# Hazards 5

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The primary hazard associated with a seismic event is an earthquake:

• Earthquakes are caused by the tension that builds up at all three types of plate margin.
• When the plates jerk past each other, it sends out shockwaves. These shockwaves are the earthquake.
• The shockwaves spread out from the focus. The focus doesn't have to be a single point - for example, it could be along a fault line. Near the focus the waves are stronger and cause more damage.
• The epicentre 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 rupture along the fault.

Earthquakes can be measured using three different scales:

1. The Richter scale measures the magnitude of an earthquake. It doesn't have an upper limit and it's logarithmic meaning that an earthquake with a magnitude of 5 has an amplitude ten times  greater than one with a magnitude of 4. Major earthquakes are above 7.

2. The moment magnitude scale (MMS) is based on the total amount of energy released by an earthquake. Like the Richter scale, it is logarithmic and has no upper limit. It is more accurate than the Richter scale, especially for large earthquakes, so it's more widely used.

3. The Mercalli scale measures the impacts of an earthquake using observations of the event. The scale is between 1 and 12, with 1 being an earthquake that's only detected by instruments, and 12 being an earthquake that causes total destruction.

Earthquakes can cause a range of different hazards, including:

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 seabed to move, which displaces water. Waves radiate out from the epicentre of the earthquake. The greater the movement of the sea floor, the greater the volume of water displaced, and the bigger the wave produced.
• A tsunami will usually be more powerful if it starts close to the coast. This is because the waves lose energy as they travel towards land. So, the closer to the coast the waves start, the less energy they will lose.
• The waves travel very fast in deep water so they can hit the shore without much warning. This means that they can cause a high 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 ground material, making it easier for water to infiltrate. The weight of the extra water may trigger a landslide even after ground shaking has stopped.

Soil liquefaction:

• When soil is saturated with water, the vibrations of an earthquake can cause it to act like a liquid.
• This makes the soil weaker and easier to deform, so it's more likely to subside, especially where it has a heavy…