Tristan : Dr. Vicky Hards: A Scientific Assessment of Tristan's Recent Seismic Activity
Submitted by Tristan Times (Juanita Brock) 25.10.2004 (Current Article)
Dr. Vicky Hards has returned to the UK after spending approximately a month on Tristan da Cunha assessing the recent seismic activity that began 29/30 July 2004.
Photo (c) James Glass (Tristan Times) - A view of the 1961 volcano and Edinburgh of the Seven Seas from the base.
DR. VICKY HARDS: A SCIENTIFIC ASSESSMENT OF TRISTAN’S RECENT SEISMIC ACTIVITY
By J. Brock (SARTMA - TdC)
A view of the 1961 volcano and Edinburgh of the Seven Seas.
Dr. Vicky Hards has returned to the UK after spending approximately a month on Tristan da Cunha assessing the recent seismic activity that began 29/30 July 2004. Prior to leaving the Island, Dr Hards submitted a preliminary report to the people and spoke to the senior students at St. Mary’s School in Edinburgh of the Seven Seas. Copies of the report were left on the Public Notice Board and were made available to anyone on the Island that wanted one. Following is a complete summary of Dr. Hard’s preliminary report.
The report states that the tremors felt on 29/30th July and since were volcanic in origin. Nevertheless, Dr. Hards continues, a significant eruption is not imminent and there appears to be no immediate threat to the Settlement. Tristan itself and the neighbouring islands; Inaccessible and Nightingale (The Tristan da Cunha Group) is entirely volcanic in origin, representing the uppermost parts of large volcanoes, Tristan being the youngest and still active. The Tristan da Cunha group is accepted by scientists to represent the surface volcanic expression of a deep-seated hotspot.
There is no sign that the 1961 “volcano” is about to re-awaken. Even in recent years there were three sites at the top from which steam would emanate. These sites are now much cooler than when previously examined by my colleague - Peter Dunkley - in 2001, and simply what would be expected for a volcano cooling down 40+ years after the most recent eruption. James Glass and Dr. Hards went to the rim of the 1961 volcano in early September and found that the temperature was 35C cooler than when it was last measured in 2001.
People were concerned when the recent tremor episodes were felt most strongly in houses on the eastern side of the settlement – closest to the old “volcano”. Nevertheless, analysis of the data from the two earthquake monitoring stations (seismometers) installed by the UN indicates is consistent with the source of these earthquakes being c.25km away from the seismometers, the simplest explanation, that they were at depth below the volcano was first given, but it is now considered more likely that they were horizontally displaced to the southeast.
Why then, were tremors felts where they were? This appears to be due to the presence of two perpendicular structural axes along which the island was built up, the principle one lying NNW-SSE (N170°). Thus the earthquake energy is preferentially transmitted the along a plane through the centre of the island (i.e. from Stony Hill to the old (1961) volcano). Additionally local site conditions can make a big difference to whether an earthquake can be felt or not and very similar buildings may affected in different ways. Buildings on unconsolidated ground may shake more than those on solid rock.
Pale grey floating rock found at the beginning of August is pumice, a volcanic rock resulting from eruption of gas-rich lava. Old deposits are of similar material can be found to the SW of Hottentot Gulch, and can be seen as a creamy-coloured horizon near the base of the cliffs. It is most likely that this was erupted at some distance away from Tristan, to the SW, given where it was first found and the prevailing currents. The earthquakes of 29/30 July were the type expected from rocks breaking under pressure on the rock [below the seabed] from rising magma (what lava is referred to prior to its eruption at the surface), prior to its eruption as pumice, which then floated to the surface.
Earthquake activity in the region was ongoing at the time Dr. Hards wrote her report, although at too low a level for tremors to be felt and at reduced frequency in comparison with the past two months. Data from the UN seismometers on Tristan continue to be analysed on a daily basis by seismologists at the British Geological Survey. Prior to any significant volcanic activity, warning tremors of far greater intensity than those felt to date would be expected. Additionally scientists are able to differentiate different kinds of earthquake, which are indicative of different volcanic processes thus allowing a better understanding of the state of the volcano than has been possible in the past. It should therefore be possible to give ample warning of any resumption in volcanic activity on the island.
Given the nature of the Tristan volcano (the whole island) direct risks to the settlement from volcanic eruption are fairly localised – witness the size of the area actually affected by the 1961/2 eruption. Should an eruption take place, say on the far side of the island, the threat to the settlement would be limited to the threat posed by the accompanying tremors, which could cause damage to property and potentially disrupt power and water supplies, or cause large rock-falls from the cliffs behind the settlement.
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