Seismic Hazards

Question 1

What is the focus?

Answer 1

The shockwaves spread out from the focus. The focus doesn’t have to be a single pint - it could be a fault line. Near the focus the waves are stronger and cause more damage.

Question 2

What is the epicentre?

Answer 2

The epicentre is the point on the Earths surface where the earthquake is felt first. Its straight above the focus.

Question 3

What does the Richter scale measure?

Answer 3

It measures the magnitude of an earthquake. It is logarithmic - this means that an earthquake with a magnitude of 5 has an amplitude ten times greater than one with a magnitude of 4.

Question 4

What is the moment magnitude scale based on?

Answer 4

Based on the total amount of energy released by an earthquake. It is more accurate than the Richter Scale.

Question 5

What is the Mercalli scale?

Answer 5

It measures the impacts of an earthquake using observations of the events (reports and photos). The scale is between 1 and 1, with 1 being an earthquake that’s only detected by instruments and 12 being an earthquake that causes total destruction.

Question 6

What is a tsunami?

Answer 6

• 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.

Question 7

What is a landslide?

Answer 7

• 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.

Question 8

What is soil liquefaction?

Answer 8

• 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 weight on top of it (e.g. a building).

Question 9

What three factors affect the nature of an earthquake?

Answer 9

Margin type, rate of movement and depth of focus.