Y12 MB - Earthquakes Flashcards
How do seismic hazards occur?
The movement of tectonic plates cause pressure to build up in the lithosphere. When the pressure is released, a series of tremors called earthquakes occur
What percentage of earthquakes occur on plate margins?
90%
The rest are intraplate
What is the focus?
The point in the ground at which the breaking point of the earthquake is
What is the epicentre?
The point on the ground surface directly above the focus
Here, the most intense ground shaking is experienced and it dissipates the further away from the epicentre you go
What are faults?
Fractures in the rock when the pressure that builds up in the lithosphere is suddenly released
What focus depth of earthquakes causes the most damage? (Shallow, intermediate, deep)
Shallow faults (0-70km deep) account for 75% of all earthquake energy released
Give the common focus depth of earthquakes at each of the plate boundaries
Constructive - shallow
Conservative - shallow
Collision - shallow / intermediate
Destructive subduction - shallow / intermediate / deep
What are the different types of shock (seismic) waves?
Body waves (primary and secondary)
Surface waves (love and Rayleigh)
What are primary waves?
The fastest waves, reaching the surface first
They travel through both liquids and solids so can travel through both the crust, mantel and core to the opposite side of the earth
Compressional waves (cause expansions and compressions of the ground)
What are secondary waves?
Half as fast as primary waves
Cannot travel through liquids such as the outer core
Transverse waves (they shake the ground up and down)
Why are surface waves slower than body waves?
They cannot travel through the mantel and the core
What are love waves?
The slowest waves but the most destructive
Shake the ground from side to side (causing damage to infrastructure and buildings)
What are rayleigh waves?
Travel out from the epicentre near the surface
Move in a rolling motion so can break up the ground surface
Are seismic shock a waves primary or secondary hazard of an earthquake?
Primary
What are potential secondary hazards of an earthquake?
Tsunamis
Landslides
Liquefaction
What is a tsunami?
Giant sea waves generated by shallow focus underwater earthquakes which cause the movement of the seabed
What are the features of a tsunami?
Have along wavelength and low wave height in the open ocean
Series of waves that travel extremely quickly
On approaching the cost they slow down and pile up as a massive wall of water before breaking (around 30m high)
Results in a dramatic reduction in sea level (known as a drawdown) when the sea appears to drain away
What percentage of tsunamis are associated with the ‘Pacific Ring of Fire’?
Up to 90%
Give an example of a recent destructive tsunami
December 2004 ‘Boxing Day’ tsunami
What is liquefaction?
Occurs in unconsolidated sediments that are saturated with water. The sediments are made of loosely arranged particles so when shaken by seismic waves, act like a liquid (buildings can sink into the sediments)
How can the impacts of liquefaction be reduced?
Using vibrating rods to shake the soils beforehand
What is a landslide?
A rapid movement of earth materials down a slope (in this case as a result of ground shaking)
Why may intraplate earthquake occur?
Reactivation of an old fault line
Mining
Fracking
Building large reservoirs
What are the ways in which the magnitude of an earthquake can be measured?
The Richter Scale (logarithmic - starts at 0 and each interval is 10x the magnitude of the one before it)
The Moment Magnitude Scale (adaptation of the Richter Scale which looks at the total energy released by the earthquake)
Metcalli scale (12 point scale that measures the intensity of the event and its impact instead of the magnitude)
How can earthquakes be predicted?
Impossible to predict when or where
What is ground rupture?
Primary impact of earthquakes, where there is visible breaking and displacement of the Earth’s surface
Has the potential to damage large buildings and infrastructure such as dams, bridges and power stations
What percentage of deaths during an earthquake are caused by collapsing buildings?
75%
What are life-safe buildings?
Sophisticated engineered buildings which have features such as cross-bracing and springs which act as shock absorbers to strengthen them so they are less likely to collapse during an earthquake
