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why are some areas more at risk to tectonic hazards (2) - Coggle Diagram
why are some areas more at risk to tectonic hazards (2)
earthquakes
-Benioff zone: area where earthquakes originate in the subduction zone
-process to cause an earthquake:
1- 2 tectonic plates converge or diverge causing friction
2- friction builds up between plates and they slow or stop creating a slip deficit
3- pressure from mantle convection, basal drag, etc. is being applied to the plates until the pressure overpowers the friction between plates
4- the plates 'jolt' into position releasing seismic waves
-earthquakes are on all 3 plate boundaries
-but the most are on destructive at destructive plate boundaries
-only 95% of earthquakes occur at plate boundaries
-constructive earthquakes:
-shallow focus (depth < 70 km)
-due to plate movement
-low risk - low magnitude
-destructive earthquakes:
-shallow, intermediate and deep earthquakes
-friction in Benioff zone
-high risk - high magnitude
-can be caused by plate crumpling
-shallow focus = higher damage
-conservative earthquakes:
-shallow earthquakes
-caused by friction as plates pass by each other
-high risk due to higher mgnitudes
-shallow focus - no subduction
-focus: the point inside the lithosphere where seismic waves radiate from
-epicentre: the surface of the earth right above the focus
-P waves: (primary/pressure waves)
-body waves (travel through earths body)
-from the focus
-push through the crust, mantle and core
-shaking in an backwards to forwards motion
-fastest as the first to reach the epicenter
-only damaging in the most powerful earthquakes
-S waves:(secondary/shear waves)
-body waves
-travel from the focus
-shake through crust and mantle
-travel in a sideways motion at 90' angles to direction of travel
-60% slower than P waves - reach epicentre later
-cause more damage than P waves - amplitude larger
-L waves:(love/surface waves)
-travel along the earths surface
-travel from epicenter triggered by S waves
-shake along ground surface only
-in a sideways motion at right angles to direction of travel
-slower than P and S waves
-cause more damage than P and S waves
volcanic eruptions
-there are 60 globally every year (1500 volcanoes active today)
-they kill 500 people / year
-volcanic eruptions:
-openings of the crust through which magma erupts as lava
-plate movement allows magma to rise at some boundaries:
-constructive - in rift between plates
-destructive - where subduction ocures
-hot spot volcanoes:
-may occur in interplate locations away from their margins
-located over a mantle plume (rising current of unusually hot magma burns through lithosphere)
-eg, Killauea, Hawaii - 1983 to present day:
-hot spot in the middle of the Pacific plate 3000 km from nearest plate boundary
-primary hazards:
-lava flows:
-up to 1170'c can take years to cool, destroy everything in their path
-pyroclastic flows:
-dense hot rock and lava, ash, gasses
-tephra and ash flow:
-pieces of rock and ash that blast into the air
-smallest pieces travel far causing poor visibility and slippery roads
-secondary hazards:
-lahars:
-masses of rock, mud and water that travel quickly
-largest can be hundreds of metres
can flow at 10s of m/s
-caused when snow / ice melt or heavy rainfall causes a surge downslope
-jokulhlaup:
-eruption melts glacier ice
-causes floods
-also called glacial outburst floods
-can catch people unawares
-gas eruptions:
-dissolved gases in magma released in eruptions
-include water vapour (80%) , CO2 and sulphur dioxide
-can travel 1000s of metres
-volcanic explosivity index (VEI):
-measures eruption magnitude
-factors in:
-volume and height of erupted material
-duration of eruption
-description (gentle or violent)
-ranges from 0 (non explosive), to 8
-VEI 0-3:
-basaltic magma at constructive boundaries, hotspots
-low viscosity, gas escapes freely
-effusive low risk eruptions
-VEI 4+:
-andesitic magma at destructive boundaries
-high viscosity, traps gases
-eruptions explosive high risk
-predicting volcanic eruptions:
-volcanic eruptions are easier to predict than earthquakes
1- monitor frequency and strength of seismic tremors
2- thermal imagery (IR) - changes in temp = volcanic eruption
3- gas detection - ppm of sulphur dioxide
4- ground deformation - magma changes volcano shape (tiltmeter and GPS)
tsunamis
-seismic sea waves
-a series of abnormally large waves over several hours
-small tsunamis occur almost every day with little impacts
-large tsunamis are rare but impacts are enormous
-where do tsunamis occur:
-originate along plate margins - the Pacific 'ring of fire'
-72% are a secondary hazard of submarine mountain ranges
-other causes of water displacement:
-volcanic eruptions (5%) - Anak Krakatua, 2018, Indonesia
-landslides (coastal / submarine)
-glacial breakup and meteorites
-Lituya bay 1958 was the largest wave in recorded history caused by earthquake triggered landslide
-large tsunamis can be very destructive:
-most deaths from drowning
-other causes include:
-building collapse
-floating debris strikes eg, trees
-land, food and water are contaminated with pollutants, waste and seawater
-DART: (tsunami warning system)
1- recorder on seabed monitors changes in pressure it can detect tsunamis as small as 1cm
2- acoustic link transmits data to moored surface buoy
3- data relayed to satellite
-because of the Indian ocean 2004 tsunami DART has been installed across the Indian ocean
-in the Sulawesi 2018 tsunami the warning system failed to save 1000s
multiple hazard zones
-multiple hazard zones:
-aka disaster hotspots
-very vulnerable area:
-prone to a large range of natural hazards both geophysical and hydrometeorological
-hazards can interact compounding into worst risks
-hurricanes, cyclones and typhoons are the same
-Haiti and Philippines are MHZs:
-mount Pinatubo Philippines 1991:
-VEI 6
-20,000 lives saved by successful management
-super typhoon Haiyan Philippines 2013:
-worlds strongest storm ever recorded at landfall (320km/hour winds)
-Philippines:
-poor communites live in high risk locations - steep hillsides
-Pinatubo1995 eruption and typhoon yunya 1991:
-lead to rain saturated ash - lahars
-ash roof collapse
-Bohol 2013 earthquake:
-mag 7.3
-222 dead
-270,000 homeless
-mixed with typhoon Haiyan rising death toll to 6,201 dead