GeoLog

seismology

Imaggeo on Mondays: On the way to Tristan’s penguins

Imaggeo on Mondays: On the way to Tristan’s penguins

Tristan da Cunha is a remote volcanic island in the south Atlantic Ocean. In fact, it is the most remote inhabited archipelago in the world. Tristan is still volcanically active; the last time it erupted was in 1961. After the eruption, which luckily did not have any casualties, the whole population of around 260 people evacuated the island for some time, but they all returned back to the island because it was home.

I took this photo while aboard the ISOLDE research cruise associated with the GEOMAR Helmholtz Centre for Ocean Research in Germany. The ISOLDE project focuses on investigating the electromagnetic, gravimetric and seismic activity present on this little island.

There are several reasons why this area is particularly interesting for multi-disciplinary geophysical studies. First, the island is a prominent candidate for a deep-rooted hot spot. A hot spot is a volcanic region believed to be fed by mantle plumes, which bring considerable heat from deep in the Earth. Deploying ocean bottom seismometers (OBS) should help investigate the presence (or absence) of a whole-mantle plume beneath the island. Second, geophysical analysis in this region can help scientists better understand the tectonic processes involved in the extension of the South Atlantic margins and the formation of the Walvis Ridge.

In 2012, the ISOLDE (as part of the SAMPLE project) research cruise aimed to acquire a year’s worth of data on the marine electromagnetic activity, active and passive seismicity, gravity and bathymetry around Tristan da Cunha. Among others, there were 24 OBS deployed on the sea floor (around 3000-4000 m in depth). These instruments stay on the ocean bottom for one year and continuously record seismic signals.

After one year, in 2013, I joined the recovery cruise. This was my second time on a research vessel, but it was the first time I actually worked as a technical assistant on OBS.

The cruise started from Walvis Bay, a coastal town in Namibia. After a one-week transit from the harbour to the first station, we spent around seven days recovering 12 OBS around Tristan da Cunha.

The process of recovering the instruments is usually straight forward. To start, you head to the location where you first deployed the instrument, put a transducer into the water and then ping the OBS. If you get a response, you enter a code that sends an acoustic signal to release the main instrument from its steel anchor. The floating units attached to the instrument then take care of bringing the OBS back to the sea surface. Depending on the depth, it can take up to an hour until the OBS resurfaces (e.g. this is a simple calculation: 3000m deep, rising velocity of 1 m/s).

This would be a perfect recovery procedure, but you know, it rarely happens like this! After recovering half of the instruments over the course of about a week, the team got a well-deserved day off on Tristan.

Tristan da Cunha is such a small, beautiful, strange and lonely island. I was almost expecting to find a lost native tribe there, but in truth, it looked like any small town in England, with tiny gardens in front of their houses. Once we arrived at the island we had the choice between taking a touristic tour of the potato fields, where the Tristanians go in summer for holidays, or exploring the island independently.

I decided to go off to the north of the island. It was a perfect day, sunshine with no clouds in the sky, which was surprising for the South Atlantic. I wandered off past the remains of the famous 1961 eruption and the island’s own dumping place until I couldn’t go further. I arrived at a stony beach, from where I could see our ship, the M/S Merian, in the distance, anchored before the island’s coast, since our vessel was too big for Tristan’s small harbour.

I spotted the three penguins standing next to each other sun bathing. ‘Chilled guys’, I thought; and even when I drew closer to take the shot, they looked entirely relaxed and barely noticed me. It’s not like they had seen so many tourists around here! After taking the picture, I placed myself next to them (it’s surprising how smelly they are) to enjoy the view and the sun. Further down the beach, I also spotted a big mama seal and its adorable small fluffy baby. Right in front of me an orca emerged from the waters, properly trying to get to the seals. It flashed its fin before diving down again.

All in all, it was a surreal experience sitting on the remotest island on Earth surrounded by animals I usually only see in a zoo. After one wonderful day on Tristan da Cunha, we went back onboard to continue recovering the remaining OBS from the deep ocean.

By Maria Tsekhmistrenko, University of Oxford (UK)

References

SAMPLE webpage

ISOLDE project description

OBS provided by DEPAS pool in AWI

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

April GeoRoundUp: the best of the Earth sciences from the 2018 General Assembly

April GeoRoundUp: the best of the Earth sciences from the 2018 General Assembly

The 2018 General Assembly took place in Vienna last month, drawing more than 15,000 participants from 106 countries. This month’s GeoRoundUp will focus on some of the unique and interesting stories that came out of research presented at the Assembly.

Mystery solved

The World War II battleship Tirpitz was the largest vessel in the German navy, stationed primarily off the Norwegian coastline as a foreboding threat to Allied armies. The ship was 250 metres in length and capable of carrying around 2,500 crewmates.

Despite its massive size, the vessel’s presence often went unnoticed as it moved between fjords, masked by a chemical fog of chlorosulphuric acid released by the Nazi army.

Ultimately the ship sank and the war ended, but evidence of the toxic smog still lingers today, in the tree rings of Norway’s nearby forests.

Claudia Hartl, a dendrochronologist from the Johannes Gutenberg University in Mainz, Germany, made this discovery unexpectedly while sampling pines and birches near the Norwegian village Kåfjord. She and her research team presented their findings at the General Assembly in Vienna last month.

The German battleship Tirpitz partly covered by a smokescreen at Kaafjord. (Image Credit: Imperial War Museums )

Hartl had been examining wood cores to draw a more complete picture of past climate in the region when she noticed that some trees completely lacked rings dating to 1945,” reported Julissa Treviño in Smithsonian Magazine.

The discovery was odd since it is rare for trees to have completely absent rings in their trunks. Tree ring growth can be stunted by extreme cold or insect infestation, but neither case is severe enough to explain the missing tree rings from that time period.

“A colleague suggested it could have something to do with the Tirpitz, which was anchored the previous year at Kåfjord where it was attacked by Allied bombers,” explains Jonathan Amos from BBC News.

The researchers indeed found physical and chemical evidence of the smokescreen damage on the trees, demonstrating the long-lasting impact warfare can impart onto the environment.

 

What you might have missed

Seismicity of city life

Researchers use seismometers to record Earth’s quakes and tremors, but some seismologists have employed these instruments for a different purpose, to show how humans make cities shake. “This new field of urban seismology aims to detect the vibrations caused by road traffic, subway trains, and even cultural activities,” reports EGU General Assembly Press Assistant Tim Middleton on GeoLog.

With seismometers, Jordi Díaz and colleagues at the Institute of Earth Sciences Jaume Almera in Barcelona, Spain have been able to pick up the seismic signals of major football games and rock concerts, like footballer Lionel Messi’s winning goal against Paris Saint-Germain and Bruce Springsteen’s Barcelona show.

Seismic record captured by the seismometer during the Bruce Springsteen concert. The upper panel shows the seismogram, while the lower panel shows the spectrogram where it is possible to see the distribution of the energy between the different frequencies. (Image Credit: Jordi Díaz)

Díaz’s project first began as an outreach campaign, to teach the general public about seismometers, but now he and his colleagues are exploring other applications. For example, the data could help civil engineers with tracking traffic and monitoring how buildings withstand human-induced tremors.

Antarctica seeing more snow

Meanwhile in Antarctica, snowfall has increased by 10 percent in the last 200 years, according to new research presented at the meeting. After analysing 79 ice cores, a research team led by Liz Thomas from the British Antarctic Survey discovered that Antarctica’s increased snowfall since 1800 was equivalent to 544 trillion pounds of water, about twice the volume of the Dead Sea.

It has been predicted that snowfall increase would be a consequence of global warming, since a warmer atmosphere can hold more moisture, thus resulting in more precipitation. However, these ice core observations reveal this effect has already been happening. The new finding implies that Earth’s sea level has risen slightly less than it would have otherwise, but only by about a fifth of a milimetre. Though overall, this snowfall increase is not nearly enough to offset Earth’s increased ice loss.

Ocean’s tides create a magnetic field

Also at the Assembly, scientists presented new data collected from a team of ESA satellites known as Swarm, In particular, the satellite observations recently mapped magnetic signals induced by Earth’s ocean tides. As the planet’s tides ebb and flow, drawn by the Moon’s gravitational pull, the salty water generates electric currents. And these currents create a tiny magnetic field, around 20,000 times weaker than the global magnetic field.

Scientists involved with the Swarm project say the magnetic view provides new insight into Earth’s ocean flow and magnetic field, can improve our understanding of climate change, and help researchers build better Earth system models.

When salty ocean water flows through Earth’s magnetic field, an electric current is generated, and this in turn induces a magnetic signal. (Credit: ESA/Planetary Visions)

 

Other noteworthy stories:

 

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Shaking in the city

Shaking in the city

Bruce Springsteen was playing at Barcelona’s football stadium on 14th May 2016. 65,000 people were there to hear him as he launched into an encore including “Born in the USA”, “Dancing in the Dark” and “Shout”. But unknown to Springsteen, just 500 metres away, in the basement of the Institute of Earth Sciences Jaume Almera (ICTJA), Jorde Díaz and his colleagues were also listening in via their broadband seismometer. “We have beautiful recordings of rock concerts,” says Díaz, the scientific director of the Seismic Laboratory at ICTJA, part of the Spanish Scientific Research Council (CSIC).

The first global seismic networks, installed in the 1960s and 1970s, were set up, not to record earthquakes, but to listen in on human activities. Their primary goal was to monitor nuclear tests during the height of Cold War tension. Since then, the same devices have been used extensively and successfully to record the Earth’s natural vibrations, allowing scientists to study earthquakes and volcanoes, as well as map the interior of the Earth in remarkable detail. But researchers are now turning their attention to human activities again; this new field of urban seismology aims to detect the vibrations caused by road traffic, subway trains, and even cultural activities.

“Our motivation for installing this station was mainly for outreach,” Díaz says, “to show [people] how a seismometer works.” But Díaz soon realised that there might be useful information buried within the seismic noise at this new station. “We identified a number of signals and we wanted to know the origin of these signals. Some of them are quite amusing,” he recalls.

Some of these less conventional signals were so-called “foot-quakes”, tremors associated with goals scored at the Barcelona football stadium. “We can get information every time there is a goal,” says Díaz. “Or at least every time there is a Barcelona goal. Not the other side! People jump and then the shaking is recorded at our instrument.” Indeed, ever since the famous Gol del terremoto, Earthquake’s Goal, in Argentina in 1992, we have known that football fans could be picked up by seismometers.

Springsteen’s concert was another of the less orthodox events that the seismologists were able to study. As well as a simple seismogram of the whole four-hour show, which shows the magnitude of the shaking through time, Díaz also plots his data on a spectrogram. The spectrogram reveals the different frequencies present in the vibrations and how they change over time.

Seismic record captured by the seismometer during the Bruce Springsteen concert. The upper panel shows the seismogram, while the lower panel shows the spectrogram where it is possible to see the distribution of the energy between the different frequencies. (Image Credit: Jordi Díaz)

The colour on this diagram then corresponds to the amplitude of the shaking. “You can see that every single song has a particular pattern,” explains Díaz, “and you can even define from the seismic data when we are moving from one song to another.” The vertical stripes in Díaz’s spectrogram correspond to the different songs, whilst the horizontal, red stripes indicate the main frequencies that are present in each track. “In the goal celebrations… the energy is distributed all over,” says Díaz, “while here [at the concert] you can see what we call harmonic structures. You have energy localised at precise [frequencies]. This is because people are dancing, moving in a coordinated way.”

As Díaz explains in his paper, published last year in Scientific Reports, the harmonic structures are likely to be because of a phenomenon known as the Dirac comb effect. As the audience dance to a track with a specific beat, they create a series of equally-spaced pulses in time. This then transforms to a series of “evenly spaced harmonics in the frequency domain,” which is the series of horizontal stripes for each track. Furthermore, faster songs tend to produce higher frequencies.

Rock concerts in a football stadium might sound light-hearted, but Díaz’s work is not without important applications. The majority of the concert vibrations are in the range of 1.8 to 2.5 Hz. Meanwhile, building codes suggest that, structures should not be built with resonant frequencies higher than around 6 Hz. As Díaz and his team have demonstrated, the precise vibrations that the stadium experiences vary depending on the activity occurring. But some of the higher harmonics at the rock concert are close to the suggested building limit such that, if structures were to have resonant frequencies close to this limit, then there might be the potential for damage to the building. “Additional work, following a more engineering approach, is required to know if structure excitation has a significant contribution to the total shaking,” says Díaz.

The shaking in the city that Díaz and his colleagues have been observing is not only good fun, but also potentially of significant importance for civil engineers.

By Tim Middleton

Imaggeo on Mondays: Chilean relics of Earth’s past

Imaggeo on Mondays: Chilean relics of Earth’s past

As Earth’s environment changes, it leaves behind clues used by scientists to paint portraits of the past: scorched timber, water-weathered shores, hardened lava flows. Chile’s Conguillío National Park is teeming with these kind of geologic artifacts; some are only a few years old while others have existed for more than 30 million years. The photographer Anita Di Chiara, a researcher at Lancaster University in the UK, describes how she analyses ancient magnetic field records to learn about Earth’s changing crust.

Llaima Volcano, within the Conguillío National Park in Chile, is in the background of this image with its typical double-hump shape. The lake is called Lago Verde and the trunks sticking out are likely remnants from one of the many seasonal fires that have left their mark on this area (the last one was in 2015).

The lake sits on pyroclastic deposits that erupted from the Llaima Volcano. On these deposits, on the side of the lake, you can even track the geologic record of seasonal lake level changes, as the layers shown here mark the old (higher) level of the lake during heavy winter rains.

The lake also overlaps the Liquiñe-Ofqui Fault, which runs about 1000 kilometers along the North Patagonian Andes. The fault has been responsible for both volcanic and seismic activity in the region since the Oligocene (around 30 million years ago).

I was there as field assistant for Catalina Hernandez Moreno, a geoscientist at Italy’s National Institute of Geophysics and Volcanology, studying ancient magnetic field records imprinted on rocks. We examined the rocks’ magnetised minerals (aligned like a compass needle to the north pole) as a way to measure how fragmented blocks of the Earth’s crust have rotated over time along the fault.

From this fieldwork we were able to examine palaeomagnetic rotation patterns from 98 Oligocene-Pleistocene volcanic sites. Even more, we concluded that the lava flows from the Llaima Volcano’s 1958 eruption would be a suitable site for studying the evolution of the South Atlantic Anomaly, an area within the South Atlantic Ocean where the Earth’s magnetic field is mysteriously weaker than expected.

By Anita Di Chiara, a research technician at the Lancaster Environment Centre in the UK 

References

Hernandez-Moreno, C., Speranza, F., & Di Chiara, A.: Understanding kinematics of intra-arc transcurrent deformation: Paleomagnetic evidence from the Liquiñe-Ofqui fault zone (Chile, 38-41°S), Tectonics, https://doi.org/10.1002/2014TC003622, 2014.

Hernandez-Moreno, C., Speranza, F., & Di Chiara, A.: Paleomagnetic rotation pattern of the southern Chile fore-arc sliver (38°S-42°S): A new tool to evaluate plate locking along subduction zones. Journal of Geophysical Research: Solid Earth, 121(2), https://doi.org/10.1002/2015JB012382, 2016.

Di Chiara, A., Moncinhatto, T., Hernandez Moreno, C., Pavón-Carrasco, F. J., & Trindade, R. I. F.: Paleomagnetic study of an historical lava flow from the Llaima volcano, Chile. Journal of South American Earth Sciences, 77, https://doi.org/10.1016/j.jsames.2017.04.014, 2017.

 

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submittheir photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.