Seismic Hazards Flashcards
The primary hazard associated with a seismic hazard is an earthquake.
How is an earthquake created?
> Earthquakes are caused by the tension that builds up at all three types of plate margins.
> When the plates jerk past each other it sends out shockwaves (vibrations). These vibrations are earthquakes.
> The shockwaves spread out from the focus. Near the focus, the waves are stronger and cause more damage.
> The epicenter is the point on the earth’s surface where the earthquake is felt first. It’s straight above the focus.
> Earthquakes cause the ground to shake, and sometimes to split apart along the fault.
How can earthquakes be measured? (3 scales)
The Richter scale:
> This measures the magnitude of the earthquake. It doesn’t have an upper limit and it’s logarithmic- this means that an earthquake with a magnitude of 5 has an amplitude (wave size) ten times greater than one with a magnitude of 4.
The moment magnitude scale:
> It’s based on the total amount of energy released by an earthquake. It is more accurate than the Richter scale, especially for large earthquakes.
The Mercalli scale:
> This measures the impacts of an earthquake using observations of the vent e.g. reports and photos. The scale is between 1-12 will with 1 being an earthquake thats only detected by instruments and 12 being total destruction.
What other seismic hazards can earthquakes cause?
Tsunamis:
> Tsunamis are large waves caused by the displacement of large volumes of water.
>They can be triggered by underwater earthquakes. The earthquakes cause the sea bed to move, which displaces water.
>Waves radiate out from the epicenter of the earthquake. The greater movement of the sea bed, the larger displacement, meaning a large wave is produced.
> A tsunami will be more powerful if closer to the coast. This is because the waves lose energy as they travel toward land.
> The waves travel very fast in deep water, so they can hit the shore with little warning. This means it can cause a higher death toll.
Landslides and Avalanches:
> Shaking of the ground can dislodge rock, soil, or snow, causing landslides or avalanches that move downslope quickly.
> Shaking can also loosen the ground, making it easier for water to infiltrate. The weight of the extra water may cause a landslide even after the ground shaking has stopped.
Soil liquefaction:
> When soil is saturated with water, the vibrations of an earthquake can cause it to act like an earthquake.
>This makes the soil weaker and easier to deform, so it’s more likely to subside, especially where it has a heavy weight on top of it e.g a building.
The nature of an earthquake and its magnitude is affected by three main factors:
Margin type:
> The biggest earthquakes occur at destructive plate margins, where one plate is forced beneath another at the subduction zone. The subduction of a plate causes massive pressure to build up, causing a huge earthquake when it is released.
>Earthquakes at constructive margins tend to be of lower magnitude than at destructive or conservative margins.
Rate of Movement:
> Tectonic plates move in relation to each other at different rates, between about 1 and 15 cm per year.
> There’s no clear relationship between the rate of movement and earthquake magnitude.
Depth of Focus:
> An earthquake’s focus can be close to the earth’s surface or deep below it.
> Deep-focus earthquakes tend to be of higher magnitude than shallow-focus earthquakes. However deep-focus earthquakes generally do less damage than shallow-focus earthquakes- this is because shock waves generated deeper in the Earth have to travel further to reach the surface, which reduces their power.
