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Imaggeo on Mondays: Shaken, not stirred – sediment shows signs of past earthquakes

23 Jun

Nore Praet, a PhD student from Ghent University in Belgium, brings us this week’s Imaggeo on Mondays. She sets the scene for an investigation into past earthquakes and explains how peering through a lake’s icy surface and its murky waters, and into the sediment below can help scientists find out more about the impact of earthquakes in the future…

Early this year, I set off with a group of scientists (including Koen De Rycker, Maarten Van Daele and Philipp Kempf) from the Renard Centre of Marine Geology to conduct fieldwork in South-Central Alaska. The reason for our stay was the search for past megathrust earthquakes (earthquakes produced by the subduction of an oceanic tectonic plate under a continental plate), which had an unusually high magnitude and destructive power.

The most recent Alaskan megathrust earthquake was the Great Alaskan Earthquake in 1964, which represents the second largest earthquake ever instrumentally recorded (9.2 on the moment magnitude scale). In order to have an estimate when such a large earthquake may strike again, we need to study the recurrence pattern of past earthquakes. Since megathrust earthquakes typically have recurrence periods of several centuries, historical archives will not suffice. This is where natural archives, like lake sediments, come in. These records have the advantage of going much further back in time, and they are what brought us to Alaska’s Eklutna Lake.

Zooming in on some individual ice crystal aggregates (few centimeters across) and geometric frost patterns on the frozen surface of Eklutna Lake in Southern Alaska. (Credit: Nore Praet via imaggeo.egu.eu)

Zooming in on individual ice crystal aggregates (few centimeters across) and geometric frost patterns on the frozen surface of Eklutna Lake, Alaska. (Credit: Nore Praet via imaggeo.egu.eu)

Lake sediments can contain very distinct earthquake traces because seismic shaking produces underwater landslides that leave well-defined sediment deposits in the lake basin.

Long sediment cores, extending some 15 metres through these deposits, make it possible to construct a palaeoseismic record. From this we can make an estimate of the recurrence rate of megathrust earthquakes. This will be crucial for understanding seismic hazard in South-Central Alaska and, in particular, in the densely populated city of Anchorage.

This photo, taken in early February, captured the moment where we officially started the fieldwork, after checking the thickness of the ice and its stability.

The constant struggle with the almighty Alaskan cryosphere was the real common theme during the fieldwork. The freezing cold, together with ice formation on the coring equipment, seriously hampered the efficiency of the coring operation. It took some time to accept these conditions and adjust to the demanding laws of this harsh wilderness, but once you are willing to invest the energy into working here, every day nature surprises you with her astonishing beauty.

By Nore Praet, Ghent University

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: Plate it up – a recipe for sea ice errors

16 Jun

Last week, a team of cryospheric scientists published a paper in The Cryosphere that showed how tiny plates of ice can lead to spurious estimates of sea ice thickness. This week, we’re featuring their findings, as well as some spectacular sea ice images in the latest in our Imaggeo on Mondays series…

Viewing the poles from above is a stunning sight – a seemingly endless expanse of brilliant white, ridged blue crests, and delicate grey fringes that stretch like lace over the ocean. Such a vantage point also allows scientists to get to grips with what’s happening to these delegate fringes, seeing how far the sea ice stretches from year to year and how its thickness changes over time.

One of the best ways to measure how thick a large expanse of sea ice is, is to measure its elevation, comparing it to the level of the surrounding water to see just how much is floating on the ocean. This can be done swiftly using satellites, which have the added bonus of keeping a continuous record of change over time. But recent research reveals there may be a problem with this technique.

Spying on sea ice from some 20,000 feet above the surface. (Credit: NASA)

Spying on sea ice from some 20,000 feet above the surface. (Credit: NASA)

Beneath sea ice you find a fine crystalline mush composed of thin ice crystals, or platelets. These platelets bridge the boundary between sea ice and the sea below. Because ice is buoyant, this icy mush (aka the sub-ice platelet layer) pushes the sheet of sea ice upwards, increasing its elevation. Small differences in the proportion of platelets below the ice could make it appear thicker than it really is, leading to inaccurate measures of sea ice thickness.

Looking out over Antarctic sea ice. (Credit: Andrew Chiverton via imaggeo.egu.eu)

Looking out over Antarctic sea ice. (Credit: Andrew Chiverton via imaggeo.egu.eu)

To know just how big a difference these platelets make, first you need to know how much solid ice is present in the mush. Using drill hole data collected in 2011, a team of scientists from New Zealand and Canada estimated that solid platelets made up about 16% of the mush beneath Antarctic sea ice. It may not sound much, but this many platelets could cause ice thickness to be overestimated by almost 20%.

You also need to know just how dense the platelets are. If they have a very low density, they can buoy the ice up more, and if they’re denser, they will have less of an affect on sea ice thickness. The findings mean a fair bit of ground-truthing will be needed to get better estimates of sea ice thickness from satellites in the future.

By Sara Mynott, EGU Communications Officer

Reference:

Price, D., Rack, W., Langhorne, P. J., Haas, C., Leonard, G., and Barnsdale, K.: The sub-ice platelet layer and its influence on freeboard to thickness conversion of Antarctic sea ice, The Cryosphere, 8, 1031-1039, 2014.

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: Soil and water conservation in the Dogon Plateau, Mali

10 Jun

Velio Coviello, a scientist from the Research Institute for Hydrogeological Protection, Italy, and one of the winners of the EGU 2014 Photo Contest, brings us this week’s Imaggeo on Mondays. He sheds light on his winning image and the problems associated with conserving soils and water in Western Africa… 

This picture was taken on Mali’s Dogon plateau during the dry season, in the course of a late sandstorm day. Between November and March, a hot, dust-laden Harmattan haze frequently persists over the whole of  Western Africa. The Harmattan is a hot, dry wind blowing from the Sahara, carrying large amounts of dust and transporting it for hundreds of kilometers. Here, we see two men drawing water from a deep and narrow well excavated by hand. This latter is a task commonly carried out by children, who climb down to dig the well bottom.

Men and children drawing water for irrigation in the Dogon plateau during a sandstorm. (Credit: Velio Coviello via imaggeo.egu.eu)

Men and children drawing water for irrigation during a sandstorm. (Credit: Velio Coviello via imaggeo.egu.eu)

Mali has a low population density, most settlements are concentrated in the southern part of the country and along the Niger River, where the climate is less harsh and water availability is higher. In the north, Mali is arid and only those who raise livestock can make a living.

One of the most important tourist attractions in Mali is the Dogon Plateau, which sits in the central part of the country, east of the Niger River. The plateau gently descends westward to the river valley and ends in abrupt cliffs on the southeast. These cliffs reach an elevation approaching 1,000 meters at Bandiagara, the main village of the Pays Dogon (Land of the Dogon). These geological, archaeological and ethnological interests, together with the striking landscape, make the Dogon Plateau one of West Africa’s most impressive sites.

Ensuring the population has safe and sustainable access to water is one of the major challenges in the Sahelian region. Facing recurring drought events and encroaching desertification, Sahelian countries are currently heavily affected by climate change. Extreme rainfall events and high rainfall intensity are the main cause of soil erosion and land degradation. Consequently, high rates of soil transport can lead to reservoir siltation and the reduction of water availability for agriculture. To cope with these issues, traditional soil and water conservation (SWC) measures like hillside terracing, permeable rock dams, stone lines, earth basins, planting pits and earth mounds have been regularly employed in the Sahelian area. The Dogon Plateau is home to a broad variety of these measures, implemented to deal with the acute shortage of soil and water. As the population urgently needs support to preserve soil fertility and reduce soil erosion, SWC measures need to be improved and adopted more widely. However, most donors fund short-term projects without considering the maintenance that is needed to ensure SWC measures remain effective long-term.

The first lesson is that there is much to learn from the traditional ways of doing things and SWC projects should always begin by looking at what the people are doing for themselves. Secondly, the international cooperation actors should set up long-term funding programs improving the participation and inclusion of local communities. The final goal would be to ensure the stakeholders are not permanently dependent on international aids.

by Velio Coviello, Research Institute for Hydrogeological Protection (IRPI) and Italian National Research Council (CNR)

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: Long-lived lakes have a lot to tell

2 Jun

The world’s oldest, deepest freshwater lake lies in southeast Siberia: Lake Baikal. Stretching some 600 kilometres across the Russian landscape, Baikal marks what the very early stages of a new ocean – an ancient rift that cleaved the centre of Asia apart throughout the Palaeozoic, Mesozoic and Cenozoic. Today, there are still signs of tectonic activity and the rift continues to diverge 4 mm further apart each year. Much like the East African Rift, Baikal provides scientists with a window into the way oceans are formed.

Where the Selenga River meets Lake Baikal. (Credit: Galina Shinkareva via imaggeo.egu.eu)

Where the Selenga River meets Lake Baikal. (Credit: Galina Shinkareva via imaggeo.egu.eu)

The Selenga River, which snakes across Asia for over 900 km, brings 30 cubic kilometres of water and more than 3.5 million tonnes of sediment to the basin each year, feeding a rich wetland ecosystem at the northern end of the lake. What’s more, the lake’s age and isolation has led to the establishment of a unique world of underwater flora and fauna – 70% of the lake’s inhabitants are found nowhere else in the world. This combination has earned the lake the nickname ‘The Galapagos of Russia’ and its designation as a World Heritage Site, as well as providing a prime site for evolutionary studies.

Not only that, but the lake is a key spot for climate science too. Baikal’s high latitude location means that it’s particularly sensitive to comate change, leading to numerous investigations into how climate has varied over the last 250,000 years. No doubt we have a lot more to learn from this incredible environment.

By Sara Mynott, EGU Communications Officer

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

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