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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

Lots of work remains to be done after we recover all the OHA-GEODAMS data in early 2027. These will paint a finer picture of how the event unfolded, whether it caused significant « post-rifting » deformation, and how hydrothermal activity responded to it… stay tuned!

A picture of R/V Marion Dufresne in the middle of the Indian Ocean

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

This could explain why mid-ocean ridge normal faults generally appear seismically quiet compared to their transform (or continental) counterparts.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

The earthquake swarms alone cannot account for the amount of fault slip that we estimated with seafloor geodesy. This is why we believe fault slip occurred primarily as aseismic transients.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

We estimate that the event accommodated 2-4 m of plate separation, of which ~10% corresponded to slip on axis-bounding faults.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

We later modeled the horizontal and vertical displacements that had been measured by the geodetic array. While many models can fit the data, all of them point to substantial drainage of a magma pocket several km deep, meter-scale opening of a dike, and slip on adjacent faults.

Sub-seafloor mechanisms that may have caused the observed seafloor displacements (Royer et al., 2026)

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

By comparing the bathymetric map we acquired in 2025 vs. the one from 2024, we calculated that ~160 million cubic meters of lava had poured out onto the seafloor. That’s over 60,000 olympic pools worth of lava!

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

Lava effusion continued for several weeks, with a few bursts of activity. When we returned to the site in early 2025, we were shocked to find that up to 100 m thick lava flows had been emplaced along the axis.

cross-sections of seafloor bathymetry measured in 2024 and in 2025, highlighting the difference (in blue). (Royer et al., 2026)

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

The temperature of the deep ocean also rose by a fraction of a degree, a clear sign that hot stuff was flowing out of the seafloor!

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

Seafloor subsidence continued for a few days, reaching a total of 4.2 meters. Meanwhile, our hydrophone network started picking up "impulsive" sounds characteristic of lava encountering water.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

At the same time, our acoustic beacon array measured over 1 m of extension between the middle of the axial valley and the first abyssal hill to the SW: a combination of dike-accommodated stretching and normal fault slip.

schematic description of extension across the axis of the southeast indian ridge

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

While the second dike was propagating, the axial valley floor subsided by about 1 meter in ~40 minutes, reflecting the drainage of an underlying magma reservoir.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

The swarm migrated to the SE, and was quickly followed by a second one, which migrated to the NW at ~2 m/s, along the ridge axis. This is a typical signature of magma-filled fractures (dikes) propagating from a magmatic segment center.

map of the first earthquakes in the April 2024 sequence (Royer et al., 2026)

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

On April 26, 2024 at about 8 pm UTC, a big swarm of (mag~5) normal faulting earthquakes started in the middle of segment I1, at 37°S on the Southeast Indian Ridge… just below our geodetic network!

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 08/07/2026

The news is out today in @nature.com ! The OHA-GEODAMS observatory had the incredible luck to capture a major seafloor spreading event back in 2024, just 2 months after its deployment. You can read all about it at: www.nature.com/articles/s41... Here’s how it unfolded 👇

nature.com

Anatomy of a seafloor spreading event captured by in situ seismogeodesy - Nature

By combining hydroacoustic, direct-path ranging and bottom-pressure measurements, in situ observations of a rifting event at a segment of the Southeast Indian Ridge are reported, providing insight int...

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 09/02/2026

What a day! We visited Kerguelen station, met king penguins and elephant seals, and walked on the "Desolation Island". Just a few days before reaching the OHA-GEODAMS area, this was an unforgettable stop in the middle of the Southern Ocean. Cheers to the penguins 🐧

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 03/02/2026

On our way to the Southeast Indian Ridge and it’s all hands on deck 🚢🔧 From building tripods for acoustic beacons to installing the transducer and modem in the ship’s forward hold, operational prep is well underway!

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 24/01/2026

The 2026 GEODAMS team safely arrived close to Possession Island in the Crozet archipelago a few days ago. A penguin crowd welcomed us under a very bright sky and with the traditional croziflette that follows... 🐧 🍽️ We’ve had our fill, both for the eyes and the stomach.

Penguin crowd at la Manchotière from Possession Island

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 20/01/2026

The OHA-GEODAMS team is back to document how the Indian Ocean grows. Cruise #3 on board Marion Dufresne left La Réunion Island a few days ago to maintain the multi-sensor observatory on the South East Indian Ridge. Follow us on this new adventure! (credit photo: Ewen & Didier)

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 01/02/2025

If it all goes well, our now-complete observatory will be the first to document seafloor spreading and transform faulting events with geodesy, hydro-acoustics, and seismology! 🤞

Schematic illustration of the OHA-GEODAMS seafloor observatory. Credit: J.-A. Olive (LG-ENS) & J.-Y. Royer (Geo-Ocean)

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 01/02/2025

But pressure at the seafloor can fluctuate for many reasons! Tides, ocean dynamics, etc... To avoid misinterpreting an oceanographic signal as a tectonic signal, we deployed a mooring that will help us assess how the weight of the water column changes through time.

Expected pressure variations solely due to tides at our study area for the entire year 2024, from the EOT20 model of https://essd.copernicus.org/articles/13/3869/2021/. Credit: J.-A. Olive, LG-ENS

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 01/02/2025

The A-0-A lets us measure vertical displacements of the seafloor: if the ground swells, the instrument will go up, feel less water weight, and the pressure will decrease.

The A-0-A pressure sensor in its frame, hanging from a cable on the starboard side of the ship at night, waiting to be deployed. Credit: J.-A. Olive, LG-ENS / FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 01/02/2025

Last, but not least, we also recovered and re-deployed an A-0-A pressure sensor, right in the middle of the ridge's axial valley. This state-of-the-art instrument corrects its own drift by regularly re-calibrating itself against an inner chamber where the pressure is known.

An A-0-A pressure sensor, encased in a frame attached to four buoys floating on the ocean surface, waiting to be picked up. Credit: J.-A. Olive, LG-ENS / FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

But last week we succeeded in recovering and re-deploying 1 beacon from the ridge network, and 1 from the transform network. The first year of data did not disappoint! #ToBeContinued

Water color drawing of an acoustic transponder at the seafloor. Credit: J.-A. Olive, LG-ENS

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

Some deployments can take up to 11 hours, as we go through trial-and-error on different target sites with the tripod hanging from the ship on a 2-km long cable.

Jean-Yves eagerly waiting for the transponder to respond. Credit: J.-A. Olive, LG-ENS / FOF

The control center from where we follow the transponders' deployments. Credit: J.-A. Olive, LG-ENS / FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

Deployments are tricky because we can only communicate with the beacon through an acoustic modem, and sometimes they're in no mood to talk to us (or the ship is just too noisy).

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

When we redeploy a transponder+tripod, we must always make sure it's standing upright on the seafloor, and that it can still ping its colleagues.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

Once the beacon is on board, we download the data, and place it atop a new tripod, to be re-deployed from the back deck, by cable. @jbeesau.bsky.social @pyraumer.bsky.social @annebriais.bsky.social @sismolise.bsky.social

Julie, Anne and Edgar carefully assembling a metal tripod to redeploy an acoustic beacon. Credit: S. Furst / FOF

Preparing new tripods for redeploying acoustic transponders. Credit: S. Furst / FOF

Lise, Julie and Pierre-Yves admiring a reconditioned acoustic transponder, awaiting its tripod for redeployment. Also visible on the left in the ship's hangar: a couple of OBSs! Credit: J.-A. Olive, LG-ENS / FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

Spotting the beacon when it surfaces can be challenging! It's essentially a meter-long pink cylinder encased in a yellow buoy. Lucky for us, the bridge crew has sharp eyes!

a yellow float containing an acoustic beacon just surfaced near the ship. Credit: J.-A. Olive, LG-ENS / FOF

A yellow buoy containing a acoustic beacon is being recovered on the starboard side of R/V Marion Dufresne. Credit: J.-A. Olive, LG-ENS / FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

Last week, we paid a visit to our beacons and were delighted to find them all alive and pinging! Downloading the data from the ship with an acoustic modem was possible, but excruciatingly slow. That's why we recovered and re-deployed a couple of beacons to get a year's worth of geodetic data.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

By measuring acoustic travel times and the speed of sound in the ocean, we can infer how the mid-ocean ridge axis stretches, and the adjacent transform slips.

Simulation of the path of an acoustic ray at the bottom of the ocean, from an emitter (green star) to a receiver (red star). Black curve shows the ocean floor topography. Colors indicate the attenuation of the acoustic signal. Credit: J.-A. Olive, LG-ENS / CNRS

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

As tectonic plates drift further and further apart on either side of the mid-ocean ridge, sound waves take a longer and longer time to travel from beacon to beacon.

A schematic illustration of our acoustic ranging network across the Southeast Indian Ridge. Credit: J.-A. Olive & J.-Y. Royer, LG-ENS / Geo-Ocean / CNRS

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 31/01/2025

The core of the OHA-GEODAMS project is to measure active deformation for 3 years on the Southeast Indian Ridge and the Amsterdam transform fault. To this end, in February 2024, we deployed 15 acoustic beacons that have been pinging each other every few hours ever since.

2 iXblue/Exail Canopus acoustic transponders encased in yellow floats, atop 2-m tall metal tripods, waiting to be deployed on the back deck of R/V Marion Dufresne during the 2024 GEODAMS cruise. Credit: J.-A. Olive, LG-ENS / FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

They also record whale songs, and even the sound of icebergs grinding against each other! See for example this great explainer video from @noaa.gov : oceanexplorer.noaa.gov/explorations...

oceanexplorer.noaa.gov

NOAA Ocean Explorer: Submarine Ring of Fire 2003: Hydroacoustics Animation

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

Once comfortably settled in the SOFAR channel, hydrophones record the sound made by earthquakes near and far, and let us relocate them with much better accuracy than distant seismometers on land.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

We then release the weight, sinking the entire mooring down to the seafloor in a matter of minutes.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

We start by deploying the float containing the hydrophone from the back deck, then unroll the ~2 km long mooring...

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

Hydrophones are basically big microphones moored ~1000 m below the sea surface, in the SOFAR channel, where acoustic waves can travel thousands of kilometers across an ocean basin with minimal attenuation.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

We also recovered and redeployed five hydrophones that were first deployed last year during the #GEODAMS 2024 cruise.

a hydrophone encased in an orange float just surfaced right in front of the R/V Marion Dufresne, on a calm sea. Credit: J.-A. Olive, LG-ENS / FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

We send an acoustic signal that releases the instrument from its anchor. The buoys bring it to the surface at a little under 1 m/s, and we find it with the help of flashing lights and radio signals (but that’s for next year!)

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

Using broadband OBSs, we will also carry out compliance measurements: that's looking at how the oceanic crust responds to changes in water pressure at the seafloor, due to tides and ocean dynamics. This should tell us whether magma is present at depth in the crust.

A broadband OBS being deployed from the side of the ship. The green sphere is the instrument, kept away from the noise of the batteries and data loggers by the white "arm". Credit: L. Retailleau, IPGP/FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

This local network will let us locate the events with high precision to characterize how mid-ocean ridge faults slip.

Deployment of a short-period OBS, hanging on a cable over the ocean, from the side of the ship. Credit: L. Retailleau. IPGP/FOF

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

We deployed 7 Ocean Bottom Seismometers (OBSs) to record earthquakes and other signals emitted by the Southeast Indian Ridge and the Amsterdam transform fault.

water color by D. Pacaud illustrating the deployment of an ocean bottom seismometer from the Marion Dufresne

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 30/01/2025

For the last ~10 days, we've been hard at work on and around the Southeast Indian Ridge at 37ºS to recover and redeploy a bunch of geophysical instruments. Let's break it down 👇

Regional map of the Southeast Indian Ridge near Amsterdam Island

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 20/01/2025

We will recover it at the location of our last hydrophone, West of Amsterdam, next week.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 20/01/2025

This glider is equipped with a hydrophone (48kHz) to record the underwater soundscape, and in particular the cetaceans (i.e., blue whales, fin whales, killer whales, sperm whales, etc.). The glider will navigate south and west of Amsterdam, diving down to 1000 m, driven by buoyancy changes.

Deploying the glider from the workboat, with the Marion Dufresne in the background

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 20/01/2025

In the meantime, Julie stayed on the Marion Dufresne, to manage communications between the pilot team in Brest and the team on the workboat.

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 20/01/2025

In order to prevent the glider from colliding with the Marion Dufresne, it had to be launched at a certain distance. To do so, 3 brave members of our GEODAMS team, Anne, Séverine and Diane, boarded a workboat with 2 LDA crew members.

Séverine, Anne and Diane all dressed up to deploy the glider from the workboat

Deploying the workboat, with the glider in the front

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 20/01/2025

A few days ago, we deployed a Sea Explorer glider developed by ALSEAMAR and owned by ENSTA Bretagne at the location of our first hydrophone, southeast of Amsterdam island.

Julie & Jean-Arthur bringing the glider to the workboat for deployment. Credit: E. Klein

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 19/01/2025

Measuring travel time and knowing the speed of sound in water, the echosounder provides depth measurements along swaths, which we combine to create bathymetric maps, revealing underwater landscapes, submarine volcanoes, and ridges.

The difference between satellite-based bathymetry (left, Smith & Sandwell, 1997) and ship-acquired multibeam bathymetry of the Southeast Indian Ridge (Loubrieu et al.)

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The OHA-GEODAMS Seafloor Observatory @geodams.bsky.social · 19/01/2025

As we make our way to our study area, we map the seafloor under the ship! 🌊 Our multibeam echosounder sends acoustic waves to the ocean floor and records how much time they take to bounce back.

Data coming live from the ship's multibeam echosounder

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