Dave told us that a landslide occurs when a slope collapses. He showed us a picture of a huge landslide in New Zealand which left two cows and a calf marooned on an untouched meadow that was surrounded by landslide debris. Sometimes there is no obvious reason that one section of a slope collapses but other parts are stable.
There are different types of slide and different scales of slide.
In Alaska a mountain slide slid 15 km across a glacier, but in a cliff fall in Staithes a block about the size of a coffee cup hit a girl on the beach and killed her.
Probably the landslide that most people know about was the disaster at Aberfan in 1966.
The spoil tip, located above the town, collapsed and slid down into the school and killed 118 people in there. He said that this was a shameful event, because the spoil could have been taken away but the tip was situated where it was because that was the cheapest place to put it. The Coal Board did remove it later, but it took money raised from the public appeal for the town to pay for the removal.
Sometimes landslides have reached the sea and caused tsunamis and three massive slides in Brazil in 2019 killed 400 people as the slide front became chaotic.
There are five key processes involved in landslides.
1/ The role of water.
Friction is a key control. Water creates buoyancy and reduces friction.
2/ Liquefaction.
As earth materials deform they go from solid to fluid and the block picks up material and speeds up.
(Dave showed a picture of a car half-buried in a road in Alaska, 2018. He said that the embankment foundations liquefied as the slope slipped.)
3/ Rocks with defects are much weaker that those without defects.
Multiple horizontal defects or cracks are more likely to fail.
4/ Earthquakes in mountains generate many slope failures. Nepal has multiple slides.
5/ We don’t understand why the biggest slides travel so far and so fast.
The slides are frequent in springtime when snow and ice melts.
They are very chaotic, they slide far and then continue to creep. We don’t know why they become chaotic and break up in to much smaller particles and ’liquefy’.
These conditions cannot be simulated in the laboratory.
Big rock falls are not understood.
Landslides are often part of a highly complex chain of events.
A landslide in Sulawesi in 2018 was probably caused by an earthquake and possibly irrigation contributed.
In the north of India, as the continent travels northwards on the tectonic plate pushing up the Himalayas, earthquakes are caused. Dave showed satellite photos on which the fault line and fault ruptures were shown. The are stretched for over 200 km. These faults cause earthquake waves.
He showed photos of an area along this 200km fault line where he said 40 – 50% of the landscape had slipped. In one place the mountain had slipped and flowed down the valley. More that a cubic kilometre (i.e. 2.5 billion tons) of material blocked the valley, 500metres deep.
In 2007, in a panda conservation area in China, an earthquake and landslide caused great damage to Beichuan town. Dave showed a picture of a crane that had the arm that holds the counterweight bent upwards. The crane had been jolted as it dropped over 6.5 metres in the earthquake. The landslide and buried Beichuan Middle School, killing 700 people. The school was a primary and secondary school so all the school-age children in the town were in it at the time. As Chinese parents were allowed to have only one child this was a major catastrophe. 10 minutes later a whole hill slid down, bulldozing buildings and causing a massive loss of life
In 2005 in Kashmir a landslide killed 600 people. Two women, who were cutting grass at the top of the slope, were on an unbroken mass of material which slid down. The women ‘surfed’ over 2.5 km on the landslide and survived.
The chaotic mass blocked the valley and a lake built up behind the dam. As water loosens the material the dam could collapse at any time, so the Army evacuated all the 1 million people who lived there until they could drain the dam.
A question was asked about avalanches, which are much easier to predict and therefore could be studied in more detail than landslides. Dave said that they were similar but different because the materials in an avalanche were snow, ice and water so much less friction that in a landslide, and as the solid materials slid they melted and the water made them even more frictionless.
This was a very interesting talk that was well illustrated with slides and a video.
If you want to know more about landslides Dave has a blog:
https://blogs.agu.org/landslidesblog/