9.1. Earthquakes

Question 1

Global Distribution of Earthquakes

Answer 1

• broad belts of earthquakes are associated with subduction zones
• collision boundaries are also associated with broad belts of earthquakes
• exceptions include intraplate earthquakes (found in the centre of plates that might be on old fault lines or weaknesses that are often unknown

Question 2

Focus

Answer 2

the point where the rocks start to fracture

Question 3

Epiccentre

Answer 3

the point on the earth’s surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins

Question 4

Cause of Earthquakes

Answer 4

• Movement of plates on all types of plate boundaries
• When plates move and get stuck, there is friction and this builds up
• There is increasing tension, as convection currents still try to move the plates
• When the friction is released (seismic waves), the ground shakes in the form of an earthquake
• There are two types of seismic waves released: P waves and S waves

Question 5

Primary Waves

Answer 5

• first seismic waves released from earthquakes
• longitudinal waves
• relatively weak
• causes the surface to move in a back and forth motion by stretching and compressing the material they pass through
• can pass through solids and liquids (able to move pass all layers of the Earth)

Question 6

Secondary Waves

Answer 6

• next seismic waves to arrive from earthquake
• transverse waves
• cause the crust to move side to side or up and down as they pass
• can only pass through solids (does not go through the liquid outer core

Question 7

Earthquake Magnitude

Answer 7

Total amount of energy released by a earthquakes

Question 8

Richter scale

Answer 8

• Measures magnitude (energy released)
• Has a logarithmic scale
• Mercalli scale - measures damage and destruction

Question 9

Mercalli scale

Answer 9

• Measures damage and destruction of Earthquakes

Question 10

Factors affecting earthquake damage

Answer 10

1) magnitude
2) depth of focus
3) bedrock nature
4) location (population density, distance from epiccentre)
5) building structure
6) earthquake preparedness
7) secondary effects
8) levels of development

Question 11

How magnitude of earthquakes influence their damage

Answer 11

• measured by RIchter scale (1 to 10)
• has a logarithmic scale
• higher magnitude –> more destruction

Question 12

How depth of earthquakes influence their damage

Answer 12

• shallow earthquakes that occur close to the surface tend to result in a greater intensity of surface shaking and often cause the greatest loss of life and damage to property
• often associated with destructive margins

Question 13

How bedrock nature influences earthquake damage

Answer 13

• some materials are vulnerable to become ‘jelly-like’ when shaken
• commonly associated with clay and silts
• this is liquefaction
• building foundations become unstable and slopes potentially undergo mass movements

Question 14

How location (population density, distance from epiccentre) influences earthquake damage

Answer 14

• earthquakes are only hazards when it impacts human activity
• earthquakes that occur at places with high population densities –> more damage
• If closer to the epiccentre of the earthquake there would be more damage as waves are strongest there

Question 15

How building structure influences earthquake damage

Answer 15

• “earthquakes don’t kill people, buildings do”
• vast majority of suffering results from the collapse of building structures such as buildings and bridges
• appropriate building designs to counteract earthquakes can reduce impacts

Question 16

How earthquake preparedness influences earthquake damage

Answer 16

• in wealthier countries where earthquakes are common, there are regular earthquake drills in schools, offices
• people are informed of potential dangers and how to respond when earthquakes occur
• supplies of food, water are stored in recognised safe areas ready for coping with an earthquake aftermath
• response of rescuing people and avoiding spread of disease is faster

Question 17

How secondary effects of earthquakes influence their overall impact

Answer 17

• tsunamis, landslides and fires

- all can make earthquake impacts more hazardous

Question 18

How countries’ levels of development influence earthquake impacts

Answer 18

• LICs suffer much more from effects of earthquakes than HICs
• LICs have poor building standards, lower standards of education and healthcare, poverty, and so are less able to cope with earthquake aftermaths

Question 19

Primary hazards of Earthquakes

Answer 19

• Ground shaking - passage of P and S waves
  Bedrock shakes least and shortest
  Mud-rich soils shake the most and longest
• Surface faulting

Question 20

Secondary hazards of Earthquakes

Answer 20

• Ground failure and soil liquefaction
• Landslides, rockfalls and avalanches
• Fires
• Tsunamis

Question 21

Earthquake Impacts

Answer 21

• Loss of life
• Loss of livelihood
• Total or partial destruction of building structure
• Interruption of water supplies
• Breakage of sewage disposal systems
• Loss of public utilities such as electricity or gas
• Floods from collapsed dams
• Release of hazardous material
• Fires
• Spread of chronic illness

Question 22

Earthquake predictions

Answer 22

* no accurate method - not 100% reliable *
Short term
1) foreshocks and animal behaviour 
2) radon gas emissions
3) laser and levelling
Long term
1) seismic gap theory

Question 23

foreshocks and animal behaviour in predicting earthquakes

Answer 23

• Generally, major earthquakes are preceded by minor shocks.
• risk of false alarms are high as plates break at different positions - not a guaranteed precursor
• strange animal behaviour due to responding to P waves (twice as fast as S waves)?
• almost all cases are anecdotes and depends on impression of “strange animal behaviour”

Haichiang earthquake 1975

• there was increased seismicity in the area Dec 1974 to Feb 1975
• Hibernating snakes were abandoning their hideouts

Question 24

radon gas emisisons in predicting earthquakes

Answer 24

• Uranium-bearing rocks in the Earth’s crust emit minute amounts of radon (colourless, odourless gas) that forms from the decay of uranium
• As faults shift in the days leading up to earthquakes, they create new openings from which radon can escape
• has a very short half-life meaning that increased concentrations must have occurred recently
• Due to reliance on cracks in the crust for radon to move through rocks, it makes sense that high radon content implies an earthquake is approaching
• However, tests in the 1970s and 1980s found the radon concentrations unreliable - often there are higher radon concentrations when there are no earthquakes

Question 25

laser and levelling in predicting earthquakes

Answer 25

• on one side of the fault, there would be an obvervatory housing a laser that emits a laser beam to a laser reflector on the other side
• it can measure changes in the relative elevation of land either side of a fault land
• accurate methods
• however, there are very few of these set along major fault lines (San Andreas fault), for tiny variations of elevation, to be measured thoroughly enough to reliably predict an earthquake

Question 26

seismic gap model in predicting earthquakes

Answer 26

• in the seismic gap model, the net big earthquake should be expected not in the segments where recent seismicity has relieved the strain, but in the intervening gaps where the unrelieved strain is the greatest
• However, there may be no information in seismic gaps about the time of occurrence or the magnitude of the next large event in the region

Question 27

Monitoring earthquakes

Answer 27

1) seismometers
If more are installed in a local area, earthquake locations for local events can be determined from these local stations
2) ground deformation
small-scale uplifts, subsidence or ground tilt measured by tilt meters
3) radon gas emissions
increase –> likely there will be earthquake

Question 28

Earthquake preparations to reduce impacts

Answer 28

1) Hazard-resistant buildings
2) Hazard mapping & land-use planning
3) education and communication preparedness
4) emergency service, insurance and aid

Question 29

Hazard-resistant building structure

Answer 29

3 types

1) putting large rubber shock absorbers in building foundations which allow movement of the building
2) large concrete weight on the roof which is controlled by a computer program and will move in the opposite direction of the earthquake to counteract stress
3) adding cross-bracing to the structure to hold it better when it shakes

Question 30

Hazard mapping and land-use planning

Answer 30

• Hazardous areas with previous tectonic activities should be identified and regulated in terms of land use
• certain types and qualified building should be built in those areas in order to reduce death rate
• certain areas with land vulnerable to liquefaction should not be built on
• important places like hospitals and schools should be surrounded by open spaces and away from fires

Question 31

What are Hazard Maps?

Answer 31

• Hazard maps show the distribution of risks that have a certain probability of occurring in an area.
• These maps were created to provide accurate and detailed information to assist engineers in designing buildings, bridges and highways that will withstand hazards.
• These maps are used to create and update building codes that are now used by many governments.
• A normal member of the public may be interested in hazard maps to avoid living in a location where significant shaking or eruptions are probable.

Question 32

Limitations of Hazard Maps

Answer 32

• However, hazards don’t really PREPARE people for what is likely to occur during a quake or eruption.
• Other forms of preparedness are more effective for assisting people in the event of a quake or eruption.
• Generally hazard maps have limited use for the general public. In the case of Japan…