Seismic hazards Flashcards
1
Q
Nature of seismicity
A
- Plates do not perfectly fit into each other, meaning they do not move in fluid motions. At all boundaries, plates can become stuck due to the friction between plates
- When the plates are stuck, the convection currents in the asthenosphere continue to push, which builds the pressure.
- It builds so much that it cannot be sustained and the plates eventually give
way.
All of this pressure is released in a sudden movement, causing a jolting motion in the plates.
- This jolt is responsible for seismic movement spreading throughout the ground in the form of seismic waves (or shock waves).
2
Q
Spatial distrubution
A
- Clear line of earthquakes along the centre of the Atlantic ocean between the African and American plates and around the pacific ocean at the edge of the pacific plate
- Broad linear chains occur
- Conserative plate boundaries where plates are sliding past each other, give a relatively narrow band of earthquakes
3
Q
Magnitude
A
- Seismicity is measures using the Richter Scale which is a measure of the strength of seismic waves
- The Modified Mercalli Intensity Scale is also used, which is a rate of the destruction caused. The Mercalli scale is subjective, meaning sometimes it is disputed as it is dependent on human development being present rather than the strength of the seismic waves.
4
Q
Frequency
A
- Earthquakes are frequent around the world and occur every day at boundaries.
- Hundreds of smaller magnitude earthquakes that cannot be felt by humans occur every day, whereas the larger earthquakes are less frequent.
5
Q
Regularity
A
- Earthquakes follow no pattern and are random so there is irregularity between events
6
Q
Predictibility
A
- Earthquakes are almost impossible to predict.
- Microquakes may give some indication but the magnitude cannot be predicted as how strong they are is random.
7
Q
Hazards caused by seismic events : Shockwaves
A
- When two plates move side by side, friction builds up and pressure increases; this pressure is stored as potential energy, it cannot move so it just builds up
. - When the pressure becomes too much, the plates eventually move. All of the energy that has been built up must go somewhere, so it is transferred into kinetic energy, which is released and vibrates throughout the ground
. - The further away from the focus, the weaker the shockwaves, as the energy is transferred into the surroundings.
8
Q
Hazards caused by seismic events : Tsunamis
A
- When an oceanic crust is jolted during an earthquake, all of the water above this plate is displaced.
- The water travels fast but with a low amplitude (height).
- As it gets closer to the coast, the sea level decreases so there is friction between the sea bed and the waves
- This causes the waves to slow down and gain height, creating a wall of water that is on average 10 feet high, but can reach 100 feet.
9
Q
Hazards caused by seismic events : Liquefaction
A
- When soil is saturated, the vibrations of an earthquake cause it to act like a liquid.
- Soil becomes weaker and more likely to subside when it has large weight on it.
10
Q
Hazards caused by seismic events : Landslides and avalanches
A
- Movement in soil or snow will cause it to become unstable.
11
Q
Primary effects of a seismic hazard
A
- Earthquake can cause fault lines which destroy the environment
- Liquefaction
- Businesses destroyed
- Buildings collapse, killing/injuring people and trapping them.
- Government buildings destroyed
12
Q
Secondary effects of a seismic hazard
A
- Radioactive materials and other dangerous substances leaked from power plants
- Saltwater from tsunamis flood freshwater ecosystems
- Soil salinisation
- Economic decline as businesses are destroyed
- High cost of rebuilding and insurance payout
- Sources of income lost
- Gas pipes rupture, starting fires which can kill
- Water supplies are contaminated as pipes burst, spreading disease and causing floods
- Tsunamis which lead to damaging flooding
- Political unrest from food shortages or water shortages
- Borrowing money for international aid
- Can be initial chaos and ‘lawlessness’ e.g. looting
13
Q
Prevention of seismic hazards
A
- Majority of seismic hazards can’t be prevented
- Liquefaction of soils can be prevented through soil stabilisation
- Avalanches can be prevented through controlled explosions
14
Q
Preparedness of seismic hazards
A
- Earthquake prone areas have extension awareness strategies and education in places
- Earthquake warning systems and tsunami warning systems after an earthquake
- Evacuation plans and training
15
Q
Mitigation of seismic hazards
A
- Search and rescue, immediate emergency aid, evacuation (short term)
- Demolishing older, unsafe buildings
- Tsunami wave breaks and sea walls
