3.b. Earthquakes generate distinctive hazards

Question 1

On average, how many significant earthquakes are there each year?

Answer 1

100.

Question 2

Between 2000 and 2015, earthquakes kill an estimated how many people?

Answer 2

Estimated 900,000 people.

Question 3

Population densities in tectonically active areas has increased, so what has this also increased?

Answer 3

Population densities in tectonically active areas has increased, so number of people at risk is higher.

Question 4

What are the 4 distinctive earthquake hazards?

Answer 4

• Tsunamis.
• Ground shaking and displacement.
• Landslides and avalanches.
• Liquefaction.

Question 5

What is ground shaking and ground displacement?

(Ground shaking and ground displacement)

Answer 5

The vertical and horizontal movement of the ground.

Question 6

The severity of ground shaking depends on what?

(Ground shaking and ground displacement)

Answer 6

• Earthquake magnitude.
• Distance from the epicentre.
• Local geology.

Question 7

Where is ground shaking and ground displacement worst?

(Ground shaking and ground displacement)

Answer 7

In locations that are close to the epicentre of a high-magnitude quake and where the surface layers are relatively unconsolidated and have a high water content will experience extreme ground shaking.

Question 8

The length of the shaking is also important. Where is this most severe?

(Ground shaking and ground displacement)

Answer 8

Most buildings can withstand vertical movements better than horizontal ones, as the swaying of structures is dangerous to stability.

Question 9

Outline human impacts of ground shaking and ground displacement.

(Ground shaking and ground displacement)

Answer 9

Ground movements that cause displacement of rocks along fault lines can rip apart pipelines and sewers, sever rigid structures such as railway tracks and roads, and cause buildings to collapse.

Question 10

Outline environmental impacts of ground shaking and ground displacement. How does this create further human impacts?

(Ground shaking and ground displacement)

Answer 10

The displacement of the surface can also disrupt natural drainage, diverting streams and rivers and affecting the movement of groundwater in aquifers.

This can have serious implications for public water supplies and irrigation for agriculture.

Question 11

What is the named example for the ground shaking and ground displacement?

(Ground shaking and ground displacement)

Answer 11

Tohoku earthquake, (2011).

Created intense shaking for over 5 minutes.

Question 12

When does liquefaction occur? What happens as a result?

Answer 12

When an earthquake strikes an area with surface materials of fine-grained sands, alluvium and landfill with a high water content, the vibrations can cause these materials to behave like liquids.

As a result, these materials lose their strength and structures tilt and sink as their foundations give way.

Question 13

What is the named example for liquefaction?

(Liquefaction)

Answer 13

Kobe earthquake, (2011) - 7.2 MW event.

Liquefaction was a major issue as much of the port had been built on reclaimed land in Osaka Bay.

Just under 200 berths in the port were destroyed, affecting not just the Japanese economy but trade throughout the world.

Question 14

What two earthquake hazards cause slope failure? What areas are the most vulnerable to this?

(Landslides and avalanches)

Answer 14

Both ground shaking and liquefaction can cause slope failure.

Steep slopes in mountainous regions are notoriously unstable and vulnerable to landslides.

Question 15

Some areas are more prone to landslides and avalanches, what are they?

(Landslides and avalanches)

Answer 15

Areas with increased deforestation and heavy monsoon rains, so that even small tremors can trigger landslides.

Question 16

What is the named example for landslides and avalanches?

(Landslides and avalanches)

Answer 16

Nepalese earthquake, (2015).

Triggered large numbers of landslides and avalanches caused by ground shaking.

Landslides blocked transport routes in mountainous regions where accessibility is already difficult.

Question 17

What can the movements of soil and rock on slopes also do? What threat does this pose?

(Landslides and avalanches)

Answer 17

Block rivers.

These natural dams create temporary lakes, which can threaten areas downstream with catastrophic floods.

Question 18

Give an example of when a landslides/ avalanches blocked rivers.

(Landslides and avalanches)

Answer 18

Kashmir, (2005) and Sichuan, (2008):

Soils and rocks blocked many rivers due to an earthquake. This occurred in several locations.

Question 19

Upland valleys are often favoured sites for reservoirs. How may this pose threats when landslides/ avalanches occur?

(Landslides and avalanches)

Answer 19

Should an earthquake create a landslide on slopes above a reservoir, the displacement of water and the waves generated could weaken and overtop the dam.

Question 20

Give an example of when landslides/ avalanches have overtopped reservoirs.

(Landslides and avalanches)

Answer 20

Northern Italy, (1963).

It led to the collapse of a hillside above the Vaiont reservoir, generating a 100 m wave which swept over the dam and down the valley of the Piave river, drowning nearly 3000 people.

Question 21

How do underwater earthquakes generate tsunamis?

(Tsunamis)

Answer 21

They cause the sea bed to rise vertically.

This displaces the water above, producing powerful waves at the surface which spread out at high velocity from the epicentre.

Question 22

Outline the process of tsunami waves approaching the coast.

(Tsunamis)

Answer 22

Low height (< 1 m) and very long wavelength (up to 200 km), mean they can pass underneath a ship out at sea unnoticed.

However, wave height increases greatly as they approach the shore and enter shallow water.

Before the wave breaks, water in front of the wave is pulled back out to sea, a process known as drawdown.

Finally the tsunami wave rushes in as a wall of water that can exceed 25 m in height.

Question 23

Give a named example for tsunamis?

(Tsunamis)

Answer 23

Aceh province in Sumatra, (2004).

Estimated to have delivered about 1000 tonnes of water per metre of shoreline.

Question 24

What affects the local height of a tsunami? What determines tsunami inland distance?

(Tsunamis)

Answer 24

Shape of the sea bed and the coastline.

The relief of the coastal zone determines tsunamis variable distance inland spread.

Question 25

What can underwater landslides cause? What can this create?

(Tsunamis)

Answer 25

Can displace water and create tsunami waves.

When a large volume of rock is shaken and slides downslope, water is dragged in behind it from all sides and collides in the centre.

This can generate a tsunami wave which radiates outwards.

Question 26

What is the difference between the effects of underwater earthquakes and underwater landslides?

(Tsunamis)

Answer 26

Underwater earthquakes produce waves that have enough power to cross oceans.

Underwater landslides result in waves with devastating local effects.

Question 27

What is the named example for an underwater landslide triggering a tsunami?

(Tsunamis)

Answer 27

Papua New Guinea, (1998).

2200 villagers living in coastal communities were killed by a local tsunami generated from an underwater landslide triggered by an earthquake.

This is because as events were local, warning times were short, making them particularly hazardous.