The Mountain That Jumped
The GPS station installed at the top of Brown Mountain last summer recorded that the mountain "jumped" 7mm during a recent nearby seismic event. The GPS station was originally installed to measure land uplift and tectonic plate movements. The recording of the effect of nearby earthquakes was not one of the expected outcomes within the first year of the installation.
Nevertheless, measurements during seismic activity along the South and North Scotia Ridge Transforms on the 17th and 24th November 2013, respectively, were recorded. The large earthquake of the 17th had an epicentre to the west of the South Orkney Islands (60.27°S, 46.40°W) along the South Scotia Ridge Transform, the most active section of the entire plate boundary of Antarctica. The resulting seismic waves travelling through the Earth's crust shook Brown Mountain displacing it by 7mm before settling back to its normal position. Although this is sufficient for people to feel the earth shaking, no one at KEP recollected being aware of it, possibly because it occurred at night. Much closer to the epicentre, the Argentine science station at Orcadas in the South Orkney Islands was seriously shaken and permanently displaced by 60cm towards East, 20cm towards South and 0.3cm up!
The GPS station on Brown Mountain is linked to the tide gauge fixed to the jetty at King Edward Point (KEP) and the data shows that a small tsunami wave followed the earthquake. The tsunami arrived at KEP at 8:34, around two and a half hours after the earthquake, reaching a maximum wave height of 15 cm. Since the very large earthquakes in recent years (Indonesia, Chile, Japan), it is being realized that the analysis of readings from a network of GPS stations recording data every second can lead to a better estimation of the magnitude of these seismic events than from data of the specialist seismic stations. A quick and reliable estimation of the magnitude is important for the very large events that trigger tsunamis. That said, GPS will not replace the specialist seismic instruments which are far more sensitive and can detect even the slightest tremors; there is a seismic station on KEP too.
The GPS data is also being used to analyse the effect of greenhouse gases. To enable the position of the mountain to be measured to the millimetre, the GPS signals need to be processed very carefully; en-route the signal travels through the Earth's atmosphere but the signal is affected by water vapour in the atmosphere. Water vapour is a potent greenhouse gas; as the atmosphere warms it can hold more water vapour which increases global warming. This effect of the water vapour on the GPS signal is measureable, giving scientists another powerful tool to monitor climate change and any resultant global warming.
During their visit in March the two scientists, Dr Norman Teferle and Dr Addisu Hunegnaw of the University of Luxembourg, which is funding the project, installed a further GPS station on KEP. The combined measurements recorded by the two stations will now enable more accurate and comparative results to be obtained. The first year results from the station on Brown Mountain indicate that Brown Mountain is steadily moving north and possibly subsiding, though the movements are only at the millimetre level and much more data is needed to be sure. With another station now installed they will be able to see if both stations are recording the same land motion or not. It will be possible, for instance, to see if there are changes within the gravel spit that forms KEP or if there is subsidence of the jetty due perhaps to heavy traffic use. The latter affects the readings of the tide gauge measuring sea level.
The GPS stations have been installed because relatively little is known about the motion of the Scotia Sea Plate, and to test a theory that South Georgia could be a micro-continent. What is known is that relative to the Scotia Plate, the South American Plate is moving westward (8-9 mm/yr), and the Antarctic Plate is moving eastward (6-7 mm/yr). The terminus of the Scotia plate is marked by the East Scotia Rift, separating the Scotia Plate from the South Sandwich Plate upon which the chain of seismically active volcanic islands - the South Sandwich Islands - sits. To better cover the area Dr Norman Teferle hopes to return and install other GPS stations in the region.
Other work done by the visiting scientists during their stay included reactivating a GPS station on the slopes of Mt Duse and installing a tide board on the KEP jetty. In another side-use of the GPS data, the GPS signals sent down from the satellites are partly bounced off the sea surface before being received by the GPS station on KEP. Under certain circumstances this bounce is also measurable and can be used to record the sea level, giving a neat check to the tide gauge data.
http://www.sgisland.gs/index.php/%28h%29South_Georgia_News_and_Events
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