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    Earth-Shattering Events
    By Douglas Blane | March 2nd 2007 04:40 PM | 1 comment | Print | E-mail | Track Comments
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    Research physicist originally, working on nuclear reactors, autonomous underwater vehicles, smart materials etc. Now journalist, writer and teacher...

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    Snail-racing is an action-packed spectator sport compared to watching the drift of Earth’s continents, which usually move just a few centimetres a year – about as fast as fingernails grow. But occasionally events quicken dramatically, and become interesting enough for a "desk-bound geophysicist" like Dr Tim Wright to pack his bags and go camel-trekking across a desert.

    "What’s happening in the Afar region of northern Ethiopia is unprecedented," says the University of Leeds researcher. "A 60-kilometre long rift opened up there in just a couple of weeks. In fact some of the faults that slipped at the surface moved overnight. Things are happening incredibly fast there in geological terms."

    As the only place on the planet where continents are being ripped apart at the boundary between tectonic plates, the Afar desert has presented a multi-national team of scientists, alerted by experts at Addis Ababa University, with a wonderful opportunity to explore the science of the Earth’s surface.

    "There was a similar sequence in Iceland in the 1970s at a place called Krafla on the mid-Atlantic ridge," says Wright. "But that was much smaller. The amount of opening that took place in 10 years there was similar to what we saw over 10 days in Ethiopia.

    "This is the first time we have been able to use modern instruments – broadband seismometers and satellite techniques like radar interferometry – to study such an event. The data show that the ground is still moving very rapidly."

    While the broad picture of the Earth’s surface as a mosaic of tectonic plates, which violently collide, rip apart or slide past each other, was painted in the 1960s, much of the detail remains unsketched, says Wright.

    "We do not understand the physics of the processes going on in the crust and the mantle. What we are seeing in Ethiopia is the last stage of a continent splitting apart. The big question is how that happens and what is the role of magma – molten rock.

    "We believe Afar is sitting on a hot-spot, where the mantle beneath the crust is upwelling. We know from simple calculations that magma helps break the crust apart. But we don’t understand where that magma forms, how it moves through the crust, what its composition is, how it changes, or how it gets into the crack between the plates.

    "Nor do we understand the bigger picture. Is what we see here typical? Should we always expect rifting to take place with these very dramatic events every 500 years or so?"

    As principal investigator on a five-year, multi-national research study, Tim Wright is now aiming to answer as many of these questions as possible. "We’re putting seismometers around the whole region. These will let us look at seismic waves coming through it from all over the Earth."

    Seismic waves travel at speeds that depend on temperature – the cooler the medium the faster they move through it. So times and directions of arrival from known sources will let the researchers gradually build up a detailed 3-dimensional model of the region, down to a depth of about 200 kilometres, says Wright.

    "Essentially it is the same technique that doctors use when they want to build up a picture of the inside of your body – computer tomography. But we are using seismic waves instead of X-rays."

    Besides exploring below the surface with seismometers, the researchers will also map the surface itself from space, using a conceptually simple but highly accurate technique called radar interferometry.

    "By imaging the ground with satellite radar at different times, and looking at the phase difference of the reflected waves, we can work out the changing distance between satellite and ground to a precision of a few millimetres. It really is an extraordinary technique.

    "This is the first of these large events since the technique was developed. So we expect to learn a great deal. It is very exciting."

    Radar interferometry is normally used to measure the strains and work out the stresses that build up between earthquakes at particular locations, says Wright. "A good analogy is a brick pulled by an elastic band. This stretches for a time until the brick suddenly slides – that’s the earthquake.

    "So for years now I’ve been using these very small signals, in places like Tibet and Turkey, to measure how the ground deforms during and between earthquakes."

    The complexity of the physical processes associated with tectonic plate motion, together with chaos in the dynamics and a severe lack of data about what Earth’s surface has done in the past, means the ability to predict the occurrence of earthquakes will not be acquired any time soon.

    But detailed information about stress and strain on the surface of the Earth does have important practical applications: "We can see where stresses are building up fastest, and where earthquakes are most likely to happen in the next few years. So the government of a country like Iran can use that information to decide where to spend its money to minimise the damage.

    "Designs and materials to make schools, hospitals and houses safe from earthquakes are available now, and are not all that expensive – maybe 10% more than the usual cost. It’s not earthquakes that kill people. It’s buildings."

    Earthquakes are not the only danger faced by the international team of scientists in northeast Ethiopia. Afdera, one of the few towns in the sparsely-populated Danakil depression – where the cataclysmic events are taking place – is the hottest continuously occupied place on the planet, with a summer temperature that can reach 60° Celsius.

    Even more worrying is that not long after Dr Wright spoke to us about his research two groups of tourists were reported missing, believed kidnapped, in the Afar region.

    The Earth has exposed the machinery that drives its most violent surface processes in a barren, hazardous and inaccessible location. But it is likely nonetheless to yield many secrets in the next few years to modern instruments, international cooperation and good old scientific curiosity.

    "I really want to understand what is happening out there," says Tim Wright.

    Photos from Afar  











      Figure 1. Topographic relief of the 60 km-long Dabbahu rift segment within the Afar Depression. Inset shows directions of plate divergence between the stable African (Nubian), Arabian, and Somalian plates. Cynthia Ebinger, Royal Holloway University of LondonFigure 2. 3D view of satellite radar measurements of how the ground moved in September 2005. Over about 3 weeks, the crust on either side of the rift moved apart by as much as 6 metres, with molten rock filling the crack between the plates. Satellite radar data is from the European Space Agency's Envisat satellite. Figure was prepared by Tim Wright, University of Leeds using Google Earth. Images can be viewed in Google Earth by following the instructions here.  
      Enhanced Landsat Thematic Mapper image of the Dabbahu rift segment prior to the September 2005 events. These satellite images have been enhanced to show subtle differences in rock type invisible to the naked eye. (Bands 7, 4, 1, decorrelation stretch). Ellen Wolfenden, Royal Holloway, University of London. Photo looking N of the explosive vent that opened on September 26 after two days of nearly continuous seismic activity. To the right of the ~60 m-wide vent lies a 200 m-wide, 4 km-long zone of open fissures and normal faults that may mark the subsurface location of the dyke. The fault zone continues to the top of the photo to the right of the small rhyolite centre. Photo Elizabeth Baker, Royal Holloway, University of London.  
      Aerial photograph looking NW toward the September 26 volcanic vent. The area of buff-coloured material is some of the volcanic ash deposited after the volcanic eruption. Note the open fissures both to the north and south of the vent. Additional cracks are located to the east (right). Photo Elizabeth Baker, Royal Holloway, University of London. 3 m of movement in the September-October episode (Figure 10). The faults displace basaltic lavas (dark rocks) and small pockets of windblown ash and dust (white rocks). Photo by Cindy Ebinger, Royal Holloway, University of London." width="185" height="139">  
      Aerial photo of 0.5 m-wide cracks and faults that formed in September, 2005. These cracks formed above the zone where molten rock rose into the plate, reaching to within approximately 2 kilometres of the surface. Photo by Julie Rowland, University of Auckland. Volcanic vent that opened September 26, 2006. The vent was about 500 m long. View to the south from the north end of the vent - notice the tunnel at the southern end. Notice the layers of ash that built up over a periods of days around the vent. The rhyolitic rocks in the foreground were blown out of the vent. Photo by Julie Rowland, University of Auckland.  

     


    Comments

    Cash
    That thematic mapper pic is amazing. 60 KM in a few weeks ... it's like something out of a science fiction movie.