Hazards: Seismic Hazards Flashcards
What is an earthquake?
- a gradual build up of pressure and tension in the earths crust
- the release of seismic energy in the form off seismic waves
—> when plates move past each other there can be points where there is friction and and jerk past each other and this sends out shockwaves (vibrations) - which is the earthquake.
What is the focus?
The focus is the point underground where the earthquake originates from.
- the shockwaves spread out from the focus, yet nearer the focus the waves are stronger and cause more damage
What is the epicentre?
The epicentre is the area above ground, directly above the focus and where the earthquake is felt first.
What is ground deformation and how is it a primary hazard from earthquakes?
Ground deformation and fracture is the primary hazard problem in all earthquakes.
— buildings and infrastructure collapse leads to injuries and fatalities
—> e.g. Loma Prieta (1989) in California. 2/3s of all deaths occurred on raised freeway.
How is fires a secondary hazard to earthquakes?
- fires often caused by fractured gas pipes and torn electricity cables
—> e.g. Kobe - Japan
How is mass movement (e.g. landslides and avalanches) a secondary hazard?
Shaking of the ground can dislodge rocks, soil or snow, causing landslides or avalanches that move downslope quickly. Shaking can also loosen ground material, so easier for water to infiltrate - the weight of the extra water may trigger a landslide even after ground shaking has stopped.
—> e.g. the Alaska earthquake in 1964 - 56% of all damage and 37% of deaths was caused by landslides
What is liquidifcation and how is it a secondary hazard?
- water saturated sediment loses strength when vibrated by seismic wave energy.
- material that is ordinarily a solid behaves like a liquid
- mostly observed in saturated, loose (low density or uncompacted), sandy soils.
- it makes the soil weaker and easier to deform, so more likely to subside, especially where it has a heavy weight on top of it (e.g. a building)
— loss of bearing strength under buildings causing settlement and tilting (e.g. Niigata, Japan)
— opening of fissures and cracks, leading to failure of pipes, in the ground and structures on the surface (e.g. dams)
How are Tsunamis a secondary hazard to earthquakes?
Tsunamis are large waves caused by the displacement of large volumes of water.
- triggered by underwater earthquakes which displaces water upwards, generating tsunami at ocean surface. Cause a vertical displacement.
-greater the movement of seafloor, greater the volume of water displaced - bigger waves produced.
- more powerful when close to coast, as they lose energy while travelling - however faster in deep ocean - so therefore hard to warn
— horizontal movements of tectonics plates do not generate a tsunami.
—> e.g Sumatra, Indonesia - Indian Ocean (2004) —> 9.1mg, 230,000 deaths and 13 countries affected
What are shockwaves?
(Seismic waves) - when 2 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 are, as energy transferred into surroundings
What are primary waves?
These waves are faster, longitudinal (expand-contract motion parallel to the direction of the wave) e.g. slinkies
What is a secondary wave?
These waves are slower and more transverse (‘up-down’ motion, at right angles to the direction of travel). Cannot travel through air or liquids as easily
What is the spatial distribution of seismic hazards?
- along the plate boundaries - but deeper earthquakes at destructive (as well as shallow and intermediate) - this is because the huge amount of pressure built up from subduction releases. only shallow earthquakes and constructive plate boundaries (e.g. mid Atlantic ridge) - as well as conservative.
Why is the depth of the focus important?
- deep focus tends to be a higher magnitude than shallow focus earthquakes
—> however, deep focus earthquakes generally do less damage than shallow focus earthquakes - as shockwaves generated deeper in the earth have to travel further to reach the surface, which reduces their power.
How are earthquakes measured (magnitude)?
- Richter scale - measures magnitude or energy released and is more objective - open-ended logarithmic scale (meaning that an earthquake with a magnitude of 5 has an amplitude (wave size) 10x greater than a one with magnitude 4). - major earthquakes are above 7
- Mercalli scale - measures the impacts (intensity or degree of damage) of earthquakes using observations (reports or photos) - the scale is between 1 and 12, with 1 being little affect, and 12 being total destruction.
- There is also the movement magnitude scale (MMS) which is based on the total amount of energy released by the earthquake - logarithmic and has no upper limit
What is the ‘Ring of Fire’?
The ring of fire accounts for 90% of the worlds Earthquakes (the Circum-pacific belt)
—> after the ring of fire, the Alpine-Himalayan belt accounts for 5-6% of the worlds earthquakes
What’s the frequency of seismic hazards?
Earthquakes are frequent around the world and occur every day at boundaries. Hundreds of smaller magnitude earthquakes that cannot be felt by humans occur everyday, whereas the larger earthquakes are less frequent
