3.3.1 Seismic Hazards

Question 1

Earthquakes

Answer 1

1) Earthquakes are caused by the tension that builds up at all three types of plate margins.

2) When the plates jerk past each other it sends out shockwaves (vibrations). These vibrations are the earthquake.

3) 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.

4) The epicentre is the point on the Earth’s surface where the earthquake is felt first. It’s straight above the focus.

5) Earthquakes cause the ground to shake, and sometimes to rupture (split apart) along the fault.

Question 2

What is the primary hazard associated with a seismic event?

Answer 2

An earthquake

Question 3

Scales used to measure earthquakes

Answer 3

• Richter scale
• Moment magnitude scale (MMS)
• Mercalli scale

Question 4

Richter scale

Answer 4

This measures the magnitude of an earthquake (how powerful the shaking is).

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 length) ten times greater than one with a magnitude of 4.

• Major earthquakes are above 7.

Question 5

Moment magnitude scale (MMS)

Answer 5

This is based on the total amount of energy released by an earthquake.

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.

Question 6

Mercalli scale

Answer 6

This measures the impacts of an earthquake using observations of the event (e.g. reports and photos).

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.

Question 7

Seismic hazards

Answer 7

• Earthquakes
• Tsunamis
• Landslides and avalanches
• Soil liquefaction

Question 8

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

Landslides and avalanches

Answer 9

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 forw ater to infiltrate. The weight of the extra water may trigger a landslide even after ground shaking has stopped.

Question 10

Tsunamis

Answer 10

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.

Question 11

Magnitude and frequency of seismic hazards

Answer 11

Hundreds of low magnitude earthquakes happen around the world every day. Fortunately, earthquakes of very high magnitude occur much less often. The number of earthquakes that occur globally also varies from year to year.

Question 12

Regulatity of seismic hazards

Answer 12

Earthquakes and other seismic hazards don’t seem to follow any clear pattern or trend — their occurrence is largely random.

Question 13

Predictability of seismic hazards

Answer 13

Scientists can monitor the movement of tectonic plates to predict which areas are at risk from seismic hazards. However, it’s currently impossible to tell when an earthquake will strike a particular place, and what magnitude it’s likely to be.

Question 14

The distribution of seismic hazards

Answer 14

Most seismic hazards occur around destructive and conservative plate margins, but they can also occur around conservative margins.

Question 15

What factors affect the magnitude of earthquakes?

Answer 15

• Margin type
• Depth of focus
• Rate of movement

Question 16

How does margin type affect the magnitude of earthquakes?

Answer 16

• 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 lower magnitude than at destructive or conservative margins.

Question 17

How does the rate of movement affect the magnitude of earthquakes?

Answer 17

• Tectonic plates move in relation to each other at different rates, between 1 and 15cm per year.
• There’s no clear relationship between rate of movement and earthquake magnitude.

Question 18

How does the depth of focus affect the magnitude of earthquakes?

Answer 18

• An earthquake’s focus can be close to the Earth’s surface or deep below it.
• Deep focus earthquakes tend to be 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.

Question 19

What percentage of earthquakes are found in the Pacific Ring of Fire?

Answer 19

70%

Question 20

Earthquake waves

Answer 20

• P-waves (primary waves): These waves arrive first, have a short wavelength and travel quickly through the crust (usually at 8-km/s). They travel quickly through both solids and liquids and are only damaging in the most powerful earthquakes. These waves are body waves.
• S-waves (secondary waves): These waves move much more slowly, at around 4km/s. They have a longer wavelength and only travel through solids. They vibrate at right angles to the direction of travel and are therefore more destructive than P waves. S waves are also body waves.
• L-waves (love waves): Slowest but most devastating type of seismic wave. Love waves are surface waves, which means they focus all of their energy on the surface of the crust. They have a high amplitude and shake the ground from side-to-side.
• R-waves (rayleigh waves): These waves radiate from the epicentre in complicated low-frequency rolling motions.