Introduction to seismicity - earthquakes

Question 1

What is elastic rebound theory (Henry Fielding)?

Answer 1

1. Before an earthquake, the build up of stress in the rocks, either side of a fault causes gradual deformation (high pressure).
2. The deformation exceeds the frictional force holding the rocks together, this causes a slip occur along the fault.
3. Stress is released and the rocks return to their original shape, but are offset

Question 2

Describe how a normal fault is formed.

Answer 2

1. The hanging wall moves downwards (angle) compared to the footwall
2. the 2 blocks of rock diverge
3. A fault scarp is formed when pieces of rock are exposed at the surface.

Question 3

Describe how a reverse fault is formed.

Answer 3

1. The two blocks or rock are pushed together by compressional forces
2. The hanging wall is forced upwards compared to the footwall

Question 4

Describe how a strike-slip fault is formed.

Answer 4

• One block of rock slides laterally past the other, these occur at conservative plate boundaries.
• NO upwards movement

Question 5

How are earthquakes formed?

Answer 5

1. As tectonic plates move past each other, they can get stuck and friction can build up between them.
2. This causes pressure to also build up and eventually this becomes so great that friction is overcome.
3. This pressure is released at a critical point as the plates snap past each other.
4. Forming an earthquake and seismic waves (released from focus/benioff zone)

Question 6

How do P waves move ?

Answer 6

As longitudinal waves through the compression and expansion of the crust surface.

Question 7

Describe some characteristics of P waves.

Answer 7

• Travel through solids and liquids
• Exist as body waves
• Travel faster than S waves
• Felt as a thump

Question 8

How do S waves move?

Answer 8

As transverse waves through the vertical, upwards and downwards motion of the crust.

Question 9

Describe some characteristics of S waves.

Answer 9

• Travel more slowly than primary waves
• Exist as body waves
• Causes the ground to shake at right angles to the direction of the wave.

Question 10

What are the two types of surface waves?

Answer 10

1. Love waves
2. Rayleigh waves

Question 11

How do Love waves move?

Answer 11

Travel at right angles to the direction of movement.

Question 12

Describe some characteristics of Love waves.

Answer 12

• Fastest surface waves
• Move from side to side
• The most destructive surface wave

Question 13

How do Rayleigh waves move?

Answer 13

Travel in a rolling motion.

Question 14

Describe some characteristics of Rayleigh waves.

Answer 14

• Both horizontal and vertical movement
• Slowest surface waves
• Cannot travel through liquids

Question 15

What does earthquake intensity depend on?

Answer 15

• the magnitude of the earthquake
• the distance away from the earthquake
• the local rock and soil conditions at the earthquake.

Question 16

Define intensity in the context of a hazard.

Answer 16

The amount of damaged caused by an earthquake.

Question 17

What does the Richter Scale measure? How?

Answer 17

The size of the seismic waves that travel through the earth. It involves a system which classifies seismic wave power, where each step is 10x the power of the last.

Question 18

What are the negative aspects of the Richter Scale/ why is it no longer used?

Answer 18

• Does not take into account that damage varies dependent on the rock composition and economic stability of a place.
• The Richter scale has a tendency to underestimate large earthquake magnitudes.

Question 19

How does the Mercalli scale measure intensity?

Answer 19

It measures the amount of damage caused by an earthquake in a particular location.

Question 20

What does the Moment Magnitude scale measure? How?

Answer 20

The amount of energy released by an earthquake event. A formula is used which includes:
- the rigidity of the rock affected
- the distance moved by earthquake
- the size of the area where movement takes place.

Question 21

How deep are shallow focus earthquakes?

Answer 21

0-70km

Question 22

What is a basic definition of an earthquake?

Answer 22

Shock waves cause the ground to shake and move, this movement can cause damage to be caused

Question 23

What is a landslide in the context of an earthquake?

Answer 23

A hazard where cliff slopes weaken and fail due to shaking. They can travel several miles from their source, growing in size and collecting debris.

Question 24

What is a forest fire in the context of an earthquake?

Answer 24

A hazard where fires occur in urban areas, caused by ruptured gas pipes and electricity pylons.

Question 25

What is ground deformation in the context of an earthquake?

Answer 25

A hazard which occurs when an earthquake destabilises material.

Question 26

Describe fully the process of liquefaction in the context of an earthquake.

Answer 26

1. When ground deformation occurs in soil with a high water content, rigid rock particles lose their structural strength and separate.
2. These particles break down into smaller, unconsolidated rock particles, causing the soil to lose its mechanical strength.
3. The soil begins to act like a liquid.

Question 27

What is a tsunami in the context of an earthquake?

Answer 27

Offshore earthquakes can cause giant waves which displace hundreds of kilometres of water when the seabed is shifted several metres. This causes giant waves which travel through the sea

Question 28

Give an example of a landslide in the context of an earthquake?

Answer 28

The Khait landslide (1949) occured in Tajikstan after a 7.4 magnitude earthquake.

Question 29

Give an example of a forest fire in the context of an earthquake.

Question 30

Give an example of ground deformation in the context of an earthquake.

Question 31

Give an example of liquefaction in the context of an earthquake.

Question 32

Give an example of a tsunami in the context of an earthquake.