Hazards Flashcards
what is a hazard?
it is a potential threat to human life and property. it can either be hydro-meteorological or geophysical
what are hydro-meteorological hazards?
caused by climatic processes
what are geophysical hazards?
caused by land processes
where do geophysical hazards occur?
they occur near plate boundaries. these plates moved different speeds and directions which can cause collisions, earthquakes and volcanic activity.
where do earthquakes mainly take place?
greatest in the Pacific Ocean along the ring of fire. also the entire ocean is ringed by areas known as subduction zones.
other than near plate boundaries where else can earthquakes occur?
near the middle of plates (called an intra-plate). this is when pre-existing weaknesses become reactivated, forming seismic waves. for example, an intraplate earthquake may occur if solid crust, which has weakened over time, cracks under pressure
explain volcanic hotspots
- these occur ‘intra-plate’ meaning amongst the centre of plates
- this occurs when a localised area of the lithosphere (earth’s crust and upper mantle) has an unusually high temp due to the upwelling of hot molten material from the core. this was first theorised by Tuzo Wilson in 1963.
- This produces isolated plumes of convecting heat, called mantle plumes, which rise towards the surface, generating basaltic volcanoes that tend to erupt continuously.
- This over time can produce a chain of volcanic islands.
e.g. Kilauea, Hawaii
where do the most powerful earthquakes generally happen?
at convergent or conservative boundaries
what is the OFZ (oceanic fracture zone)?
this is a belt of activity through the oceans and along the mid-ocean ridges through Africa, the Red Sea and the Dead Sea
what is the CFZ (continental fracture zone)?
this is a belt of activity along the mountain ranges from Spain through the alps to the Middle East and to the Himalayas
what are the tectonic trends since 1960?
- the total number of recorded hazards, one of the main reasons for this is due to CC and global warming.
- the number of fatalities has decreased due to the increased protection and preparation
- the total number of people being affected by tectonic hazards is increasing, due to pop growth, increased density has increased vulnerability
- the economic costs associated with hazards and disasters has increased significantly. this is partly due to increases in development as infrastructure in more developed countries costs more to repair. increasing number of insurance policies, especially in developed countries, heightens the costs.
why is reporting disaster impacts (e.g. fatalities) very difficult and controversial?
- depends on whether you look at the direct deaths so those killed straight away or indirectly by looking at how many people died of diseases that spread after the disaster
- the location is important as rural and isolated areas are hard to reach and so it may be hard to collect data from them.
also areas of high pop densities may be hard to collect from - the number of deaths quoted by a government may be subject to bias. in the 2004 tsunami, the Burmese government claimed there were 0 deaths in Burma. this may be to try and show the government is doing a good job.
what are the four sections of the earth?
crust
mantle
outer core
inner core
what is the crust like?
also known as the lithosphere. the uppermost layer of the earth which is thinnest, least dense and lightest. oceanic crust is only 7km thick, whereas continental crust can be up to 70km thick
what is the mantle like?
may also be called the asthenosphere. widest layer. the mantle is semi-molten and a temperature gradient towards the core generates convection currents. this may contribute to the lithosphere’s plate tectonic movement. the mantle is at a depth from 700km to 2890km below the crust.
what is the outer core like?
dense, semi-molten rocks containing iron and nickel alloys. at a depth of 2890km to 5150km below the earth’s surface. temps of 4500-6000ºC
what is the inner core like?
similar composition to the outer core. its over 5150km below the earth’s crust. the inner core is solid iron due to the extreme pressures it experiences. temperature of over 5000ºC. the core’s high temperature is a result of:
- radiogenic heat produced form radioactive decay
- primordial heat left over from the earth’s formation
what is the lithosphere?
consists of the crust and the rigid upper section of the mantle and is approximately 80-90km thick. this is the section of the earth that is divided into seven very large plates and a number of smaller ones. the plates are divided into oceanic and continental plates.
what is the asthenosphere?
this lies beneath the lithosphere and is semi-molten on which the plates float and move
what causes tectonic plates to move?
radioactive reactions occur inside the core which produces convection currents in the mantle. this causes the tectonic plates to move. The friction between the convection current and the crust causes the tectonic plate to move.
where are volcanoes predominately located?
along the ring of fire which borders the indo-australian plate to the south west, the Eurasian plate to the north west and the North American plate to the west
- they are usually found at plate boundaries where plates converge or diverge, anomalies include hotspots.
what was wegner’s continental drift theory?
- in 1912 he published his theory that a single continent that he called Pangea, existed about 300 million years ago
- he states that the shapes of South America and Africa seem to fit together so were once part of a supercontinent
- he suggested mountains formed when the edge of a drifting continent collided with another causing the floor to crumble and fold
- took 150 years to be accepted as scientists still believed mountains are causing by cooling of the earth as well as it being difficult to find viable evidence
what are examples of Wegner’s continental drift theory?
- similar fossils found where these plates could have fit, but are now separated by oceans. remains of the reptile Mesosaurus found in both South America and southern africa. it is unlikely that the reptile could have developed over both areas or migrated across the Atlantic
- rock sequences in northern Scotland closely agree with those found in eastern Canada, indicating that they were laid down under the same conditions in one location
- land masses fit like a jigsaw
other than Wegner what is another piece of evidence for continental drift theory?
how seismic waves travel through the earth. along the wadati-benioff foci, the depth of waves shows subdiction of the denser basaltic oceanic plates into the upper mantle.
what are divergent plate boundaries?
- when plates move apart causing magma to rise through the cracks to form new land
- they form mid-ocean ridges (e.g. mid-atlantic ridge)
- sub-marine volcanoes form as magma rises, less explosive
- ridge push (the bubbling magma causes plates to move apart)
- creates great Rift Valley (continents) where crust is stretched and land between faults collapses
e.g. mid-atlantic ridge
what are convergent (oceanic - continental) plate boundaries?
- subduction occurs at the Benioff zones and an earthquake occurs from the friction of the two plates
- the oceanic plate is denser than the continental plate which causes the mantle to melt through wet partial melting
- this causes an explosive eruption of andesitic lava with high gas and silica content, volcanic plug
- fold mountains occur when the continental crust is pushed up
- tsunamis due to mega thrust, water column displacement
An example of a destructive plate boundary is where the Nazca plate is forced under the South American Plate
what is a locked fault?
when two interlocked rocks create pressure to then grind and create pressure and tension
what is a convergent (oceanic - oceanic) plate boundary?
- very similar to continental to oceanic but the oceanic plate that is denser is subducted leaving an ocean trench.
- through the friction in the Benioff zone it causes the melting of the mantle causing submarine volcanoes
- this creates island arcs and eventually an atoll, this is when lava cools above sea level and creates new land
an example of an island arc is the Japanese islands
- fold mountains will also occur
- tsunamis (low-lying coastlines)
what is a convergent (cont-cont) plate boundary? (collision boundary)
- aka collision margin
- because they have the same density less subduction occurs but instead a lot of pressure builds up which can result in a very powerful earthquake
- fold mountains (Everest) formed on top of lithosphere due to pressure from continental crust as sediment crumbled and is formed upwards
- ancient oceanic crust is subducted slightly, but there is no subduction of continental crust
- but if some layers are more compressed then some subduction occurs and focus is shallow
e.g. The Indian plate and Eurasian plate colliding to form the Himalayan mountain range
what is a conservative/transform plate boundary?
- aka transform boundary
- two plates moving parallel against each other in different directions or at different speeds, creating friction resulting in a high magnitude earthquake from its shallow focus
- rocks grinding past each other (San Andreas fault line) causes a locked fault which causes a transform fault (crustal fracturing), tension creates cracks which causes further earthquakes
- this can form deep trenches and ridges
e.g. san Andreas fault lin
what is paleomagnetism?
- It results from magma locking in the earth’s magnetic polarity when it cools. Scientists can use this to reconstruct past plate movements.
- The discovery in the 1960s by Hess of magnetic strips in the oceanic crust of the seabed which are palaemagnetic signals from past reversals of the earth’s magnetic field.
- It proves that new ocean crust is created by the process of sea-floor speading at mid-ocean ridges.
- He discovered using sonar that the seafloor isn’t flat, there were mountain ranges in the middle of it.
- This led to the discovery that the ocean floor is getting progressively older the closer to the coast and that newer coast must be produced by molten rock inside.
- He also found that the pushing at the ridge and the pulling at the convergent margin resulted in sea floor spreading
what are intraplate earthquakes?
- middle of the interior a plate, old fault lines can react
- earthquakes occur in mid-plate settings, are usually associated with major ancient fault lines being retracted by tectonic stresses
or when collision of plates can fracture crust in the interior - happens in the earth’s interior
- perhaps old collision theories, already has cracks and faults
what are intraplate hotspot volcanoes?
- volcanoes at a distance from the boundary
- isolated plumes of convecting heat (mantle plumes) rise towards the surface generating basalt volcanoes that erupt continuously, the plume is stationary but the plate moves slowly above it
- produces a chain of volcanic islands = atoll
- e.g. Kilauea, Hawaii
- causes shield volcanoes, low viscosity and effusive lava
- mantle plume goes through the cracks and melts the crust
what are the different mechanisms that could cause plate movement?
1) mantle convection- radioactive elements in the core of the earth decay which produce a lot of thermal energy. this causes the lower mantle to heat up and rise, as the magma rises it cools down becomes more dense and begins to sink back down to the core. these are CONVECTION CURRENTS. they push the plates
2) slap pull- This is where older and denser oceanic crust will subduct beneath less dense continental plate. The density of the oceanic plate pulls itself into the mantle. The pulling action drags the rest of the plate with it (first theorised by Dan McKenzie)
how have thoughts on the primary cause of late movement changed?
previously, convection currents were thought to be the primary cause of plat movement. however, researches now believe that slab pull is the primary mechanism for plate movement. connection currents seem to weak to move massively dense plates.
explain the forming of earthquakes
A locked fault on a plate which creates tectonic strain as they can’t grind past each other easily. This leads to a build up of pressure which eventually exceeds the strength of the plates leading to elastic rebound, which eventually causes the pressure to be released in the form of seismic waves which radiate from the hypocentre in the form of an earthquake. this can lead to crustal fracturing and the ground to shake.
what is the epicentre?
the area above ground that is directly above the hypo centre (shaking tends to be worse here)
what is the hypo centre?
the point at which tension and friction releases seismic waves
where do the most powerful earthquakes occur?
at destructive and conservative boundaries
how does an earthquake happen at a constructive boundary?
Plates move at different speeds, which builds pressure until plates crack, causing fault lines. This results in the release of energy in the form of seismic waves, producing earthquakes.
how does an earthquake happen at a destructive boundary?
one plate is forced under the other, getting stuck due to friction which produces energy. As plates suddenly jerk past one another, this energy is quickly released as large seismic waves, forming a powerful earthquake.
how does an earthquake happen at conservative boundaries?
plates lock with one another which, when pressure is built, produces sudden seismic waves.
explain the theory of plate tectonics
convection currents happen due to radioactive elements in the core of the earth decay (into uranium) which produce a lot of thermal energy. this causes the lower mantle to heat up and rise, as the magma rises it cools down becomes more dense and begins to sink back down to the core.
- sea floor spreading occurs (youngest at ridge and oldest at subduction zone)
- proved by palaeomagnetism (orientation of magnetic minerals to earth’s magnetic force)
- ridge push: magma pushes u[ as crust splits and pushes crust outwards
- slab pull: heavy weight of the crust pulls it downwards towards subduction zone at edge of ocean
what are the 4 types of seismic waves?
primary
secondary
Rayleigh
love
what are primary waves?
- they are caused by compression pushing and pulling in the direction of travel
- travels fastest at 4-8 km/s and arrive first
- side to side movement (crust, mantle and core)
what are secondary waves?
- Vibrate at right angles to direction of travel
- Travels only through solid rocks (up and down movements)
- Travels slower at 2.5-4 km/hr, 60% slower than P waves
- Shaking (crust and mantle only)
what are love waves?
- Only travels through surface of the crust
- Fastest surface waves with high amplitude
- Move from side to side
- Causes most damage due to longer wave length and focus of energy at the surface
- Travels at 2-6 km/hr
what are the strongest types of seismic waves?
Secondary and Love waves are the most destructive as they have large amplitudes.
Due to their different speeds, these different waves will hit a location at different speeds. The aftershocks the survivors feel are these different types of waves arriving after each other.
These waves can also result in crustal fracturing (producing faults), ground shaking and secondary hazards such as landslides, avalanches and liquefaction.
when will the intensity of seismic waves decrease?
they will decrease further from the epicentre as waves lose energy as they travel. However, this does not mean that impacts felt or damage caused will always decrease further from the epicentre as other factors affect a location’s vulnerability
what are the factors affecting vulnerability for an area?
- Geology
- Geographical location (whether the earthquake occurs nears the sea or intraplate)
- Education of locals
- Durability of buildings
- Mitigation
how are earthquakes measured?
moment magnitude scale
modified mercalli scale
what are the factors explaining the intensity of an earthquake?
- Seismic intensity = what you feel
- Proximity to the epicentre
- Types of soil / rock and distance from the epicentre
- Magnitude= the size of the earthquake and the energy intensity released
- Depth of focus
- Shallow hypocentre means higher intensity
- Soil substrate, if it is wet, soft or sandy then intensity will rise, liquefaction, soft saturated bay mud is x5 more damaging than bay rock
what is the mercalli scale?
- intensity reflected by effects/damage
- lower numbers correspond to little damage
- Very subjective scale based on where you are, closer to the epicentre there is more shaking
- Usually, the stronger the magnitude the greater the intensity but local factors can modify this, e.g. being closer to the epicentre, type of geology can amplify shaking
what is peak acceleration?
how quickly the ground shook, how swiftly it changed direction
what is a secondary hazard?
a hazard that is a result of a primary hazard, e.g. landslides, flooding, sinkholes, tsunamis
what is a primary hazard?
caused by the initial process, e.g, the earthquake, ground shaking, crustal fracturing
what are the secondary hazards of earthquakes?
soil liquefaction
landslides
slope processes/mass movement
tsunamis
explain the secondary hazard of earthquakes: soil liquefaction
Earthquake shaking causes the water pressure to decrease and the soil to lose mechanical strength and so the structure is lost and the soil acts as a liquid and bubbles up to the surface, which can cause building subsidence or landslides.
- Affects poorly compacted sand and silt
- Water moisture within the soil separates from the soil particles and rises to the surface
- The upwelling of liquid means that the soil underneath the buildings becomes muddy and the sink collapses. If it’s on a hill, the liquid soil can cause mud/landslides
explain the secondary hazard of earthquakes: landslides
- The shaking caused by the earthquake can weaken or damage cliff faces, hills and snow material
- Soil is loosened by rain or gravity, ground eroded by heavy rain causes rock to fall
- Unconsolidated material or loose rocks can collapse
- Landslides can travel several miles and accumulate material on the way
- Risk varies with topography, rainfall, soil and land use
explain the secondary hazard of earthquakes: tsunamis
- Tsunamis have a long wavelength (150-1000 km) and a low height in comparison (5m) until they reach the shallow bed and grow. The shallow sea bed slows down the lower proportion of the wave till the upper particles catch up causing it to grow taller.
- Tsunamis present additional damage to vulnerable communities such as accelerating coastal erosion, which is a case in Malibu in California
- They are generally generated in subduction zones at convergent plate margins. Most tsunamis are found along the ring of fire, hence the most vulnerable countries are often located in Asia or Oceania. 90% occur within the Pacific basin (biggest and deepest)
what is the formation of tsunamis?
1) Tsunamis are produced by sub-marine earthquakes at subduction zones causing tectonic strain and elastic energy to be build up in the Benioff zone causing water displacement and deep trough waves
2) When an oceanic crust is jolted during an earthquake, all of the water above this plate is displaced, normally upwards forming tall waves with a high crest
3) This water is then pulled back down due to gravity. The energy is transferred into the water and travels through it like a wave.
4) The water travels fast along the ocean water column but with a low amplitude (height).
5) As it gets closer to the coast, the sea level decreases so there is friction between the sea-bed and the waves, causing the wave to slow down.
6) This creates a wall of water that is on average 10 feet high but can reach 100 feet. This will eventually crash onto the shore and once the energy is lost, the water drags debris back into the ocean.
what are the primary hazards of earthquakes?
- ground shaking
- crustal fracturing
what is moment magnitude (magnitude) for earthquakes?
- the moment magnitude scale measures from the energy released by an earthquake, usually in terms of the amount of fault slip
- better for higher magnitudes
- Based on energy released (compared to TNT/Kg) from all shock waves and the area of rupture and plate movement
- More accurate than the original Richter scale as that only measure P and S waves measured on a seismograph
- Both MMS and Richter are logarithmic
what is modified mercalli scale (intensity) for earthquakes?
- looks at relative affects due to shaking
shaking depends on: - magnitude
- distance from epicentre
- depth of focus
- rock type (amplification)
- Measures intensity using the amount of damage caused by shaking
- Very subjective scale based on where you are, closer to the epicentre there is more shaking
- Geology can affect shaking, e.g. In Haiti, unconsolidated sediment shook violently, saturated foundations in Christchurch caused liquefaction
- Usually, the stronger the magnitude the greater the intensity but local factors can modify this, e.g. being closer to the epicentre, type of geology can amplify shaking
what human factors influence the impact of a tsunami?
- Quality of Warning and Evacuation Systems
- Population density of area hit, if the population is higher then more people are likely to be affected, tourism, industry
- Coastal defences (e.g. sea walls)
- Level of economic and human development
what physical factors influence the impact of a tsunami?
- Wave amplitude, amount of water displaced, velocity and distance travelled
- Degree of coastal ecosystem buffer (e.g. mangrove trees, barrier island, coral reef)
- Timing at which the tsunami hits (day or night)
- The topography of the land, water depth, bays will funnel and concentrate tsunami waves
what are the high risk zones for a tsunami?
- Coastal settlements around the pacific ocean as it’s the biggest and deepest ocean
- Countries within the ring of fire as volcanic activity can also cause earthquakes which lead to tsunamis
- Places near/on destructive plate boundaries
explain the intensity scale for tsunamis
- 12 point intensity scale created in 2001
- Corresponds to current earthquake scale
- Arranged based off a tsunami’s effect on humans, effect on objects (e.g. boats) and damage to buildings\rough correlation with wave heights
- Scale 1: isn’t felt, Scale 12: is a 32 + metre wave which is completely devastating
explain the use of DART for researching tsunamis
DART (Deep-ocean assessment and reporting of tsunamis)
- Sensors on seabed detects crust movement and monitors change in pressure (as small as 1cm tsunamis)
- Data is transmitted to the moored surface buoy which detects wave action
- Transmits data to ground stations and informs at risk areas
- These are in place in the Indian and Pacific Ocean, they use seismic waves to detect undersea earthquakes, they use additional technology as not all earthquakes cause a tsunami
- The more the research available the less the damage (The 2004 Indian Ocean tsunami had not warning systems and 31,000 people died but an early warning system has been in place since 2006)
- Japan has lots of warning systems so when a 9.0 quake struck Japan in 2011, the meteorological agency were ready but part of it failed and so people were not expecting such a large tsunami so they did not prepare or evacuate.
how does an eruption occur at the different plate boundaries?
- They can occur at convergent (destructive) margins where subduction creates friction to then melt the plate at the Benioff Zone. This often happens along the ring of fire (geological fault belt)
- They can also occur at divergent (constructive) margins where magma rises as plates pull apart, e.g. Iceland
- They can occur on hotspots where a hot magma plume breaks a hole in a plate to form new land.
what are the primary hazards of a volcano?
Lava flows
pyroclastic flows
tephra and ash flows
volcanic gases
these tend to have a long geographical reach (pyroclastic flows can travel up to 10,000 km)
explain the primary hazard of lava flows
streams of lava that have erupted on the Earth’s surface. Fast flowing lava can be very dangerous. This depends on the lava’s viscosity which depends on the silicon dioxide content
explain the primary hazard of pyroclastic flows
This is a mixture of hot dense rock, lava, ash and gases which move very quickly along the surface of the volcano. Due to their high speeds, pyroclastic flows are extremely dangerous and can cause asphyxiation for anyone unfortunately caught by the flow.
explain the primary hazard of tephra and ash flows
when pieces of volcanic rock and ash are blasted into the air. This can cause serious damage to buildings, which can collapse under the weight of ash or tephra
explain the primary hazard of volcanic gases
gases like sulphur dioxide and carbon monoxide are released into the atmosphere. Due to their potency, volcanic gases can travel long distances.
what are the secondary hazards of a volcanic eruption?
Lahars
Jokulhlaup
Acid rain
explain the secondary hazard of lahars
combination of rock, mud and water which travel quickly down the sides of volcanoes. These can occur when the heat of the eruption causes snow and ice to melt or alternatively when an eruption coincides with heavy rainfall.
explain the secondary hazard of jokulhlaup
snow and ice in glaciers melt after an eruption which causes sudden floods that are very dangerous, affecting accessibility, transport of aid
explain the secondary hazard of acid rain
caused when gases such as sulfur dioxide are released into the atmosphere.
what are the 3 types of lava?
rhyolitic
andesitic
balsaltic
what is rhyolitic lava like?
- Seen in supervolcanoes, composite volcanoes
- Coolest form of lava at (650-800ºC)
- High silica and gas content, low iron and magnesium
- Formed by melting of the lithosphere and slabs of previously subducted plate
- very viscous with violent eruptions, cataclysmic eruptions
what is andesitic lava like?
- Composite cone locations, Benioff zones (Pinatubo)
- Medium heat of about (800 – 1000ºC)
- Intermediate levels of silica, gas content, magnesium and iron
- Low SO2, high water and hydrochloric acid
- Forms when subducted oceanic plate melts and mixes with seawater, lithosphere and continental plate rocks
- Intermediate viscosity which is slow and traps gases
- Eruption is violent and moderately explosive
what is basaltic lava like?
- Ocean hotspots, shield volcanoes (Kilauea)
- Hottest lava (1000-1200ºC)
- Low silica, water, gases and aluminium, high levels of Co2, Iron and magnesium
- Formed by melting of mantle materials (mostly from upper zone but some from core mantle boundaries
- Eruptions are gentle and effusive
what are shield volcanoes like?
- basaltic lava (less viscous due to less silica) so doesn’t trap gases so not explosive, but effusive
- Shallower gradient due to the lower viscosity of lava meaning it can travel further before cooling (basaltic lava)
- Usually at oceanic hotspots or mid-ocean ridges
- Formed from layers of lava
what are composite volcanoes like?
- Cone volcanoes form from viscous lava (rhyolitic and andesitic) and traps gases so is more explosive
- Usually at subduction zones
- Steep side and formed of layers of lava and ash
- More likely to have pyroclastic flows
what is the way of measuring volcanoes?
Volcanic explosivity index (VEI)
- VEI is a relative measure of the explosiveness of volcanic eruptions done by Chris Newhall
- Classification ranges from “gentle” to “mega-colossal”, scale of 1-8
- It is of no use in connection with non-explosive eruptions of lava
- Volume of tephra ejected, ash cloud height and qualitative observations used to determine the explosivity value
- The scale is logarithmic, increasing by x10 each level
- the greater the VEI, the less frequent the event occurs, e.g. In Hawaii eruptions are happening all the time and they have very gentle lava flows. This makes them more predictable and less threatening.
what is elastic energy in an earthquake?
when a plate is subducted and friction builds to such a point that it exceeds the pressure of the overhead plate causing it to jolt.
what is the focal depth?
the distance between the hypo centre and surface of the earth
what is an intraplate?
when tectonic activity happens in the middle of a plate away from the main boundary
what is a lahar?
saturated soil infused with ash which travels in a channelled motion down a volcano resembling a landslide
what is mantle convection?
when plumes of heated magma rise to the surface and cool, going sideways and sinking at the trenches to be reheated convection currents
what is tectonic strain?
when seismic energy creates pressure and friction
what is a hazard-management cycle?
a theoretical model of hazard management as a continuous four stage cycle involving: mitigation, preparation, response and recovery
what is rapid onset for a hazard?
a hazard that happens very slowly with plenty of evidence and warning (e.g. drought)
what is the pressure and release model?
a tool used to work out how vulnerable a country is to hazard
what is sea-floor spreading?
It is the movement of the oceanic crust away from a constructive (divergent) plate boundary at the mid-oceanic ridge as recorded by the magnetic stripes in the basaltic rock. This causes the spreading of the sea floor.
what is an earthquake in a developed country?
Christchurch earthquake, New Zealand
what are the key facts about the earthquake of Christchurch, 2011
- 22nd February 2011
- happened in the Canterbury region on New Zealand
- the epicentre was 6 miles SE of Christchurch and the focus was very shallow at 3.1 miles
- found on the ring of fire
- 8 on the mercalli intensity scale