3.1.5.4 Seismic Hazards

Question 1

What are seismic Hazards?

Answer 1

Plates do not move in fluid motions.

At any boundaries, plates become stuck due to the friction between plates.

The pressure builds so much that it cannot be sustained and the plates eventually give way.

The pressure is released quickly, causing a jolting motion in the plates.

This jolt is responsible for seismic movement spreading throughout the ground.

Question 2

Focus Point

Answer 2

The focus is the point underground where the earthquake originates from.

Question 3

Epicentre

Answer 3

The epicentre is the area above ground that is directly above the focus

Question 4

Secondary effects of seismic hazards

Answer 4

• Shockwaves
• Tsunamis
• Liquefaction
• Landslides

Question 5

Primary effect of Seismic Hazards

Answer 5

• Earthquakes - ground shaking by the fracturing of rocks.
• Sudden movements along fault lines.
• Ground rupture

Question 6

Shockwaves

Answer 6

The shifting of rock in an earthquake causes energy to be released which vibrates through the ground creating shockwaves.

There are two types:

• P-waves
• S-waves (high amplitude, most destructive)

Question 7

Tsunamis

Answer 7

A tsunami is a giant sea wave generated by shallow-focus underwater earthquakes, volcanic eruptions and large landslides into the sea.

Tsunamis have a long wavelength and low wave height in the open ocean.

They travel at speeds of over 700kmh but when they reach shallow water, they increase in height.

Question 8

Liquefaction

Answer 8

This is when violently shaken soils with a high water content lose their mechanical strength and become fluid.

Question 9

Landslides

Answer 9

This is when there is slope failure as a result of the ground shaking.

Question 10

Spatial distribution of earthquakes

Answer 10

The majority of earthquakes occur along plate boundaries, the most powerful at destructive margins and subduction zones.

There is a clear line of earthquakes along the centre of the Atlantic Ocean between the African and American Plates and around the Pacific Ocean at the edge of the Pacific plate.

Question 11

Other places that earthquakes occur

Answer 11

Old fault lines: Some earthquakes occur away from plate boundaries and are associated with the reactivation of old fault lines.

Human activity: It has been suggested that human activity can lead to minor earthquakes, for example deep mine subsidence or fracking (fracturing of rock to release gas).

Hot spots: Isolated plumes of tectonic activity known as hot spots may give rise to seismic activity.

Question 12

Earthquake magnitude measurement

Answer 12

The magnitude of an earthquake is measured by the:

• Richter scale: logarithmic scale from 1-10 (7 is 10x than 6)
• Moment magnitude scale (MMS): this identifies the energy release on a scale of 1-10
• Mercalli scale: measures earthquake intensity through the impact on people and structures. The scale is 1-12.

Question 13

Frequency of earthquakes

Answer 13

Hundreds of low magnitude earthquakes happen around the world every day. (20000 a year)

The number of high magnitude earthquakes occur much less often. (An 8 or greater occur about once a year)

Question 14

Randomness vs regularity of earthquakes

Answer 14

Earthquakes don’t seem to follow any clear pattern or trend and their occurrence is random.

Question 15

Predictability of seismic earthquakes

Answer 15

Scientists can monitor the movement of tectonic plates to predict which areas are at risk of seismic hazards.

However it is currently impossible to tell when an earthquake will strike a particular place and what magnitude it is likely to be.

Question 16

Primary environmental impact

Answer 16

• Earthquake can cause fault lines which destroy the environment
• Liquefaction

Question 17

Secondary environmental impact

Answer 17

• Radioactive materials and other dangerous substances leaked from power plants
• Saltwater from tsunamis flood freshwater ecosystems
• Soil salinization

Question 18

Primary economic impact

Answer 18

• Businesses destroyed

Question 19

Secondary economic impact

Answer 19

• Economic decline as businesses are destroyed (tax breaks etc.)
• High cost of rebuilding and insurance pay out
• Sources of income lost

Question 20

Primary social impact

Answer 20

• Buildings collapse, killing / injuring people and trapping them

Question 21

Secondary social impact

Answer 21

• Gas pipes rupture, starting fires which can kill
• Water supplies are contaminated as pipes burst, spreading disease and causing floods
• Tsunamis which lead to damaging flooding

Question 22

Primary political impacts

Answer 22

• Government buildings destroyed

Question 23

Secondary political impacts

Answer 23

• Political unrest from food shortages or water shortages
• Borrowing money for international aid
• Can be initial chaos and ‘lawlessness’ e.g. looting

Question 24

Short term responses

Answer 24

• Evacuation
• Providing health care
• Providing emergency food and water supplies
• Providing emergency shelter

Question 25

Long term responses

Answer 25

• Prevention
• Preparedness
• Mitigation
• Adaption

Question 26

Prevention of seismic hazards

Answer 26

Earthquakes and tsunamis will occur regardless so cannot be prevented.

However liquefaction can be prevented through soil stabilization (gravel columns in the ground), and avalanches can be prevented through controlled explosions.

Question 27

Preparedness for seismic hazards

Answer 27

Earthquake prone areas have extensive awareness strategies and education in place.

Earthquake warning systems and tsunami warning systems can be put in place.

Evacuation plans and training can also be put in place.

Question 28

Prediction of seismic hazards

Answer 28

Prediction can be very difficult however regions at risk can be identified.

Attempts to predict include:

• Monitoring ground water levels
• Monitoring release of radon gas
• Strange animal behaviour
• Measuring magnetic fields

Question 29

Mitigation of seismic hazards

Answer 29

Demolishing older unsafe buildings.

Tsunami wave breaks and sea walls.

Question 30

Adaption from seismic hazards

Answer 30

Move away from areas at risk.

Capitalise on opportunities such as encouraging tourism.

Insurance if living in places of risk.

Building specially designed earthquake proof buildings.