9.1 Earthquakes Flashcards
(22 cards)
what are the different plate margins and what occurs at each
destructive: plates meet, oceanic+continental, 2 oceanic, 2 continental
constructive: plates move away
conservative: plates move past each other at different speeds
what is the global distribution of earthquakes
- 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
why do earthquakes occur?
- 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
- size of earthquake depends on thickness of descending slab and rate of movement
where are volcanoes located
- volcanoes occur on destructive and constructive margins
how are earthquakes measured
-
Richter scale:
- measures magnitude using logarithmic scale, using seismographs
- outdated for large ones, but useful for smaller -
The Moment Magnitude Scale (Mw):
- measures total energy released based on seismic movements and more accurate for large earthquakes than Richter scale -
The Mercalli Scale:
- measures intensity of earthquake, subjective
- useful for assessing damage/human impact
what are the 4 types of earthquakes
- shallow-focus: conservative/constructive
- intermediate-focus: destructive (benioff zone)
- deep-focus: destructive plate boundary - subduction (force of slab pull)
- intraplate: not at a plate boundary, e.g at a hotspot
difference between focus and epicentre and how does magnitude vary with it
- 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
Turkey-Syria earthquake 2023
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
different types of seismic waves
- 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 : earth to move sideways
Rayleigh waves: earth to move up and down
Primary and secondary hazards of earthquakes
Primary:
- ground shaking
- surface faulting
Secondary:
- ground failure and soil liquefaction
- landslides and rockfalls
- debris flow and mudflow
- tsunamis
how do tsunamis form?
- sub-marine earthquake displaces seabed vertically as a result of movement along fault line at subduction zone
- violent motion displaces large volume of water in ocean water column which moves outwards from point of displacement
- As the waves approach the land they slow and the wavelength becomes compressed as shoaling occurs, firstly drawing water from shore then hitting coast with tremendous force
- wave reaches the shore a vacuum is created and the water recedes rapidly out to sea leaving the sea bed exposed
factors effecting damage caused by landslides
- 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
what is liquefaction
- when soil is saturated, the vibrations (P-waves) of an earthquake cause it to act like liquid
- soil becomes weaker and more likely to subside when it has large weight on it
impacts of liquefaction
- 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
three ways of classifying impacts
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
Physical factors influencing impacts of earthquakes
-
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 -
depth of focus:
- shallower depth = stronger magnitude = stronger impacts, but again depends on location as impact only if people are there -
distance from epicentre:
- further away, strength of waves dissipate, strongest at centre -
frequency:
- impact effectiveness of response: more frequent = more prepared but also less time to recover - Magnitude:
- impact size of impacts -
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 -
Rock type:
- impact secondary hazards: liquefaction but also controlled more by location
- impact if landslide occurs - duration
Human factors influencing impacts of earthquakes
-
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 -
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 -
Population density:
- impact number of people affected - number of deaths
- link to wealth: informal settlement - rural vs urban
- also links to location -
Efficiency/organisation of emergency services:
- most important for response but controlled by wealth
- also linked to location, as this can influence access for recovery
how can a tsunami wall help to modify earthquake impacts? (pros/cons)
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?
ways to make a building earthquake proof
-
base isolation:
- buildings placed on flexible bearings/rubber pads to absorb seismic shocks, reducing amount of movement -
Lattice work steel:
- strengthen walls and helps build strength/resist lateral forces -
Shock absorbers:
- help to reduce swaying in high rise building by absorbing energy -
Reinforced foundations::
- prevents liquefaction -
lightweight materials:
- using wood/carbon fibre reduces force of impact if collapses and subject to smaller forces -
Flexible joints:
- in bridges/pipelines to allow movement without breaking
ways to make an earthquake proof house
-
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 -
light roofs:
- Haiti heavy concrete roofs collapsed on many homes; sheet-metal roofs on wooden trusses more resistant -
small windows:
- small, regularly spaced - fewer weak spots in walls -
reinforced walls:
- not necessarily metal - eucalyptus/bamboo, e.g Peru walls reinforces with plastic mesh -
shock absorbers:
- tyres filled with stones/sand fastened between floor/foundation - cheap -
confined masonry:
- brick walls framed/connected to roof by corner columns and crown beam of reinforced concrete - structure moves as a unit
why does depth of focus vary in different locations
Type of plate margin:
- Destructive Plate Margins (Convergent): deep-focus earthquakes are common due to the subduction process, as oceanic crust sinks into mantle, generates earthquakes at progressively deeper levels.
- Conservative Plate Margins: Shallow-focus earthquakes are more likely stress is relieved through fault movement closer to the surface.
- Constructive Plate Margins (Divergent):Where plates move apart, earthquakes are typically shallow and caused by magma rising to fill the gap between the plates.
why does level of ground shaking decrease with distance from the epicentre?
- energy released during earthquake causes ground shaking, with high level of energy and so greatest level near epicentre
- energy is absorbed, particularly by solid geology as shock waves radiate outwards from epicentre, so intensities decrease with distance
