Seismic Hazards (earthquakes)

Question 1

Where are earthquakes found and why?

Answer 1

They are found along the plate boundaries. This is because, when these boundaries slide past or subduct into one another, pressure is built up and then suddenly released. This triggers movement which results in an earthquake. They are found mainly at destructive plate margins as pressure is larger

Question 2

What is seismicity?

Answer 2

Earth shaking, which can be human induced (e.g. via reservoir construction, fracking or mining)

Question 3

What are the steps of seismic waves?

Answer 3

1) pressure builds up at the point where two plates meet
2) sudden release due to rock failure creates waves
3) the waves cause the ground to shake
4) the intensity of the shaking is dictated by the depth of the focus and the energy release
RESULTING IN AN EARTHQUAKE

Question 4

What are P waves (primary)?

Answer 4

• short pulse of energy moving forwards and backwards very quickly
• they will be picked up first by seismometers
• least damaging as they just vibrate

Question 5

What are S waves (secondary)?

Answer 5

• go up and down
• slower than p waves
• more damaging (travel like a Mexican wave)

Question 6

What are Rayleigh waves?

Answer 6

• they are much slower
• compression up and down again
• more damaging to buildings

Question 7

What is a love wave?

Answer 7

-most damaging
- have a side to side movement
- challenge to people/buildings
- relatively slow

Question 8

What are tsunamis?

Answer 8

• Large volume of water that pushes in land over time (release of energy under the sea)
• They travel across the open ocean at high speeds
• They are generated in subduction zones. Friction between plates, they get “stuck”
• Stuck plates continue to descend down into the mantle- slow distorting of overriding plate.
• Energy can accumulate in the overriding plate until it exceeds the frictional forces between the stuck plates, overriding plate snaps back and the sudden movement causes a tsunami- overlying water travels inland
• moving wave begins travelling out from where the earthquake has occurred.
• ‘wave trains’, multiple waves not just one

Question 9

What is liquefaction?

Answer 9

The mixing of sand or soil and groundwater during the shaking of an earthquake. When the water and soil mix, the ground becomes very soft (like quicksand). If liquefaction occurs under a building, it may start to lean or even sink. The ground firms up again after after the earthquake has passed and the water settles back down to underground.

Question 10

What are landslides?

Answer 10

Where the ground shakes due to earthquakes destablises cliffs and steep slopes, causing landslides and rock fall. Heavy rain or fractured rock and exacerbating factors. Relief rainfall in these upland areas means the ground is significantly more likely to become saturated and therefore more likely to fail

Question 11

What physical characteristics make areas susceptible to landslides?

Answer 11

Steep land
Land sat over unstable plate boundaries
Heavy air before an earthquake
At least a 10% land gradient

Question 12

What physical characteristics make an area susceptible to liquefaction?

Answer 12

Land near to water, soil and sand
Worse if there is sand as water can move around the particles of sand

Question 13

Why is liquefaction worse than an earthquake?

Answer 13

There’s not much you can do to adapt/mitigate liquefraction

Question 14

How can monitoring past seismic events predict future earthquakes?

Answer 14

The seismic gap theory:
Theory that earthquakes can be predicted by observing the time gap between past earthquakes in an area. The average time between past earthquakes and time since the last one can be used together to predict when the next one will be

Question 15

How can remote sensing be used to predict earthquakes?

Answer 15

Attempted to detect thermal activity. Also, attempts to relate ionosphere disturbances associated with earthquakes. They use satellites to see whether plates are moving apart or if there not then they are stuck (earthquake may occur)

Question 16

How are radon gas emissions used to predict earthquakes?

Answer 16

High concentrations of radon gas in soil or groundwater could be signs of upcoming earthquake. It’s believed that this radon is released from cavities and cracks as the earths crust is strains prior to the sudden slip of a plate

Question 17

How is animal activity used to predict an earthquake?

Answer 17

It’s been reported that animals leave their homes and head for safety several days before an earthquake. They may also exhibit strange behaviours anywhere from weeks to seconds before an earthquake (so we don’t know exactly when one might happen)

Question 18

Is predicting seismic hazards impossible?

Answer 18

Yes, almost impossible because although we get some information, we don’t know exactly when one may happpen

Question 19

Primary effects of earthquakes:

Answer 19

• panic, fear
• ground shaking, buildings collapsing, power lines collapsing, gas mains ruined
• schools, unis destroyed
• liquefaction of saturated soils

Question 20

Secondary effects of earthquakes:

Answer 20

• flooding from blocked rivers creating ‘quake lakes’
• slope failure setting off landslides and avalanches
• education suspended for immediate future
• fires caused by broken gas pipes and power lines are difficult to put out
• civil disorder/looting and direct intervention from civil authorities

Question 21

Long term effects of earthquakes:

Answer 21

• permanent disruption of natural drainage patterns . Loss of farmland and food production
• repairing building, expensive
• bodies not buried can spread disease such as cholera and injuries may result in death
• higher unemployment as not all businesses recover from damage
• trauma/grief may take months or years to recover from

Question 22

How do we measure earthquakes?

Answer 22

We use seismometers, which are used to plot a seismograph, we then read to determine just how much shaking occurred

Question 23

What two scale are used to measure intensity of an earthquake?

Answer 23

Richter scale
Modified mercalli scale (MMI)

Question 24

What does the MMI scale give us?

Answer 24

Subjective view of the effects of a seismic event

Question 25

From stage 1-5 on the MMI scale what action would you take?

Answer 25

Carry on with your normal life, might be slightly noticeable by some people.
Would give the knowledge to people that tit was an earthquake that occurred so they don’t worry about what else it may have been.
Also, if people know there has been one it may predict that one will happen again in the future/be more likely to

Question 26

From stage 6-9 on the MMI scale what action would you take?

Answer 26

More likely to need to respond to the earthquake and may need emergency services/ aid from your country to step in and help out where needed

Question 27

From stage 10-12 on the MMI scale what action would you need to take?

Answer 27

There would be lots of panic and fear. Damage will occur, most likely quite extreme. You would need lots of people to step in and help, even international aid, especially financial

Question 28

Why may people argue that the MMI scale is moor useful than the Richter scale?

Answer 28

It says the effects in writing so therefore it’s easier to know what action you would need to take in order to stay safe and be in some control, rather than just a number of hoe intense the earthquake may be

Question 29

What does the Richter scale tell us?

Answer 29

Measures the ground deformation and energy release by the earthquake. It goes from a 1-8 intensity

Question 30

What’s an important thing to remember about a Richter scale?

Answer 30

It’s a LOGARITHMIC SCALE and therefore each time you go up a number it’s actually 10x as intense as the one before.

Question 31

What happens to the magnitude and frequency as you go up the Richter scale?

Answer 31

The higher the magnitude of an earthquake, the less frequent it happens, as it takes longer for large amounts of pressure to build up again

Question 32

Why do news agencies choose to use the Richter scale rather than the MMI scale?

Answer 32

Richter scale is more well known so most people will understand it more. It’s a scientific measure of energy release, rather than than subjective opinions