Lecture 4: Landslides (mass wasting) 2

Question 1

Characteristics of triggered events

Answer 1

LANDSLIDES IN TRIGGERED EVENTS
Number: After a trigger, might be zero, one or many thousands of
landslides that occur.
Time: Landslides occur in the minutes to weeks after the
trigger occurs.
Areas: Landslide areas range over many orders of magnitude,
from m2 to km2.

Question 2

INVENTORY OF LANDSLIDES TRIGGERED BY THE 1994 NORTHRIDGE, CALIFORNIA EARTHQUAKE

Answer 2

11,111 landslide areas
Less numerous: deeper slumps and block slides
Most numerous: Shallow falls and slides of rock and debris

Question 3

Landslides Triggered by Hurricane Mitch in Guatemala

Answer 3

9,594 landslide areas

Question 4

Triggered Landslide Events- Frequency-Size Statistics

Answer 4

• Generally applicable behaviour – wherever in the world, however many landslides, the proportion of small to medium to large size landslides should be roughly the same.
• Logarithmic scale– similar to earthquakes and volcanoes.

• Not fully logarithmic – less very small size landslides
than medium size landslides.

• Still being debated why we observe this (friction and cohesion, landscape scaling, minimum dimensions?).
• Active research ongoing how we can use this behaviour to forecast impact.

Question 5

Triggered Events & Road Networks

Answer 5

Route 14, Utah, USA after a landslide destroyed 400 m of road.

Rescuers trying to reach earthquake damaged town on foot, following road blocks from landslides, Sichuan

Create a computer model to randomly drop landslides on road networks.

Use ‘general’ probability distribution for landslide areas.

Question 6

Triggered Events & Road Networks-Issues with current approaches

Answer 6

• Expensive & detailed data
• Technical capacity
required
• Consider only single road blocks, not network-wide impacts
• Many triggered landslide events in developing countries where data and expertise requirements not met

Question 7

Hazard /Susceptibility Assessment of Mass Wasting

Answer 7

 Mass‐wasting events can be extremely hazardous and result in extensive loss of life and property.
 In most cases:
(i) Areas prone to such hazards can be
recognized with some geologic
knowledge.
(ii) Slopes can be stabilized or avoided.
(iii) Warning systems can be put in
place that minimize the hazard.

Question 8

Hazard /Susceptibility Assessment of Mass Wasting

Some issues

Answer 8

• Not all high susceptibility zones will fail.
• Some low susceptibility zones will fail.
• Data intensive.
• ‘Black box-y’.
• Does not consider the
‘hazard’ to other zones.

Question 9

Hazard /Susceptibility Assessment of Mass Wasting- History

Answer 9

Many case histories of mass‐wasting disasters: looking at event in hindsight shows us that hazardous conditions and previous history of mass wasting existed prior to the event.

Question 10

Hazard /Susceptibility Assessment of Mass Wasting- planners

Answer 10

Planners can use such hazards maps to:
• make decisions about land use policies
• take steps can be taken to stabilize slopes
to attempt to prevent a disaster.