Tectonic Hazards Flashcards
Fault line
Crack/boundary between two tectonic plates
Hypocentre
Focus
The point at which the seismic waves are released
Epicentre
Directly above the hypo-centre. The point on the earths surface which has the most shaking and energy
Seismic waves
Ripples of energy released from the hypocentre
What are p waves?
They are primary waves that are caused by compression - pushing and pulling in the direction of travel
They are the fastest and arrive first
What are S waves?
Slower and only move through sold rocks, up and down movement
60% of the speed of p waves
Only through crust
What are L waves?
Love waves
From epicentre, only travel through crust, fastest surface wave
Moves from side to side as it moves forward
Causes the most damage due to longer wavelength and focus of energy at the surface
Moment magnitude scale
Measures the magnitude so the power by measuring how much energy is being released
Scale is logarithmic - each level is x10 more powerful
What are factors that explain the intensity of an earthquake?
Magnitude
Distance from the hypocentre
Bedrock and soil - pwaves travel fast through hard rock
What is the modified mercalli scale?
Reflects the effect and intensity of earthquake
Subjective scale
What does shaking depend on?
magnitude of quake
distance from epicentre
depth of focus (focal length), e.g. Haiti and
Christchurch
rock type (amplification) e.g. Haiti and
Christchurch.
Secondary hazards
Tsunami
Mudslides
Sink whole
Landslides
Primary hazards
Caused by the initial processes - ground shaking and crustal fracture
Secondary hazard
Are the side effects or the knock on impacts of primary hazards
What is liquefaction?
Loosely packed grains of soil are held together by friction, pores spaces fill with water
Shaking destabilises the soil by increasing the space between grains, with its structure lost the soil flows like a liquid
The soil then moves downslope which makes the foundation unsupported which causes buildings to sink and collapse
How do slope process and earthquake relate?
Mass movements
Occur naturally at coasts
Earthquakes with magnitude over 4 can increase likelihood as the shaking puts more stress on to the slope causing it to fail
What is the distribution of tectonic hazards?
Wide spread but mostly surrounding the ring of fire
- some hazards aren’t located due to hotspots
Intra plate
Hazard that occurs in the middle of a plate
Plate tectonic theory
The theory that the earths surface is a huge jigsaw of irregular slabs which are moving constantly
Inner core
6000 degrees
Inner - solid iron
Most dense
Outer core
Liquid nickel and iron - swirling around making magnetic swirls
4500-6000 degrees
Semi molten
Mantle
Widest layer
Asthenosphere = solid below this is molten where plates float
Crust
Continental plates are older, thicker but less dense
Oceanic are thin but more dense - basalt
Outer shell is solid rock
Convergent plate boundary - oc
Oceanic meets continental Subduction Mantle, melts Deep ocean trenches Fold mountains Frictional drag - intermediate and deep earthquakes in Benioff zone -volcanic eruptions
Convergent plate boundary - oo
Denser is subjected
deep ocean trenches form
subducted melts creating magma and rises through vent
= underwater volcanoes
Convergent plate boundary - cc
both plates have same density
less dense then the asthenosphere beneath them
neither plate sub ducted - collide and sediments crumpled and forced to form fold mountains
-Himalayas
Divergent
plates moving apart to form new crust
Oceans = forms mid ocean ridges
Continents = rift valleys
Conservative
two plates slide past each other creates major faults in crust very active and powerful earthquakes Cause stress and pressure build up Suddenly released as earthquake San Andreas
Intra plate - earthquakes
Earthquakes that occur in the interior of plates
occur along old fault lines which reactivate
collision of plates can also fracture the crust away from boundary
Intra plate - hotspots
Volcanic regions have underlying mantle lava that is hot
Position on earths surface is independant
Result from the upwelling of hot molten material from core
-high heat and low pressure at base of lithosphere enable melting of rock
-Magma rises through cracks in crust and erupts
-as the plate moves over the stationary hotspot, form chain of volcanic islands
Wegener plate tectonic theory
suggested that mountains formed when the edge of a drifting continent collided with another causing it to crumple and fold
-heat is produced by core due to radioactive decay
Harry Hess theory
Used sonar to survey the seafloor and discovered that it wasn’t flat and that there were mountain ranges in middle of ocean
- lead to the discovery that the ocean floor is getting older and closer to the coast
- newer crust must be getting produced by molten rock rising from inside of earth
Ridge push
Magma pushes up as earth splits and pushes crust outwards
Slab pull
Heavy weight of the crust pulls it downwards towards subduction zone at the edge of the ocean
Convection current
Creates frictional drag
radioactive decay or uranium
creates convection cells
seafloor spreading
Features of a tsunami
Long wavelengths - 150 - 1000km Low height - 0.5-5m until they reach shallow bed fast velocity series of waves Ring of fire most common 60% of wave energy reaches shore
As tsunami wave gets closer to shallow sea bed
friction of sea floor makes wave slow down but rise in height
High risk zones
East Asia, south of Asia
ring of fire
Formation of a tsunami
Subduction at the convergent margin
Elastic energy and tectonic strain builds up in Benioff zoner
Energy is released - elastic rebound and cause seabed tp thrust upwards
Shallow sea causes wave to slow down due to friction with sea bed but to grow in height
Factors affecting severity of tsunamis - human
- Quality of warning systems
- Degree of coastal development - pop size, tourism, industry
- Structural integrity of buildings to withstand wave
- Timing of the event
Factors affecting severity of tsunamis - physical
- Wave amplitude, amount of water displaced, velocity, distance travelled
- Physical geography of coast - water depth, gradient of shoreline
- ocean topography - deep water retain more energy
- degree of coastal eco-system buffer - mangroves, coral reefs, barrier islands
Liquefaction
where water rises during earthquakes causing rock to slide around more and cause buildings to collapse
Methods used to measure and research tsunamis
Recorder on seabed measure changes in pressure and can detect tsunamis as small as 1cm
- an acoustic link is transmits this data to a surface buoy
- data relayed to satellite
- satellite transmits data to ground stations
How do scientists predict tsunamis?
Systems use seismic sensors to detect undersea earthquakes
DART - Deep ocean assessment and reporting or tsunami
What is DART?
Uses seabed sensors and surface buoys to monitor changes in sea levels and pressure
when a wave is detected, the system sends the information via satellite to tsunami warning stations
-stations review and estimate the size and direction of the tsunami before warning risk areas
-Early warning systems
What is the tsunami intensity scale?
12 point scale
It is arranged according to a tsunami effects on humans, objects and damage to buildings
Rough correlation with tsunami wave heights
Which plate boundaries do volcanoes occur on?
Convergent - subduction creates friction
Divergent - magma rises as plates move apart
Hotspots- mantle plumes
Features of a volcano
Magma rises from the chamber through the crust via the conduit
Magma reaches the crater
Cloud of ash (tephra)
Layers of ash and lava make up the sides of the volcano
Fumaroles - holes in the ground that give out sulphur
Lahars, ash clouds, acid rain
Primary hazards of volcanoes tend to …
have a long geographical reach
-tend to kill less people than earthquakes but single events can be catastrophic
The more viscous the lava, the more…
explosive it is
Basaltic lava features
Hottest 1000 degrees Low in silica, water, gas and aluminium High in CO2 iron and magnesium -thin and runny -low viscosity as gases escape -eruptions are gentle and effusive
Basaltic lava - formation
By the melting of the mantle minerals (olivine) mostly from the upper zone but some from the core-mantle boundary
Basaltic lava - where is it found?
Ocean hotspots, mid-ocean ridges, shield volcanoes (Mauna Loa)
Andesitic lava - features
800 degrees
Intermediate silica, gas content, magnesium and iron
high water and hydrochloric acid
low SO2
-slow flow, intermediate viscosity traps gases
-eruption is violent and moderately explosive
Andesitic lava - formation
By sub-ducted oceanic plate melting and mixing with seawater, lithospheric mantle and continental rocks
Andesitic lava - where is it found?
Composite cone volcanoes - Chances Peak
Subduction zones
Rhyolitic lava - features
Coolest 650 degrees High silica, potassium, sodium, aluminium, gas content Low iron and magnesium -Flow is thick and stiff -High viscosity traps gases -Eruption is very violent, cataclysmic
Rhyolitic lava - formation
By melting of lithospheric mantle and sales of previously subjected plate
Rhyolitic lava - where is it found?
Super-volcanoes (Taupo), composite volcanoes
Composite cone volcano
Form from viscous lava (rhyolitic, andesitic)
Usually at subduction zones
Steep sided and formed of layers of lava and ash
More likler to have pyroclastic flows
Shield volcanoes
Formed from lava with low viscosity (basaltic)
Usually at ocean hotspots and mid ocean ridges
Less steep and very wide
Form from layers of lava
Primary hazards of volcanoes
Lava flows
Emissions of gases or steam (phreatic eruption)
Phreatic eruptions
Steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits
Secondary hazards of volcanoes
Pyroclastic flows
Lahars
Jokulhlaups
Lava
- Molten rock/magma that has reached the Earths surface
- flows - depends on type
- Can outrun unless lava lake drains
- Burns and destroys settlements
Pyroclastic flows
Dense mixture of poisonous gas and tephra 1000 degrees 700km/h Explosive from composite volcanoes Turbulent, fast-moving ash clouds
Tephra
Molten and solid rock ejected from explosive eruptions
Ash
Small fragments
Travel higher and further
Transported by jet streams
Volcanic gas
Water vapour seeps from crack, produces further fumaroles and geysers
Water mixes with sulphur dioxide - produced acid rain and particles reflect solar radiation
Lahars
Rainwater mixes with ash to create fast moving mud flows
Jokulhlaups
Floodwater resulting from rapid melting of ice caps/glaciers (E15 Iceland)
How are volcanic eruptions measured?
VEI - volcanic explosively index
Measures the volume of products, eruption cloud height, qualitative observations
Open ended scale- largest in history 8
Logarithmic - with each interval on the scale representing a tenfold increase
PAR Model
Pressure and Release model
Risk equation
Risk = hazard x vulnerability
Risk reduction equation
Risk reduction = mitigation of hazard x reduction of vulnerability and increasing capacity to cope (resilience)
Risk
The probability of harm or loss taking place
Risk/damage threshold
the point at which risk leads to damage
Mitigation
Means finding new ways of being prepared for possible tectonic hazards so that their impacts can be prevented or reduced
How to reduce vulnerability?
Management policies by government
Educate and train
Develop warning and evacuation systems
Good infrastructure for housing
Where must the action to reduce vulnerability come from?
The government as individuals or communities may lack the capacity to make significant improvements on their own, only better governance can help reduce vulnerability
Root causes
Corrupt government
Lack of access to resources
Dynamic pressures
Lack of investment
Young, dependant population
Unsafe conditions
Close to capital
Bad infrastructure
low income
Diseases
Governance and political conditions that impact vulnerability
- enforcement of building codes and regulations - determine the safety and quality of buildings
- quality of existing infrastructure - can impact recovery speed
- disaster preparedness plans - how quickly and effectively response is
- emergency services
- communication systems - inform and coordinate people in advance
- public education and practised hazard responses
- level of corruption