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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/.

Is it an earthquake, a nuclear test or a hurricane? How seismometers help us understand the world we live in

Is it an earthquake, a nuclear test or a hurricane? How seismometers help us understand the world we live in

Although traditionally used to study earthquakes, like today’s M 8.1 in Mexico,  seismometers have now become so sophisticated they are able to detect the slightest ground movements; whether they come from deep within the bowels of the planet or are triggered by events at the surface. But how, exactly, do earthquake scientists decipher the signals picked up by seismometers across the world? And more importantly, how do they know whether they are caused by an earthquake, nuclear test or a hurricane?  

To find out we asked Neil Wilkins (a PhD student at the University of Bristol) and Stephen Hicks (a seismologist at the University of Southampton) to share some insights with our readers.


Seismometers are highly sensitive and they are able to detect a magnitude 5 earthquake occurring on the other side of the planet. Also, most seismic monitoring stations have sensors located within a couple of meters of the ground surface, so they can be fairly susceptible to vibrations at the surface. Seismologists can “spy” on any noise source, from cows moving in a nearby field to passing trucks and trains.

A nuclear test

On Sunday the 3rd of September, North Korea issued a statement announcing it had successfully tested an underground hydrogen bomb. The blast was confirmed by seismometers across the globe. The U.S.  Geological Survey registered a 6.3 magnitude tremor, located at the Punggye-ri underground test site, in the northwest of the country. South Korea’s Meteorological Administration’s earthquake and volcano center also detected what is thought to be North Korea’s strongest test to date.

However they occur, explosions produce ground vibrations capable of being detected by seismic sensors. Mining and quarry blasts appear frequently at nearby seismic monitoring stations. In the case of nuclear explosions, the vibrations can be so large that the seismic waves they produce can be picked up all over the world, as in the case of this latest test.

It was realised quite early in the development of nuclear weapons that seismology could be used to detect such tests. In fact, the need to have reliable seismic data for monitoring underground nuclear explosions led in part to the development of the Worldwide Standardized Seismograph Network in the 1960s, the first of its kind.

Today, more than 150 seismic stations are operating as part of the International Monitoring System (IMS) to detect nuclear tests in breach of the Comprehensive Test-Ban Treaty (CTBT), which opened for signatures in 1996. The IMS also incorporates other technologies, including infrasound, hydroacoustics and radionuclide monitoring.

The key to determining whether a seismic signal is from an explosion or an earthquake lies in the nature of the waves that are present. There are three kinds of seismic wave seismologists can detect. The fastest, called Primary (P) waves, cause ground vibrations in the same direction that they travel, similar to sound waves in the air. Secondary (S) waves cause shaking in a perpendicular direction. Both P and S waves travel deep through the Earth and are known collectively as body waves. In contrast, the third type of seismic waves are known as surface waves, because they are trapped close to the surface of the Earth. In an earthquake, it is normally surface waves that cause the most ground shaking.

In an explosion, most of the seismic energy is released outwards as the explosive material rapidly expands. This means that the largest signal in the seismogram comes as P waves. Explosions therefore have a distinctive shape in the seismic data when compared with an earthquake, where we expect S and surface waves to have higher amplitude.

Forensic seismologists can therefore make measurements of the seismic data to determine whether there was an explosion. An extra indication that a nuclear test occurred can also be revealed by measuring the depth of the source of the waves, as it would not be possible to place a nuclear device deeper than around 10 km below the surface.

Yet while seismic data can tell us that there has been an explosion, there is nothing that can directly identify that explosion as being nuclear. Instead, the IMS relies on the detection of radioactive gases that can leak from the test site for final confirmation of what kind of bomb was used.

The figure shows (at the bottom) the seismic recording of the latest test in North Korea made at NORSAR’s station in Hedmark, Norway. The five upper traces show recordings at the same station for the five preceding tests, conducted by North Korea in 2006, 2009, 2013 and 2016 (two explosions in 2016). The 2017 test, is as can be seen from this figure, clearly the strongest so far. Credit: NORSAR.

When North Korea conducted a nuclear test in 2013, radioactive xenon was detected 55 days later, but this is not always possible. Any detection of such gases depends on whether or not a leak occurs in the first place, and how the gases are transported in the atmosphere.

Additionally, the seismic data cannot indicate the size of the nuclear device or whether it could be attached to a ballistic missile, as the North Korean government claims.

What seismology can give us is an idea of the size of the explosion by measuring the seismic magnitude. This is not straightforward, and depends on knowledge of exactly how deep the bomb was buried and the nature of the rock lying over the test site. However, by comparing the magnitude of this latest test with those from the previous five tests conducted in North Korea, we can see that this is a much larger explosion.

The Norwegian seismic observatory NORSAR has estimated a blast equivalent to 120 kilotons of TNT, six times larger than the atomic bomb dropped on Nagasaki in 1945, and consistent with the expected yield range of a hydrogen bomb.

Hurriquakes?

Nuclear tests are not the only hazard keeping our minds busy in the past few weeks. In the Atlantic, Hurricanes Harvey, Irma and Katia have wreaked havoc in the southern U.S.A, Mexico and the Caribbean.

Hurricanes in the Atlantic can occur at any time between June and November. According to hurricane experts, we are at the peak of the season. It is not uncommon for storms to form in rapid succession between August, September and October.

The National Hurricane Centre (NHC) is the de facto regional authority for producing hurricane forecasts and issuing alerts in the Atlantic and eastern Pacific. For their forecasts, meteorologists use a combination of on the ground weather sensors (e.g. wind, pressure, Doppler radar) and satellite data.

As hurricane Irma tore its way across the Atlantic, gaining strength and approaching the Caribbean island of Guadeloupe, local seismometers detected its signature, sending the global press into a frenzy. It may come as a slight surprise to some people that storms and hurricanes also show on seismometers.

However, a seismometer detecting an approaching hurricane is not actually that astonishing. There is no evidence to suggest that hurricanes directly cause earthquakes, so what signals can we detect from a hurricane? Rather than “signals”, seismologists tend to refer to this kind of seismic energy as “noise” as it thwarts our ability to see what we’re normally looking out for – earthquakes.

The seismic noise from a storm doesn’t look like distinct “pings” that we would see with an earthquake. What we see are fairly low-pitched “hums” that gradually get louder in the days and hours preceding the arrival of a storm. As the storm gets closer to the sensor, these hums turn into slightly higher-pitched “rustling”. This seismic energy then wanes as the hurricane drifts away. We saw this effect clearly for Hurricane Irma with recordings from a seismometer on the island of Guadeloupe.

What causes these hums and rustles? If you look at the frequency content of seismic data from any monitoring station around the globe, noise levels light up at frequencies of ~0.2 Hz (5 s period). We call these hums “microseism”. Microseism is caused by persistent seismic waves unrelated to earthquakes, and it occurs over huge areas of the planet.  One of the strongest sources of microseism is caused by ocean waves and swell. During a hurricane, swell increases and ocean waves become more energetic, eventually crashing into coastlines, transferring seismic energy into the ground. This effect is more obvious on islands as they are surrounded by water.

As the hurricane gets closer to the island, wind speeds dramatically increase and may dwarf the noise level of the longer period microseism. Wind rattles trees, telegraph poles, and the surface itself, transferring seismic energy into the ground and moving the sensitive mass inside the seismometer. This effect causes higher-pitched “rustles” as the centre of the storm approaches. Gusts of wind can also generate pressure changes inside the seismometer installation and within the seismometer itself, generating longer period fluctuations.

During Hurricane Irma, a seismic monitoring station located in the Dutch territory of St. Maarten clearly recorded the approach of the storm, leading to an intense crescendo as the eyewall crossed the area. As the centre of the eye passed over, the seismometer seems to have recorded a slightly lower noise level. This observation could be due to the calmer conditions and lower pressure within the eye. The station went down shortly after, probably from a power outage or loss in telemetry which provides the data in real-time.

Seismometers measuring storms is not a new observation. Recently, Hurricane Harvey shook up seismometers located in southern Texas. Even in the UK, the approach of winter storms across the Atlantic causes much higher levels of microseism.

It would be difficult to use seismometer recordings to help forecast a hurricane – the recordings really depend on how close the sensor is to the coast and how exposed the site is to wind. In the event of outside surface wind and pressure sensors being damaged by the storm, protected seismometers below the ground could possibly prove useful in delineating the rough location of the hurricane eye, assuming they maintain power and keep sending real-time data.

At least several seismic monitoring stations in the northern Antilles region were put out of action by the effects of the Hurricane. Given the total devastation on some islands, it is likely that it will take at least several months to bring these stations back online. The Lesser Antilles are a very tectonically active and complex part of Earth; bringing these sensors back into operation will be crucial to earthquake and volcano hazard monitoring in the region.

By Neil Wilkins (PhD student at the University of Bristol) and Steven Hicks (a seismologist at the University of Southampton)

References and further reading

GeoSciences Column: Can seismic signals help understand landslides and rockfalls?

NORSAR Press Release: Large nuclear test in North Korea on 3 September 2017

The Comprehensive Nuclear-Test-Ban Organization Press Release: CTBTO Executive Secretary Lassina Zerbo on the unusual seismic event detected in the Democratic People’s Republic of Korea

First Harvey, Then Irma and Jose. Why? It’s the Season (The New York Times)

NOAA  National Hurricane Center

IRIS education and outreach series: How does a seismometer work?