Seismic Hazards: Flashcards
Nature of seismicity and its relation to plate tectonics:
- Earthquakes are the primary hazard caused by seismic activity.
1) Earthquakes are caused by tension which build up at all types of plate margins.
2) Plates jerk past eachother builds up pressure which is released in the form of seismic waves (earthquake)
3) Shockwaves spread out from the focus, near the focus the waves are stronger and can cause more damage.
4) Epicentre is the point on the earths surface where the earthquake is first felt. It is directly above the focus .
5) Earthquakes sometimes rupture along the fault.
Earthquakes can be measured by using 3 different scales:
1) Richter scale - measures the magnitude of an earthquake and it’s logarithmic. Range 1-10
2) Moment magnitude scale - based on the total amount of energy released by the earthquake. Logarithmic and has no upper limit. More accurate than richter scale so more widely used.
3) Mercalli scale - measures impacts by using observations of events. Scale 1-12.
5 forms of seismic hazards:
- Earthquakes
- Tsunamis
- Liquefaction
- Landslides
- Shockwaves.
Shockwaves (4 marks):
- When pressure builds up between 2 plates this is potential energy when the pressure is too much this is transferred into kinetic energy which releases seismic shockwaves.
- Further away from the shockwaves the weaker the focus.
Different types of seismic shockwaves:
Primary - waves are the fastest and reach the surface first.
Surface love - waves are slowest but cause the most destruction.
Tsunamis (4 marks).
Can also be called tidal waves.
- When an oceanic crust is jolted during an earthquake, all of the water above the plate is displaced.
- They are triggered by underwater earthquakes, this causes large volumes of water to be displaced.
- Wave height is less than 1m then as it approaches the shore it can rise to 25m
Reach speeds between 640km and 960km.
Liquefaction: (4 marks)
- When soil is saturated the vibrations of the earthquake make the ground act like a liquid.
- Ground is more likely to subside when there is weight on it.
e.g Haiti port.
Landslides (4 marks):
Shaking in the ground can dislodge rocks, soil and snow causing avalanches or landslides to flow down slope quickly.
Shaking can make it easier for water to infiltrate, the weight of the extra water may trigger a landslide even after the earthquake.
Largest landslide ever (1980) Mount ST Helens.
Spatial distribution of seismic hazards:
- Found at all plate boundaries.
- Ring of fire accounts for 90% of all earthquake activity
Randomness:
Don’t seem to follow any clear pattern or trend.
Magnitude:
Affected by 3 main things:
- Margin type (biggest at destructive subduction plate margins)
- Rate of movement
- Depth of focus.
- Measured on the Richter Scale. Destructive earthquakes tend to be in the excess of 6.
Frequency:
Approximately, 55 earthquakes a day many are not even felt.
Around 20, 7.0 magnitude earthquakes a year.
Most frequent at destructive and conservative plate boundaries.
Regularity:
Earthquakes follow no pattern or trend.
Predictability of seismic hazards:
An understanding if tectonic hazards can help scientists identify areas with the highest risk.
However, almost impossible to predict where an earthquake will occur and when.
Primary impacts:
Environmental - Fault lines can destroy environments. Liquefaction. Fires can start releasing GHG.
Social - People killed or injured and trapped.
Economic - Businesses destroyed and repairing infrastructure.
Political - Governmental buildings are destroyed.
Secondary impacts:
Environmental - Radioactive material leaked from power lines. Saltwater floods, so mix with freshwater. Soil salination.
Social - Fires, food shortages flooding.
Economic - Economic decline, costs of rebuilding
Political - Political unrest over food and water shortages. Interational aid and looting.
