SENAT
Report n° 117 (2007-2008) by M. Roland COURTEAU, Senator (for the parliament office for the evaluation of scientific and technological choices) - Appendix to the minutes of the 7 December 2007 session
Disponible au format Acrobat (21 Moctets)
(2) Landslides
The Bureau de Recherches Géologiques et Minières (BRGM) defines a landslide as the shearing off and displacement of a mass of loose or rocky ground along a rupture area, often corresponding to a pre-existing discontinuity. Landslides are provoked by a combination of factors, that can be:
- either permanent in other words, only slightly or not at all variable over time (the nature and mechanical properties of the materials, the presence of preferred fracture planes, the ground slope, etc.);
- or semi-permanent in other words, variable over time (the water content of the materials, erosion at the bottom of the slope, anthropic activity, earthquakes, the collapse of a volcano, etc.).
Landslides have caused numerous tsunamis.
On 16 December 1979, a section of the Nice Airport embankment fell into the sea. A few minutes later, after a relative lowering of the sea level, a tsunami submerged the coast and a wave with an estimated height of between 2.5 and 3.5 metres hit the beach of La Salis in Antibes.
There is little consensus over the supposed causes of this tsunami. Two competing arguments point the finger at either 1) work being carried out to expand the Nice Airport or 2) the natural geological instability of the coast aggravated by the heavy rains which fell on the region a few days prior to the incident.
The most recent studies call attention to two landslides: the first landslide (with a volume of 10 million m 3 ) consisted of the collapsed airport embankment, while the second and much larger landslide (150 million m 3 ) would have taken place off the shore of Nice.
What's more, on 13 September 1999, the collapse of a section of basalt cliff (between 2 and 5 million m 3 ) on the island of Fatu Hiva in the Marquesas Islands provoked a tsunami that struck the village of Omoa.
Landslides can also be caused by collapsing volcanoes. On 30 December 2002, on the island of Stromboli, two landslides of several million m 3 of rock blocks and ashes carried away the volcanic emissions that had accumulated since 28 December. Following a retreat of the sea, waves several metres high struck the island, injuring 6 persons and causing material damage.
While tsunamis caused by landslides can be highly destructive, they are geographically limited. Indeed, while the vertical deformation can exceed several dozen metres, the horizontal dimensions (generally a few hundred metres) rarely exceed a dozen kilometres; for this reason, the resulting waves have relatively short wavelengths.
(3) Volcanic explosions
Volcanic explosions can also produce tsunamis, by suddenly sending an immense volume of rock into the sea.
The volcanic eruption on Santorini around 1650 B.C. generated a devastating tsunami that some argue led to the demise of the Minoan civilization. The tsunami is thought to have struck the eastern Mediterranean coasts with waves estimated at around forty metres high in the vicinity of the island.
Likewise, the volcanic explosion of Krakatoa, in Indonesia, on 27 August 1883 generated waves 41 metres high which destroyed villages located along the Sunda Straight between the islands of Java and Sumatra, claiming some 36,000 victims.