Why Study Seismic Activity in Antarctica?
Who would want to install a series of seismometers, GPS stations, and gravimeters in Antarctica and for what purpose? Denis Lombardi from the Royal Observatory of Belgium explains the objectives of the GIANT-LISSA research project and gives an update on the latest activities he’s been involved while stationed at Princess Elisabeth Antarctica during the 2013-14 field season.
You’ve been installing a series of seismometers in different locations in the Sør Rondane Mountains this season. Why is it so important to set these kinds of instruments up in East Antarctica, one of the most remote places on Earth?
The overall objective of the GIANT-LISSA project is to better understand the long-term behaviour of the Antarctic continent in terms of climate change. We’re trying to determine to what extent the bedrock of the continent of Antarctica is rebounding due to the possible loss of the overlying ice sheet as a consequence of present global warming, or wether the rebound is a continued effect of former ice loss from the last ice age.
We’re investigating the different properties of the continent lithosphere and its dynamics by combining measurements from gravimeters, GPS stations and seismometers. The Earth lithosphere is composed of the crust and upper-mantle layers, is about 200 km thick and represents the most external and rigid layer of our planet.
Firstly, we’re taking gravimetry measurements done by my colleague, Olivier Francis, from University of Luxembourg to better understand how the ice mass surrounding a specific point is changing.
We combine this information with GPS measurements, taken by Nicolas Bergeot, from the Royal Observatory of Belgium, which record the three-dimensional displacement of the continent of Antarctica over time. This tells us about horizontal displacement and uplift of the continent as ice mass changes.
And lastly, we add measurements from seismometers, which provide information about the physical and mechanical properties of the lithosphere.
So you’ve been busy installing seismometers this season in order to add to the data collection needed for the project?
For me, as I work on the seismology part of the project, with the help of Alain and the Princess Elisabeth team, I’ve been quite busy installing five surface seismometers this season along a 100 km transect in the western Sør Rondane Mountains, in the vicinity of the Princess Elisabeth Antarctica station. This was our first objective this season.
Our second objective is to re-install a seismometer that had been placed at the bottom of a 13 metres borehole near the scientific shelter that houses lots of instruments just north of the station. This seismometer is very important because it provides exceptionally good quality data due to the fact that is 13 metres deep. At that depth, most of the surface background noise, such as noise you get from the wind, is minimized. So I’m looking forward to installing this seismometer in the borehole.
What do you hope to measure with these instruments?
Firstly, we’d like to study the properties of the lithosphere (crust) of the Antarctic continent. More specifically, we’d like to study the tectonics of the Sør Rondane Mountain region by using information provided by seismometers; This region of East Antarctica near the Princess Elisabeth station is quite interesting geologically. Towards the interior of the continent, beneath the ice sheet lies the East Antarctic Craton, which is one of the oldest pieces of the Earth’s lithosphere that hasn’t been modified by any tectonic activity since its formation about 3 billions years ago.
And then on the other side, more towards the coast, you have remnants of the Mozambique Belt, which makes up most of the eastern part of Africa today. About 500 million years ago, when the supercontinent Gondwana was forming, the south-eastern tip of Africa collided with the part of East Antarctica where the Sør Rondane Mountains are now located. This is when the Sør Rondane Mountains formed. When Gondwana began to break up about 180,000 million years ago, part of the Mozanbique Belt stayed behind, attached to the East Antarctica Craton. So basically the Sør Rondane Mountains lie along the old suture zone between Africa and East Antarctica.
As East Antarctica had a central position in the supercontinent Gondwana, which was a key period in the evolution of the Earth plate tectonics, it’s important to study the tectonics of Antarctica. The seismometers we’ve installed will provide an image of the present state of the Antarctica tectonic plate.
How can you take an “image” with seismic readings?
Most of what we know about the the Earth’s interior comes from seismology. Earthquakes produce lots of energy. An earthquake that registers about 6 on the Richter Scale releases the same amount of energy as an atomic bomb (but it’s not radioactive of course)! All this energy is transmitted through the Earth via seismic waves. For a small earthquake, the waves attenuate rapidly, after only a few hundred kilometres from the epicentre. But for a big earthquake, the waves travel through the Earth’s crust, mantle and core, and they can be detected by seismometers all around the planet.
By looking at the different types of waves, their velocity and their amplitude, a seismologist can tell a lot of information about the materials they travel through: rock, magma, etc. This is because waves travelling through different types of rock show different seismic signatures. This allows you to get an “image” of what the Earth’s interior look like.
Across the planet, seismologists from around the world have installed about thousands of seismometers, mostly on continents (as it’s more difficult to set up seismometers on the sea floor). Apart from Antarctica, there’s good global coverage of seismometers. If you combine all data from all seismometers around the world, you can get a detailed image of where the transition is between the core and the mantle, or where one tectonic plate is subducting beneath another.
This is the general idea of imaging the Earth with seismology. In the GIANT-LISSA project, we want to do a similar analysis at more local scale to provide an image of the tectonics of this part of East Antarctica.
And this information can give you an idea of how the tectonic plates have moved in the past?
Well, seismologists are able to provide images of the present configuration of tectonic plates at a certain depth. Then these images are analyzed by geologist and tectonicists who, by adding their own information, are able reconstruct the tectonic plate history. This is how science progresses - It’s a common effort!
What else to do use the seismic readings for?
We also use them simply to keep track of the local seismic activity in this part of East Antarctica. However Antarctica is relatively quiet seismically speaking. It’s not tectonically active like Japan or Indonesia, for example.
More interestingly, we also keep track of the seismic activity related to the flow of the ice sheet. The ice sheet is 2 km thick in that part of East Antarctica, and it flows towards the coast at a rate of 350 metres a year, which creates friction against the underlying bedrock of the continent. In some areas, the friction can be more intense due to the subsurface topography of the bedrock. And by monitoring such a seismic activity, we can get a better understanding of the dynamics of the ice sheet. Long-term monitoring the variation of seismic activity of the ice sheet will help determine how stable the East Antarctic Ice Sheet in light of climate change.
Our research is one of the few research projects looking at seismic activity of the ice sheet in East Antarctica.
Where exactly did you install the seismometers?
All but one of the seismometers we installed this season were installed on solid rock in the western Sør Rondane Mountains.
We installed one seismometer in a hole about one metre deep, which we dug into the blue ice on the plateau a bit further inland.
Will the instrument still work buried beneath that kind of ice?
All seismometers have a protective insulation cover to handle harsh winter conditions they will face.
Do you need to service the seismometers regularly once you’ve installed them?
All of the seismometers are autonomous. They run on solar panels and lithium batteries. In summer, they rely more on solar power, and switch to using the lithium batteries in winter when the sun stays below the horizon for three months (May-August). It’s a quite a challenge to run such an instrument year-round.
They usually only need to be serviced once a year, so each season when one of us in the project goes to Antarctica, we do any necessary maintenance on the seismometers and our other instruments.
What kind of logistical support was needed to get the job done?
We needed help in doing reconnaissance to find ideal sites prior to bring along and install 200 kg of equipment We had to find areas of solid rock easily accessible by skidoo.
Alain was a big help in the reconnaissance efforts as he knows the region extremely well. He knows where to go and where not to go. As you can imagine the terrain around us is quite hazardous. But in the end, we found good sites I think.
How did you manage to get the equipment to the sites?
Like I said, we had 200 kg of equipment to set up. We’d arrive at a site with skidoos and sledges carrying the seismometer along with the batteries, solar panels and electronics needed to make it work. Heading into the mountains, we had to pass through nunataks and windscoops. It wasn’t easy carrying all that equipment with us!
When we went up to the plateau to install two seismometers, we left the Princess Elisabeth base for two days. We had to take a Prinoth tractor that brought a mobile field unit behind it, which allowed us to bring all the equipment we needed with us so we didn’t have to make a return journey to the station in the meantime.
Did the Princess Elisabeth team help you to set up all the equipment?
Yes, of course. I considered this as team work not just some extra help! The other team members were especially needed when it came to carrying the solar panels and drilling anchor points to fix the equipment to the ground. Given the weather was a bit stormy at times, we appreciated having the help to make the process go faster.
Alain and at least one other team member was there each time we went out, and many times there were even four or five people in the team to help us set up the equipment. Without such a good logistical support, we would not have been able to install all these seismometers under such difficult conditions.
Picture: Denis Lombardi preparing to install a seismometer during a field expedition. - © International Polar Foundation