity structure below each station is then recovered by depth inversion of compliance.

How to cite: Aminian, M.-A., Stutzmann, E., Montagner, J.-P., and Crawford, W.: Determining the shear velocity structure of the oceanic crust from measurements of seafloor compliance, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15911, https://doi.org/10.5194/egusphere-egu23-15911, 2023.

Data from Ocean Bottom Seismometer (OBS) deployments nowadays are routinely integrated in federated seismological data centers, but often without following standardized procedures as recommended by the OBS community. This makes it difficult to locate and exploit these data mainly due to the restrictions imposed by the standard metadata format (stationXML) and the services.

In the context of the Helmholtz Metadata Collaboration (HMC), supported by OBS experts, we selected AWI and GEOMAR datasets to be archived at the GEOFON data center with standardized procedures according to the FDSN straw man proposal of W. Crawford for OBS data and metadata using also the OBSinfo tools developed at IPGP. In addition, a special emphasis was put on enhancing FAIRness for the selected datasets, for example identifiers of individual instruments were included, which are linked to instrument databases. Keywords were also added to make the data more easily findable and interoperable. From these experiences we formulated guidelines for OBS data management, which should help researchers to archive their OBS data consistently throughout EIDA data centers.

How to cite: Hillmann, L., Hemmleb, S., Strollo, A., Quinteros, J., Heinloo, A., Haberland, C., Haxter, M., Schmidt-Aursch, M., Ulmer, L., Dannowski, A., Kopp, H., and Crawford, W.: Enhancing FAIRness for OBS data within the eFAIRs project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14181, https://doi.org/10.5194/egusphere-egu23-14181, 2023.

At the Mariana Trench, the Pacific plate with increasing age to the east is subducting beneath Philippine Sea plate. This subduction zone includes a volcanic arc, back-arc spreading center and many serpentinite seamounts. The Central Mariana Trench is investigated by using seismic data from 32 OBSs and 20 stations deployed in islands. Cross-correlating ambient noise records between station pairs yields fundamental-mode Rayleigh- and Love-wave dispersion curves at periods from 3 s to 35 s and at periods from 3 s to 20 s, respectively and used a 3D reference model consisting of Crust1.0 and ak135 to obtain more accurate inversion results. In particular, we also estimate group velocities between asynchronous station pairs by using permanent stations on great circle as virtual sources. By jointly inverting Rayleigh- and Love-wave dispersion curves, we calculate an S-wave isotropic and radially anisotropic velocity model, which has resolutions down to 70 km depth for isotropic S-wave velocity model and down to 40 km depth for anisotropic model. Low velocity anomalies are imaged due to the serpentinization and volcanic arc and high velocity anomalies are caused by the stagnation of fore-arc material. On the other hand, negative radial anisotropy are observed dominantly down to 15 km depth, which may be caused by vertical dykes responsible for many seamounts in this region.

How to cite: Kim, T., Chang, S.-J., and Witek, M.: S-wave isotropic and radially anisotropic velocity structure around the Mariana Trench inferred from ambient noise tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10745, https://doi.org/10.5194/egusphere-egu23-10745, 2023.

In the Western Pacific, there are unique geologic/tectonic features such as oldest oceanic crust in the Pacific, seamount chains derived from the Caroline hotspot, the Ontong-Java plateau which is the largest large igneous province, and complex plate boundaries between major and microplates. Investigation of isotropic and anisotropic velocity structure for this region is essential to our understanding of those unique features aforementioned. We estimate an S-wave isotropic and radially anisotropic velocity model beneath the Western Pacific by applying partitioned waveform inversion to three-component seismograms collected from the Incorporated Research Institutions for Seismology Data Managing Center, the Oldest-1 Array deployed in 2018-2019 by a joint Korean-Japan research team, and the Ocean Hemisphere network Project (OJP, NM, and SSP networks). We nonlinearly invert three-component waveforms from 17,038 raypaths (Mw > 5.5) with a 3-D reference model consisting of Crust1.0 and AK135 and resulting constraints are used for iterative least-squares inversion to build an S-velocity model. Our isotropic Vs model shows low-Vs anomalies at ~40 km depth beneath the Ontong-Java Plateau indicating a thick crust, at ~200 km depth beneath the Woodlark spreading center and Caroline seamount chain and at ~600 km depth beneath the center of the Eauripik rise. High-Vs anomalies are observed beneath the center of the Ontong-Java Plateau at 40-150 km depth and at ~50 km depth beneath the West Philippine basin, the Parece-Vela basin, and the Caroline basin. Overall positive radial anisotropy anomalies are observed in the Western Pacific, but the contrast of anisotropy was found in the Pacific plate, Philippine Sea plate, and Caroline plate at ~50 km depth. Negative radial anisotropy anomalies found in the Perace-Vela basin at ~30 km depth, and strong positive anisotropy anomalies are observed at the northern boundary of the Ontong-Java Plateau and beneath the Sorol trough and Caroline seamount chain.

How to cite: Park, J., Chang, S., Witek, M., Lee, S.-M., Kim, Y., Utada, H., Shiobara, H., Isse, T., Takeuchi, N., and Sugioka, H.: S-wave isotropic and radially anisotropic velocity structure in the Western Pacific inferred from partitioned waveform inversion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11314, https://doi.org/10.5194/egusphere-egu23-11314, 2023.

Seismologists have historically focused on land-based seismic research, due to the logistical and financial challenges presented by offshore installations. Guralp has developed technology which allows the seismology community monitor offshore seismicity with greater ease, improving global seismic data resolution. This is due to systems such as the Aquarius autonomous ocean bottom seismometer (OBS) and world-leading engineering advancements in Science Monitoring and Reliable Telecommunications (SMART) cables.

Autonomous free-fall OBS units allow users flexibility in deployment and ability to redeploy a number of times in different locations. The Guralp 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 without offshore cabling, thereby reducing logistical challenges whilst maintaining some degree of real-time data transmission. Optional surface communications can be permanent (buoy-mounted), semi-permanent (wave-glider) or on demand (ship-of-opportunity or dedicated voyage).

Alternatively, cabled solutions give users access to high-resolution data in real-time via a physical link to an onshore data centre. As an example, the Guralp Orcus provides a complete underwater seismic station with observatory grade seismometer and strong-motion accelerometer in a single package. The slimline Guralp Maris also provides a more versatile solution, making use of the same omnidirectional sensor as the Aquarius and can be installed either on the seabed or in a narrow-diameter subsea borehole.

SMART cables show great potential for increasing the number of cabled ocean observatory deployments in the future with substantially reduced deployment costs to the research institute. Combining several applications into a single system, including seismic monitoring and telecommunications, large scale monitoring networks can be created cost effectively by combining efforts from several industries. Guralp is deploying a demonstration SMART Cable system to monitor volcanic and seismic activity offshore in the Ionian Sea in collaboration with Instituto Nazionale Di Geofisica e Vulcanologia (INGV). This will be the first practical demonstration of this technology and there are plans for additional projects in the future.

How to cite: Reis, W., Lindsey, J., Whealing, D., and Watkiss, N.: Novel Autonomous and Cabled OBS Solutions for Offshore Seismic Research, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6910, https://doi.org/10.5194/egusphere-egu23-6910, 2023.

The Macquarie Ridge Complex (MRC) constitutes the boundary between the Indo-Australian and Pacific plates in the southwest Pacific Ocean. It accommodates the world’s most potent sub-marine earthquakes that are not associated with ongoing subduction. To better understand the nature of MRC and its associated earthquakes, we aim to explore the crustal structures using recordings from island-based stations and ocean bottom seismometers (OBS). In particular, these OBSs, which are deployed in the surroundings of Macquarie Island from October 2020 to November 2021, enable us to image the refined oceanic structures beneath the study area. In this study, we obtain the body-wave reflections by computing phase coherence autocorrelations of both ambient noise and earthquake data. Our preliminary reflection profiles by both methods reveal coherent reflected P waves that may be related to Moho and additional structures within the crust and upper mantle.

How to cite: Ma, X., Tkalčić, H., Eakin, C., Rawlinson, N., Pham, T.-S., Winder, T., Pickle, R., Coffin, M., and Stock, J. and the Macquarie Ridge 3D Team: Crustal Structure of the Macquarie Ridge Complex Constrained by the Ambient Noise and Earthquake Autocorrelograms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10875, https://doi.org/10.5194/egusphere-egu23-10875, 2023.

The Macquarie Ridge Complex, located at the boundary between Indo-Australian and Pacific plates in the southwest Pacific Ocean, hosts the largest sub-marine earthquakes in the 20^th century, not associated with ongoing subduction. We deployed 27 ocean-bottom seismometers, of which 15 have been recovered successfully, to understand the origin of the sub-marine earthquakes and their potential earthquake and tsunami hazards to Australia and New Zealand. Additionally, we deployed five land-based seismometers on Macquarie Island.

We explore state-of-the-art processing methods to analyze the new seismic dataset from the retrieved seismic stations. One of the goals is to image the tectonic settings beneath the MRC. Here, we present a first-order tomographic model and its relevant uncertainty estimate of the region constructed from ambient noise surface waves using a probabilistic inversion framework. The tomographic image will be complemented with receiver-based imaging results such as those from P-wave coda autocorrelations and receiver functions to confirm the existence of possible geometries. The results are expected to supply a fresh understanding of the tectonic settings under the MRC and unpuzzle the origin of the significant underwater earthquakes in the 20th century.

How to cite: Pham, T.-S., Tkalcic, H., Ma, X., Pickle, R., Muir, J., Duru, K., Winder, T., Rawlinson, N., Eakin, C., Coffin, M., and Stock, J. and the Macquarie Ridge 3D Team: Probabilistic ambient noise imaging of the Macquarie Ridge Complex using ocean-bottom and land-based seismometers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10662, https://doi.org/10.5194/egusphere-egu23-10662, 2023.