3.1.5.4 Seismic Hazards Flashcards
What are seismic Hazards?
Plates do not move in fluid motions.
At any boundaries, plates become stuck due to the friction between plates.
The pressure builds so much that it cannot be sustained and the plates eventually give way.
The pressure is released quickly, causing a jolting motion in the plates.
This jolt is responsible for seismic movement spreading throughout the ground.
Focus Point
The focus is the point underground where the earthquake originates from.
Epicentre
The epicentre is the area above ground that is directly above the focus
Secondary effects of seismic hazards
- Shockwaves
- Tsunamis
- Liquefaction
- Landslides
Primary effect of Seismic Hazards
- Earthquakes - ground shaking by the fracturing of rocks.
- Sudden movements along fault lines.
- Ground rupture
Shockwaves
The shifting of rock in an earthquake causes energy to be released which vibrates through the ground creating shockwaves.
There are two types:
- P-waves
- S-waves (high amplitude, most destructive)
Tsunamis
A tsunami is a giant sea wave generated by shallow-focus underwater earthquakes, volcanic eruptions and large landslides into the sea.
Tsunamis have a long wavelength and low wave height in the open ocean.
They travel at speeds of over 700kmh but when they reach shallow water, they increase in height.
Liquefaction
This is when violently shaken soils with a high water content lose their mechanical strength and become fluid.
Landslides
This is when there is slope failure as a result of the ground shaking.
Spatial distribution of earthquakes
The majority of earthquakes occur along plate boundaries, the most powerful at destructive margins and subduction zones.
There is 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.
Other places that earthquakes occur
Old fault lines: Some earthquakes occur away from plate boundaries and are associated with the reactivation of old fault lines.
Human activity: It has been suggested that human activity can lead to minor earthquakes, for example deep mine subsidence or fracking (fracturing of rock to release gas).
Hot spots: Isolated plumes of tectonic activity known as hot spots may give rise to seismic activity.
Earthquake magnitude measurement
The magnitude of an earthquake is measured by the:
- Richter scale: logarithmic scale from 1-10 (7 is 10x than 6)
- Moment magnitude scale (MMS): this identifies the energy release on a scale of 1-10
- Mercalli scale: measures earthquake intensity through the impact on people and structures. The scale is 1-12.
Frequency of earthquakes
Hundreds of low magnitude earthquakes happen around the world every day. (20000 a year)
The number of high magnitude earthquakes occur much less often. (An 8 or greater occur about once a year)
Randomness vs regularity of earthquakes
Earthquakes don’t seem to follow any clear pattern or trend and their occurrence is random.
Predictability of seismic earthquakes
Scientists can monitor the movement of tectonic plates to predict which areas are at risk of seismic hazards.
However it is currently impossible to tell when an earthquake will strike a particular place and what magnitude it is likely to be.
Primary environmental impact
- Earthquake can cause fault lines which destroy the environment
- Liquefaction
Secondary environmental impact
- Radioactive materials and other dangerous substances leaked from power plants
- Saltwater from tsunamis flood freshwater ecosystems
- Soil salinization
Primary economic impact
- Businesses destroyed
Secondary economic impact
- Economic decline as businesses are destroyed (tax breaks etc.)
- High cost of rebuilding and insurance pay out
- Sources of income lost
Primary social impact
- Buildings collapse, killing / injuring people and trapping them
Secondary social impact
- 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
Primary political impacts
- Government buildings destroyed
Secondary political impacts
- Political unrest from food shortages or water shortages
- Borrowing money for international aid
- Can be initial chaos and ‘lawlessness’ e.g. looting
Short term responses
- Evacuation
- Providing health care
- Providing emergency food and water supplies
- Providing emergency shelter
Long term responses
- Prevention
- Preparedness
- Mitigation
- Adaption
Prevention of seismic hazards
Earthquakes and tsunamis will occur regardless so cannot be prevented.
However liquefaction can be prevented through soil stabilization (gravel columns in the ground), and avalanches can be prevented through controlled explosions.
Preparedness for seismic hazards
Earthquake prone areas have extensive awareness strategies and education in place.
Earthquake warning systems and tsunami warning systems can be put in place.
Evacuation plans and training can also be put in place.
Prediction of seismic hazards
Prediction can be very difficult however regions at risk can be identified.
Attempts to predict include:
- Monitoring ground water levels
- Monitoring release of radon gas
- Strange animal behaviour
- Measuring magnetic fields
Mitigation of seismic hazards
Demolishing older unsafe buildings.
Tsunami wave breaks and sea walls.
Adaption from seismic hazards
Move away from areas at risk.
Capitalise on opportunities such as encouraging tourism.
Insurance if living in places of risk.
Building specially designed earthquake proof buildings.
