Seismic Hazards Flashcards
Constructive margins
Shallow focus earthquakes can occur as a result of tensional forces in the crust. They often occur at mid-ocean ridges so pose little threat to people.
Collision margins
Shallow focus earthquakes can occur.
Conservative boundaries
Two sections of the earth move laterally and shallow focus earthquakes occur.
Destructive margins
Compressional forces lead to earthquakes with deeper foci. They occur in a narrow zone known as the Benioff zone.
What happens during an earthquake?
When the plates jerk past each other they send out shockwaves. They spread out from the focus, which can be a single point or a fault line.
The waves closest to the focus cause more damage because they are stronger.
The ground shakes and can sometimes rupture along the fault line.
Epicentre
The point on the earth’s surface where the earthquake is first felt- directly above the focus.
The Richter scale
Measures magnitude- how powerful the shaking is.
It does not have an upper limit and it is logarithmic, meaning that an earthquake with a magnitude of 5 is 10 times greater than one of 4.
Major earthquakes are above 7.
The Moment magnitude scale
It measures the total amount of energy released.
It is more accurate than the richter scale, especially for larger earthquakes so is more widely used.
The mercalli scale
Measures the impact of an earthquake by using observations such as reports and photos. It is measured between 1 (only detected by instruments) and 12 (causes total destruction)
Tsunamis
Large waves caused by displacement of large volumes of water.
They radiate out from the epicentre of an underwater earthquake.
The earthquake causes the seafloor to move which displaces the water.
The closer to coastline they start, the more powerful they are as they have less time to lose energy.
They travel very fast so reach the shore with little warning, and can reach up to 10m in height.
Landslides and avalanches
Shaking of the ground can casue dislodged rock, soil and snow which can move downslope very quickly.
Rocks can destroy buildings, inure people and block roads.
Shaking also causes ground material to infiltrate, meaning that extra weight can trigger landslides once the shaking has stopped.
Soil liquefaction
Soil becomes saturated with water when seismic shaking agitated ground material, causing it to act like liquid. The ground can subside, meaning that buildings can sink into it.
The combination of loose materials becoming compressed by gravity and water resisting change can build up pressure. Eventually the pressure rises enough that grains of soil become buoyant and float in the water.
Soil can remain liquefied for several hours and flooding can also occur e.g. seen in Christchurch in 2010 and 2011 to cause widespread disruption.
Spatial distribution of seismic events
90% occur on plate boundaries.
The UK experiences 20-30 earthquakes each year that can be felt by humans, despite being the in middle of the Eurasian plate.
This is thought to be due to a series of hairline fractures that are being locally compressed.
Most seismic hazards occur on destructive and conservative margins however they do also occur on constructive boundaries.
Factors affecting the magnitude of an earthquake
- Margin type - largest ones occur at destructive boundaries where huge pressures builds up when one plate is subducted.
- Depth of focus - Deep focus = high magnitude + less damage because shock waves have further to travel.
Frequency + magnitude
Frequent = low magnitude
Infrequent = high magnitude
There is around 15 volcanoes a year with a magnitude of 7-7.9.
