Napoleon's Earthquake: The Seismicity of St. Helena

Abstract

To many it might seem an exercise in futility to attempt a study on the seismicity of a small island in the middle of the Atlantic, far from the nearest monitoring station and not in an area of high seismicity. To some extent this may be true, but the fact that archival records can be found for the island going back 300 years and that these do mention some earthquakes makes this an interesting task with results worthy of record. This is particularly so as one of the earthquakes took place in the context of an important historical event: the exile of Napoleon Bonaparte to the island. ### Geography, History, and Geology St. Helena is an island in the middle of the South Atlantic Ocean, located at 15.97°S, 5.72°W (see Figure 1). Nunn (1994) classifies it as an isolated intraplate island. It is approximately 800 km east of the Mid-Atlantic Ridge. The nearest other land is Ascension Island, 1,130 km to the northwest; the shortest distances to the nearest continental land masses are 1,930 km to Africa and 2,900 km to South America. The size of the island is about 121 km2, with a maximum dimension of 17.4 km. St. Helena, with a population in 1991 of 5,632, is therefore one of the most isolated inhabited places in the world. St. Helena is the deeply eroded remnant of an extinct northeast-southwest-trending volcanic complex rising from the oceanic floor at a depth of over 4,000 m to a present-day summit at Diana's Peak of 823 m above sea level. The base of this vast volcano has a maximum diameter of over 100 km, which, according to Daly (1927), gives it a base area of over ten times that of Etna and a volume of at least twenty times that of Etna. The undersea topography …