- © 2007 by the Seismological Society of America
The remote South Atlantic island of Tristan da Cunha (hereafter abbreviated to “Tristan”) is a large oceanic shield volcano rising more than 5,500 m from the sea floor to an altitude of 2,060 m above sea level. Tristan along with the islands Inaccessible and Nightingale and a number of smaller islets forms the Tristan da Cunha group (figure 1), which lies on a submarine plateau approximately 200 km east of the Mid-Atlantic Ridge near its junction with the Walvis Ridge (Morgan 1971; Fairhead and Wilson 2005). The group is believed to represent the surface volcanic expression of a mantle plume or hotspot. The most recent eruption on Tristan was in 1961–62 (Baker et al 1964) and was preceded by two months of tremors felt by the population.
Earthquakes along the Mid-Atlantic Ridge dominate the regional seismicity of the area (figure 1). Recently, Haxel and Dziak (2005) detected low magnitude (M ≤ 3) earthquakes attributed to submarine volcanic activity on the Walvis Ridge, about 800 km northwest of Tristan, through observations of T-phases recorded on hydrophones in 2002 and 2003. We know very little about the local seismicity of the Tristan group itself because of the large distances to the nearest seismometers. However, in March 2004 the Comprehensive Nuclear-Test-Ban Treaty Organization/International Monitoring System (CTBTO/IMS) installed two seismic stations on Tristan, approximately 5 km apart, to enable hydroacoustic wave monitoring through the detection of seismic T-phases for nuclear test-ban treaty verification purposes. The locations of these stations, H09W and H09N, are …