9.1 Earthquakes Flashcards
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
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- outdated for large ones, but useful for smaller -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
Rayleigh waves
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?
- 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
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 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
volcanoes on destructive plate margins
- usually explosive due to high pressure magma is under
- composite volcanoes formed here
volcanoes on continental/oceanic destructive plate
- denser oceanic plate subducts below continental
- plate subducting leaves an ocean trench
- oceanic crust is melted as it subducts into the astenosphere
- extra magma created causes pressure to build up
- pressurised magma forces through weak areas in the continental plate
- explosive, high pressure volcanoes erupt through the continental plate
Oceanic/oceanic destructive(convergent) volcanoes
- heavier plate subducts leaving an ocean trench
- built up pressure causes underwater volcanoes bursting through oceanic plate
Volcanoes on constructive/divergent plate boundaries
- effusive
- magma is under less pressure, so lava flows more freely
- shield volcanoes are mainly formed from these eruptions
Oceanic/oceanic divergent volcanoes
- magma rises in between the gap left by the two plates separating, forming new land when it cools
- less explosive underwater volcanoes formed as magma rises
- new land on ocean floor by lava filling gaps = due to sea floor spreading
Continental/continental divergent volcanoes
- any land in the middle of the separation is forced apart = rift valley
- volcanoes form where magma rises
Type of magma at constructive PM
Basic (pahoehoe/aa)
Type of magma at destructive PM
Acidic (rhyolite)
Characteristics of lava at constructive PM
Basic:
Non viscous
Low silica
Flows further
Characteristics of lava at destructive PM
Acidic rhyolite:
Viscous
Acidic
High silica
Doesn’t flow very far
hazards at constructive PM
lava
hazards at destructive PM
Ash
Bombs
Lava
Gas
Pyroclastic flow
secondary hazards of volcanoes
Lahars
Landslides
Jokulhaups
Tsunamis
Acid rain
Global temperature change
source of water for lahars
intense rainfall
glacier melt
snow melt
dam failure
how can modification be used to protect against volcanic eruption
- reinforced roof to hold the weight of ash
- air tight buildings keeps ash out
-well maintained rain gutters - house on stilts of titanium, they may be strong enough to withstand lava
- cold water hosed onto eruptions to cool them (iceland) and reduce flow length
modification for mount etna eruption
- lava flows were bombed and diverted away from settlements
- particularly in 1992, large concrete blocks were dropped into the lava tube from which the lava was flowing
- lava stopped before it reached the town Zafferana
