4.1 Earth Hazards and their Mitigation Flashcards
What are the earth hazards you need to know about for this course?
- earthquakes
- tsunamis
- landslides
- volcanoes
What can landslides also be known as?
Mass movement
What is seismology?
The study of earthquakes
What are the different types of earthquake wave - in order of speed (most to least)?
- P waves
- S waves
- surface waves
Which type of earthquake wave causes the most damage?
Secondary
What is the Richter Scale measured by?
The ground shaking caused by the earthquake
What is tectonic unzipping?
A series of earthquakes along an active fault (over hundreds of years)
Why do earthquakes occur in San Francisco?
Due to being on the San Andreas Fault
Why does liquefaction occur?
Due to the shaking in wet sediments, which separates the grains and makes them behave like quicksand
What can be triggered by earthquakes?
Landslips and tsunamis
What effect will a high population density have on the death toll?
Increase death toll
How does the wealth of a country affect how disastrous an earthquake would be?
Wealthy country - better prepared but greater economic loss/property damage
What methods can be used to predict earthquakes?
- seismic gaps
- hazard maps
- changes in ground levels
- gas measurements
- changes in water levels in wells
- foreshocks
- animal behaviour
How can seismic gaps help to predict earthquakes?
- if area along fault has regular tremors then stress is being released - so large earthquake unlikely
- area has few tremors - stress building - could result in big earthquake
How can hazard maps help to predict earthquakes?
Maps show places most likely to suffer from major earthquakes to focus attention on areas most in need
How can changes in ground levels help to predict earthquakes?
Area around focus of earthquake may tilt due to swelling
What equipment can show change in ground levels due to earthquakes?
Tiltmeters and lasers
How can gas measurements help to predict earthquakes?
Radon, radioactive gas, moves up through cracks and can be detected in water in wells - if it ↑ then it shows rock is cracking more than usual = earthquake
How can changes in water levels in wells help to predict earthquakes?
If rocks are cracking, groundwater will sink into cracks and water levels fall
How can foreshocks help to predict earthquakes?
Number of foreshocks increases before a major events
How can animal behaviour help to predict earthquakes?
Some animals show disturbed behaviour before events - may be due to slight changes in earth’s magnetic field
How can the risk from earthquakes be reduced?
- planning
- building design
- ground or base isolation systems
- resisting shear forces
- absorbing sway
- services
Features of a shield volcano?
- basalt
- less viscous
- less gas within - so runny
- 1200 *C
- lava flow, ash, small tephra
Features of a composite volcano?
- andesite and rhyolite
- more viscous
- explosive - lava and pyroclastic flow
- huge tephra
Where do basaltic volcanoes form?
Along constructive boundaries above hot spots
Where do andesitic volcanoes form?
Above subduction zones
What type of volcano is more likely to cause major problems?
Explosive andesitic variety
What are some volcanic hazards?
- lava flow
- ash
- pyroclastic flow
- gas
- lahar
- lateral blast
Description of lava flow?
Localised distribution, depending on viscosity, and more dangerous to property than life
Description of ash?
Widespread and a health hazard to living things - cause roof collapse and destruction of farmland
Description of pyroclastic flow?
Local to volcano - very destructive and extreme hazard to life, given their temps and high velocity
Description of lahar?
Also known as volcanic mudflows - destructive in vicinity of volcano - particularly in tropical areas with heavy rainfall
Description of lateral blast?
Dangerous locally when volcano collapses and erupts sideways → fan shape of destruction
Why do volcanoes not warm the climate?
- if anything, they cool it
- produce ash - condensation nuclei
- form clouds → high albeido
What does VEI stand for?
Volcanic explosivity index
What is the VEI?
Relative measure of the explosiveness of volcanic eruptions - measured by volume of material erupted - each VEI is 10x larger than the one below
What methods can be used to predict volcanic eruptions?
- patterns of activity
- ground deformation
- gas emissions
- groundwater changes
How can ground deformation help to predict volcanic eruptions?
Tiltmeters and GPR can be used to measure changes in volcano’s size/slope angle - swelling volcano suggests magma moving closer to surface
How can gas emissions help to predict volcanic eruptions?
As magma nears surface and pressure ↓, gases escape - ↑ amounts of SiO2 suggest magma near surface
How can groundwater changes help to predict volcanic eruptions?
↑ pressure causes water levels to rise and suddenly drop - ↑ heat flow can reduce flow in aquifers
How can the risks of volcanic eruptions be reduced?
Hazards maps
How can hazard maps help reduce risks of volcanic eruptions?
- used by public officials
- create plans for evacuation, rescue and recovery
- future land use planning can be based on hazard maps
What was the major tsunami in 2011?
Japanese tsunami, 2011
What is a tsunami?
A water wave that occurs at the same time as an earthquake, but it is not a seismic wave
What is a tsunami caused by?
- the bodily displacement of a large volume of water by the movement of a large section of crust on the sea floor
- also due to the displacement of water by large landslides
In open water, what is the approximate amplitude of a tsunami?
A meter
In open water, what is the approximate wavelength of a tsunami?
Several hundred km
In open water, what is the approximate speed of a tsunami?
Up to 700 km/h
What happens when a tsunami approaches the shallow waters of the coast?
The height of the wave increases dramatically, surging across low-lying coastal areas
What is the result of tsunamis?
- buildings swept away
- people die - especially in low lying areas
- contaminated groundwater supplies
- damage to land near coast - reduce crop yield
- natural environment suffers
Prediction methods for which natural disaster are similar to that of tsunamis? Why is this?
Earthquakes as most tsunamis result from earthquakes
How can risks from tsunamis be reduced?
- good early warning system - earthquake usually arrives well before tsunami - giving several hours warning for distant epicentres
- Indian Ocean Tsunami Warning System - set up by UNESCO
How do raised coastal roads reduce effects of tsunamis?
Allows for easier evacuation
How do embankments reduce effects of tsunamis?
Acts as a barrier to water moving inland
How do buildings at right angles to the coast set on concrete piles reduce effects of tsunamis?
Keeps property above the height of the water
How do development-free zones of parks and forests reduce effects of tsunamis?
Slows down the water without damaging property
What are landslides?
Sudden movements of masses of rock and soil that may affect areas covering many square kilometers
What are the main types of landslides?
- translational
- topple
- creep
- rotational
- fall
- flow
What are the factors that will influence whether a landslide is likely or not?
- ROCK TYPE - strength, porosity and permeability, geological structures e.g. faults, weathering
- SLOPE ANGLE - clay unstable at 10° whereas other rocks can form vertical slopes
- WATER - heavy rainfall = lubricant
- PEOPLE
What will be the final cause of a landslide?
Heavy rainfall
Which human activities can increase the chance that a landslide might happen?
- adding weight by building on slope
- removing material from base of slope
- pipes leaking water and sewage
- removal of vegetation
- tarmac
What is geomorphology?
Nature of the landscape
How might glacial erosion 20,000 years prior contributed to a landslide?
Boulder clay formed which is easily eroded
What is used to gather landslide characteristics?
Before and after aerial photographs
What are the most effective methods of reducing the risk from landslides?
- planning regulations controlling location of buildings
- building codes to stabilise slopes
- control drainage
- reducing the load of overlying materials
- reducing gradients of slopes
- planting vegetation
- filling permeable rock with cement
- increasing insurance premiums on property to stop building homes in high risk areas
In general, what methods of prediction are used for natural hazards?
- hazard interval patterns (seismic gaps)
- ground deformation (tiltmeters)
- groundwater changes
- gas emissions
In general, what methods of reducing risk are there for natural hazards?
- building design and regulation
- prediction
- warning schemes and evacuation
What is the difference between lava flow and pyroclastic flow?
Lava flows are molten rock; pyroclastic flows are rock fragments, dust, ash and gases
