5. Landslides

Question 1

'’Classical’’ landslide structures from studies on land

Answer 1

Main scarp
head
minor scarp
toe
displaced mass
crown crack

Question 2

Retrogressive landslides e.g. Storegga Slide

Answer 2

Lower slope fails first, leabing mid-slope unsupported
It fails, leaving next block unsupported
Process continues until failure at head of landslide exposes the headwall

Question 3

PRACTICAL: evlauate % extension of a landslide from seismic

Answer 3

Question 4

How is the mobility of a landslide measured?

Answer 4

ratio: H/L (height of drop divided by length travelled)

Question 5

Landslide stability analysis

Answer 5

Question 6

What causes landslides?

Answer 6

The weakness of sediments seems to be more important than gradient
There is little relationship to seismicity
Landslides on the continental slope occur because of weaknesses between bed-paralle strata and are large because the continental slope is so large
Landslides on land are more limited - smaller slopes due to fluvial and glacial erosion
If temperature rises or sea-level falls solid hydrate (high P/low T CH4 combined with water) will dissociate into free gas, causing loss of sediment strength and potential landsliding
Rapid sedimentation to fast for pore waters in underlying sediment to drain -> overpressures
erosion along channels and canyon floors undermines slopes, leading to failure
steepening of slopes due to tectonics or diapirism

Question 7

Nice aiport failure

Answer 7

Possible causes of failure:
1. Loadin gby the airport contruction
2. Pore water over-pressure (rainwater on land guided by gravel layer under mud)
3. Reaction sof marine clays to fresh water

Question 8

Landslides causing tsunamis

Answer 8

landslides produce tsunamis because they incolve a rapid subsidence in their crowns and uplift in their toes, hence a coupl e applied to the overlying water
Wavelenght and period of tsunami is a function of the distance between the areas of uplift adn subsidence and the water depth

Question 9

How tsunamis propagate

Answer 9

wavelengths are large compared to the water depth so can be considered intermediate or shallow water waves depending on their periods
speeds are proportional to the square root of water depth
this means tsunamis are refracted by bathymetric features

Question 10

how waves behave in different depths

Answer 10

Deep water: particles move in circles and do not have any sense of the bed
Shallow water: particles move side to side as they are contrained by the bed
Intermediate: particles move in between the two extremes

Question 11

Recap main points

Answer 11