9.1 Earthquakes

Question 1

what are the different plate margins and what occurs at each

Answer 1

destructive: plates meet, oceanic+continental, 2 oceanic, 2 continental
constructive: plates move away
conservative: plates move past each other at different speeds

Question 2

what is the global distribution of earthquakes

Answer 2

• broad belts associated with subduction zones - dense oceanic beneath less dense continental
• narrower
  belts of earthquakes are associated with constructive plate margins, where new
  material is formed, and plates are moving apart
• also some related to hotspots
• on all margins

Question 3

why do earthquakes occur?

Answer 3

• plates do not perfectly fit into each other, meaning they do not move in fluid motions
• all all boundaries, plates can become stuck due to friction between plates
• when the plates are stuck, convection currents in the asthenosphere continue to push which builds the pressure
• builds so much cannot be sustained and plates eventually give way and all the pressure is released in a sudden movement, causing a jolting motion

Question 4

where are volcanoes located

Answer 4

• volcanoes occur on destructive and constructive margins

Question 5

how are earthquakes measured

Answer 5

1. Richter scale:
   - measures magnitude using logarithmic scale, using seismographs
   - outdated for large ones, but useful for smaller
2. The Moment Magnitude Scale (Mw):
   - measures total energy released based on seismic movements and more accurate for large earthquakes than Richter scale
3. The Mercalli Scale:
   - measures intensity of earthquake, subjective
   - useful for assessing damage/human impact

Question 6

what are the 4 types of earthquakes

Answer 6

1. shallow-focus: conservative/constructive
2. intermediate-focus: destructive (benioff zone)
3. deep-focus: destructive plate boundary - subduction (force of slab pull)
4. intraplate: not at a plate boundary, e.g at a hotspot

Question 7

difference between focus and epicentre and how does magnitude vary with it

Answer 7

• focus is point underground where earthquakes originate from, epicentres is area above ground that is directly above the focus
• magnitude/damage by earthquake increases the shallower the focus/depth of earthquake is as waves more able to reach surface and so are stronger

Question 8

Turkey-Syria earthquake 2023

Answer 8

Factors contributing to high death toll:
- freezing temps/rainfall
- only 19km below surface (shallow focus)
- epicentre in urban area
- large scale
- loss of power made recovery difficult
- wealth: Syria poorest region/conflict
- frequency: lack of experience and 2 close together
- at conservative plate boundary

Question 9

different types of seismic waves

Answer 9

• body waves come from focus, low amplitude/low impact:
  Primary waves - ground goes up and down
  Secondary waves - ground goes side to side
• surface waves, come from epicentre together, creating a twisting motion:
  Love waves
  Rayleigh waves

Question 10

Primary and secondary hazards of earthquakes

Answer 10

Primary:
- ground shaking
- surface faulting

Secondary:
- ground failure and soil liquefaction
- landslides and rockfalls
- debris flow and mudflow
- tsunamis

Question 11

how do tsunamis form?

Answer 11

• As the sea bed jolts, water is displaced and forced upwards creating a wave
• As the waves approach the land they slow and the wavelength becomes compressed
• leads to an increase in wave height: they frequently reach 5-10 metres, but can reach 30 metres
• wave reaches the shore a vacuum is created and the water recedes rapidly out to sea leaving the sea bed exposed

Question 12

factors effecting damage caused by landslides

Answer 12

• unpredictability/difficulty to manage = large amounts of damage
• development levels/wealth: informal settlements build on undesirable/unconsolidated land, likely less prepared = high vulnerability, likely to not have management strategies
• remote communities cut off - die without being directly hit

Question 13

what is liquefaction

Answer 13

• when soil is saturated, the vibrations of an earthquake cause it to act like liquid
• soil becomes weaker and more likely to subside when it has large weight on it

Question 14

impacts of liquefaction

Answer 14

• less deadly than landslide: only occurs in very specific areas: loose, saturated or sandy soil
• secondary impacts: pipes rising to surface = possible gas leaks cause fires/sewage leaks

Question 15

three ways of classifying impacts

Answer 15

hydrostatic: things picked up and carried inland by waves
hydro-dynamic: force of the water tears things apart
shock effect: debris in the water creates a battering force

Question 16

Physical factors influencing impacts of earthquakes

Answer 16

1. location of epicentre: most important:
   - links to secondary hazards: if by sea lead to tsunamis, mass movement if on unconsolidated land - rock type
   - other factors only a problem, e.g population density, depending on location
   - if there were no people = no hazards
   - most coincide with plate margins: much greater risk from destructive margins earthquakes than constructive ones
2. depth of focus:
   - shallower depth = stronger magnitude = stronger impacts, but again depends on location as impact only if people are there
3. distance from epicentre:
   - further away, strength of waves dissipate, strongest at centre
4. frequency:
   - impact effectiveness of response: more frequent = more prepared but also less time to recover
5. Magnitude:
   - impact size of impacts
6. Type/Nature of a secondary hazard:
   - arguably also most important, as secondary hazards, e.g tsunamis cannot be protected against so wealth etc doesn’t help, but location does
7. Rock type:
   - impact secondary hazards: liquefaction but also controlled more by location
   - impact if landslide occurs
8. duration

Question 17

Human factors influencing impacts of earthquakes

Answer 17

1. Level of wealth (most important):
   - link to community preparedness: more education/better communication = better drills - e.g earthquake evacuation practice in Japan on 1st September every year
   - but type/nature and distance from epicentre more important as preparation does nothing for hazards, e.g tsunami but does help with recovery/reduce long term impacts
2. Building style and landuse:
   - links to wealth: lower wealth = more agricultural land = more susceptible to mass movement
   - stronger buildings, e.g brick, more likely to survive
   - even in wealth areas, older properties still vulnerable, e.g houses in Kobe, Japan in 1995 led to high death toll of over 6,500
3. Population density:
   - impact number of people affected - number of deaths
   - link to wealth: informal settlement - rural vs urban
   - also links to location
4. Efficiency/organisation of emergency services:
   - most important for response but controlled by wealth
   - also linked to location, as this can influence access for recovery

Question 18

how can a tsunami wall help to modify earthquake impacts? (pros/cons)

Answer 18

Pros:
- protects area behind from flooding: decreasing damage to infrastructure/loss of live: ec/soc benefit
- provides security/calm for residents: social
- potential to build on it to increase its height?
- not many benefits, but the ones there are are big

Cons:
- may not be tall enough, especially over time due to climate change, sea level rise (thermal expansion): e.g Japan 2011, wall 8m high but wave reached 12 to 15m
- traps water that does reach other side of wall - prolonging recovery
- expensive to build: HIC vs LIC, local differences (rural vs urban) - rural areas likely high population density too
- inequality between areas with wall vs without and wall has to stop at some point
- not aesthetically pleasing/environmental issues of concrete
- impact on fishing industry/tourism and still have to access the coast = gap in wall at some point - tsunami funnel here?

Question 19

ways to make a building earthquake proof

Answer 19

1. base isolation:
   - buildings placed on flexible bearings/rubber pads to absorb seismic shocks, reducing amount of movement
2. Lattice work steel:
   - strengthen walls and helps build strength/resist lateral forces
3. Shock absorbers:
   - help to reduce swaying in high rise building by absorbing energy
4. Reinforced foundations::
   - prevents liquefaction
5. lightweight materials:
   - using wood/carbon fibre reduces force of impact if collapses and subject to smaller forces
6. Flexible joints:
   - in bridges/pipelines to allow movement without breaking

Question 20

ways to make an earthquake proof house

Answer 20

1. light walls and gables:
   - subject to smaller forces and so less likely to fall when ground shakes, e.g Pakistan straw buildings held together by nylon netting
2. light roofs:
   - Haiti heavy concrete roofs collapsed on many homes; sheet-metal roofs on wooden trusses more resistant
3. small windows:
   - small, regularly spaced - fewer weak spots in walls
4. reinforced walls:
   - not necessarily metal - eucalyptus/bamboo, e.g Peru walls reinforces with plastic mesh
5. shock absorbers:
   - tyres filled with stones/sand fastened between floor/foundation - cheap
6. confined masonry:
   - brick walls framed/connected to roof by corner columns and crown beam of reinforced concrete - structure moves as a unit