|
Stig A. Schack Pedersen, Geological Survey of Denmark and Greenland (Denmark)
Trine Dahl-Jensen, Geological Survey of Denmark and Greenland (Denmark)
Lotte Melchior Larsen, Geological Survey of Denmark and Greenland (Denmark)
|
|
Natural hazards caused by landslides are mainly related to the destruction caused by the collapse of the ground itself, the damage due to the struck of the rock avalanches, or the secondary effect of the landslide damming streams with a subsequent flooding, when the dam collapses. Hazardous tsunamis produced by landslides are an additional type of geo-hazards, which, although rather infrequent, should be recognised and respected. The geological setting forming the basis for this type of geo-hazards is a steep fjord or strait with deep bottom topography incised a mountainous area.
The presented example of this setting is the landslide at Paatuut, a steep coastal area at the strait Vaigat in western Greenland. The strait is up to 500 m deep and the mountains on both sides are about 1.5 km high and constitute upper Cretaceous sediments overlain by Paleocene volcanics of the Nuussuaq Basin. The unconformity between the siliclastic sediments and the lava flows is situated about 1000 m above sea level. Up to this level the slope dips moderately to steeply, above it the basalts rise vertically up to 1400 m above sea level from where rock falls commonly occur. In November 2000 freeze and thaw in the early wintertime triggered a very large landslide. An avalanche of 90 million m3 of basaltic material slid down with a velocity of 140 km/h . At the toe of the slide an alluvial fan 30 million m3 in size was dumped out from the coast into the sea and turned into a submarine slide displaced out into the deepest part of the Vaigat strait.
The landslide was recorded as an earthquake with a magnitude of 2.3 at several seismic stations in West-Greenland and surrounding regions, which provided a precise time for the landslide event. In the village Saqqaq, 40 km from the landslide, a tsunami struck the coast and destroyed a number of boats, fortunately no persons were harmed. This event took place 10 minutes after the landslide, and regarding the 4-500meters depth of the strait the figures satisfy the equation of the velocity (υ)of channel waves, υ = gd, where g is the gravitational constant and d the water depth. This implies that the submarine slide was an event taken place immediately after the landslide.
Meanwhile, the scarcity of landslide records is too few for a risk assessment. The historical evidence states that a similar event will only occur two times in a 100 years period.
|
