Friday 25 April 2014

The Robben Island ‘Earthquake’ of 7 April 1620.

Most accounts of the history of seismology in South Africa relate the earliest recorded Earthquake in the country as having occurred on 7 April 1620, as recorded by the French explorer Augustin de Beaulieu, who was becalmed near Robben Island, prior to reaching Table Bay on 15 April. De Beaulieu recorded hearing a pair of ‘startling thunderclaps like cannon shots’, which have subsequently been interpreted as evidence of an Earthquake with a Magnitude of about 4 somewhere in the Western Cape.

Robben Island, to the northwest of Cape Town. Google Maps.

In a paper published in the South African Journal of Science on 13 September 2012, Sharad Master of the Economic Geology Research Institute at the School of Geosciences at the University of the Witwatersrand disputes this interpretation of the events of 7 April 1620.

Masters observes that Earthquakes recorded in the Cape Town area have been reported by witnesses as subterranean rumbling accompanied by the sound of distant thunder, but notes that only the thunder was recorded by de Beaulieu, who, being on a ship at sea, was in a poor location to make accurate recordings of seismic activity. He further notes that de Beaulieu never claimed that this thunder was evidence of an Earthquake, rather this interpretation was made based upon de Beaulieu’s record by J Theron in 1974 in a paper published in the Seismological Series of the Geological Survey of South Africa, which seems to have been widely accepted without criticism to date.

Masters observes that the most likely explanation for the sounds is that they were thunderclaps, just as recorded, and that nothing can be inferred from de Beaulieu’s failure to observe the accompanying lightning. He further notes that there are other phenomena that could potentially cause such sounds, such as distant landslides or sonic booms generated by meteorites, and while these are not common occurrences, neither are large Earthquakes in the Western Cape, and that it is not therefore sensible to jump to the conclusion that the noises of 7 April 1690 were the result of an Earthquake. 

Augustin de Beaulieu’s ‘Fleet of Montmorency’, which put into Table Bay in 1620. From an anonymous 17th-century French engraving. Wikimedia Commons/Masters (2012).

Based upon this conclusion Masters argues that the earliest recorded Earthquake in South Africa is the 1690 event recorded in Cape Town, where actual Earth tremors were recorded by a number of witnesses.

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