3.1.5.4 - Seismic Hazards

Question 1

What are seismic hazards?

Answer 1

Earthquakes, tsunamis, landslides and liquefaction

Question 2

What is seismicity?

Answer 2

The Earth’s shaking, can be human induced or natural

Question 3

How are seismic waves formed?

Answer 3

Pressure builds up at a point where plates meet
Sudden release due to rock failure creates waves
The waves cause the ground to shake
Intensity of the shaking is dictated by the depth of focus and energy release

Question 4

What are tsunamis caused by?

Answer 4

Earthquakes under the sea, moving a column of water above it, displacing it

Question 5

What are the characteristics of tsunamis?

Answer 5

Waves less than 3ft tall, but are compress towards land, gaining height. A wave train which propagates form the earthquake.

Question 6

What is liquefaction?

Answer 6

The mixing of sand or soil with groundwater during the shaking of a moderate or strong earthquake. Ground becomes very soft and similar to quicksand.

Question 7

When is land susceptible to landslides?

Answer 7

It has experienced heavy rain
Fractured or unconsolidated rock
Fold mountains, unstable plate boundary
Steep relief

Question 8

How can earthquakes be predicted?

Answer 8

Past seismic events
Remote sensing
Radon gas emissions

Question 9

How can past seismic events be used to predict earthquakes?

Answer 9

Looking into past frequencies and magnitudes to infer future events can be used for prediction

Question 10

How can remote sensing be used to predict earthquakes?

Answer 10

GPS can be used to monitor the smallest of movements by the plates, stationary plates suggest build up of pressure

Question 11

How can radon gas emissions be used to predict earthquakes?

Answer 11

Radon escapes form cracks in the earth’s crust and a sudden increase may suggest that an earthquake may be imminent

Question 12

Are earthquakes predictable?

Answer 12

No

Question 13

What type of boundary to most earthquakes originate from?

Answer 13

Destructive - at subduction zones

Question 14

Where is liquefaction a particularly dangerous hazard?

Answer 14

In areas where groundwater is near the surface and the soil is sandy

Question 15

What is subsidence?

Answer 15

When the ground surface is lowered often during earthquakes

Question 16

What are the characteristics of P waves?

Answer 16

Fastest waves
Earth moves backwards and forwards but surface stays flat
Moves through solids and liquids

Question 17

What are the characteristics of S waves?

Answer 17

A ripple, up and down movement
Water wave, surface doesn’t stay flat
Shakes earth at right angles

Question 18

What are the characteristics of Rayleigh waves?

Answer 18

Up and down movement and side to side, moves like a snake
Low frequency, rolling motion
Radiate along the surface

Question 19

What are the characteristics of Love waves?

Answer 19

Slowest waves, surface
Causes sideways motion and most damage
One pulse, up and down and some side to side

Question 20

What is a seismograph used for?

Answer 20

To measure ground movement

Question 21

How does a seismograph work?

Answer 21

Pen attached to weight to track movements

Question 22

What is a seismometer used for?

Answer 22

Recording ground movement data digitally

Question 23

What are the issues with a seismometer?

Answer 23

So small that waves must be amplified

Question 24

What does the Mercalli scale range from and to?

Answer 24

1-12

Question 25

What is a 1 on the Mercalli scale?

Answer 25

Instrumental - not felt by many people unless in favourable conditions

Question 26

What is a 12 on the Mercalli scale?

Answer 26

Cataclysmic - total destruction

Question 27

What are the positives of the Mercalli scale?

Answer 27

Can be used for damage comparison
Relates to epicentre location
No equipment required
Idea of damage done, builds a picture
Perfect for response planning
Allows links for human need

Question 28

What are the negatives of the Mercalli scale?

Answer 28

Subjective, qualitative measure
Uncertainty and bias
Location of observer alters results
Compares effects, not the event

Question 29

What does the Richter scale measure?

Answer 29

Ground deformation and energy release

Question 30

What type of scale does the Richter follow?

Answer 30

Logarithmic

Question 31

What is the highest recorded value on the Richter scale?

Answer 31

8.9