15 July 2015
The Judgement Cliff Rock Avalanche in Jamaica
At 11:43 am on 7th June 1692 an Mw=7.5 earthquake struck the town of Port Royal in Jamaica, killing an estimated 2000 people. The greatest impacts of the earthquake occurred in the town of Port Royal itself, in which there appears to have been a combination of a submarine landslide that removed 33 hectares of the town (this is sometimes interpreted as a liquefaction event, although this seems unlikely to me) and a tsunami (which could have been related to the landslide given the nature of the strike-slip faulting in Jamaica – I would not expect this earthquake to have been tsunamigenic in its own right).
Inland, the earthquake is associated with a large earthquake, known now at the Judgement Cliff Rock Avalanche. Strangely, it is a little unclear as to whether the landslide was triggered by the earthquake or by heavy rainfall a few days later (but it seems to me that a seismic origin is most likely given the depth of the shear surface). The landslide is very large – Maharaj (1994) provides a detailed description, estimating that it has a volume of between 131 and 181 million cubic metres, and extended over 440 metres vertically and 1763 metres horizontally. The landslide is very clear in contemporary photographs:
And on Google Earth images, even 320 years after the event:
Maharaj (1994) describes a landslide body consists of highly fragmented and disrupted limestone fragments and chert nodules, suggesting that the Judgement Cliff rock avalanche was very dynamic. The runout distance is quite large – the H/L (height / length) index is 0.25, implying a flow-type mechanism. Indeed as such this is an unusually mobile rock avalanche, comparable with for example, the Elm landslide in the Alps. As such this landslide would probably benefit from a more detailed investigation.
The Judgement Cliff Rock Avalanche was reported at the time to have killed at least 19 people. Thus, for example, a report compiled in 1809 on the basis of eye-witness accounts suggested that:
“a great mountain split and fell into the level land and covered several settlements and destroyed 19 white people” (Maharaj 1994, p. 243).
My interpretation is that the loss of life would have been rather higher, given that the account only appears to be concerned about the colonial settlers.
There is a nice account of the oral history of the landslide and the mythology that is associated with it in the Jamaica Observer from a few years ago.
Maharaj. R.J. 1994. The mprphology, geometry, and kinematics of Judgement Cliff Rock Avalanche, Blue Mountains, Jamaica, West Indies. Quarterly Journal of Engineering Geology, 27, 243-56.