Tectonics Flashcards
Types of plate margins
- conservative
- constructive/divergent
- convergent (collision and destructive)
Conservative plate boundaries
- plates slide past each other horizontally
- jagged edges can catch and snag against each other causing friction and pressure
- friction and pressure may result in earthquakes, but not volcanoes
Constructive/divergent plate boundaries
- plates move apart, causing a gap between plates
- magma rises up causing volcanoes and new crust is created (sea floor spreading)
- no earthquakes
destructive plate boundaries
- dense oceanic plate subducts beneath continental plate and crust is destroyed
- rocks catch, building pressure until plates slip past each other, causing earthquakes and tsunamis
- melted crust can rise up through faults as volcanoes
oceanic plates
- dense
- basaltic rock
- 7-10km thick
continental plates
- less dense
- granitic rock
- 25-75km thick
subduction
the oceanic plate is recycled into the earth’s mantle at a destructive plate boundary
mantle plume
a large column of magma rising through the mantle that creates a hot spot on the surface
Intra-plate
occurring within a tectonic plate e.g. an earthquake in Gujarat, 2001
Convection
the exchange of heat by the movement of a liquid. Convection currents in the mantle move tectonic plates
Seismic waves
When energy from an earthquake is released from the focus in shock waves and travels through the earth’s layers. Can be P-waves, S-waves and L-waves
Focus
Where an earthquake occurs. Where the pressure is released underground and where the energy radiates out from
Epicentre
The point directly above the center of the earthquake on the earth’s surface.
Hazard
A perceived natural event that has potential to threaten life and property
Disaster
The reality of a hazard happening and causing a significant impact on a vulnerable population
Hazard risk equation
risk = hazard * vulnerability / capacity to cope
Distribution of tectonic hazards
- Distribution is uneven, some areas are at a high risk, whereas other areas are at a low risk of tectonic hazards
- Hazards happen at specific spots, usually associated with plate boundaries
Hotspots
-Hot plumes of magma rising under a weakness in a plate causes the magma to rise through the surface through the weakness
- The magma plume stays where it is as the plate slowly moves over it,
- The magma dries, a chain of volcanic islands with extinct volcanoes are produced e.g. Hawaii
Intra-plate earthquakes
- Caused by stresses within a plate
- as flat plates move over a spherical surface, zones of weakness are created
- this can lead to earthquakes e.g. 2001 Gujarat in the centre of the Indian plate
Earth’s core
- central part of the earth consisting of the inner and outer core
- made up of iron and nickel
- inner core is solid and outer core is liquid
- source of radioactive heat
Mantle
- semi-molten rock between the earth’s crust and core
- has a temperature gradient that drives convection currents
Asthenosphere
- part of the semi-molten mantle
Lithosphere
- forms tectonic plates
- consists of the crust and upper mantle
- floats on top of the asthenosphere
Ridge push
elevated altitudes of oceanic crust and convection currents at ocean ridges causes plates to slide down
Slab pull
Dense oceanic plate is pulled downwards by convection currents and then sinks down into the mantle due to its own weight. pulling the rest of the plate with it
Sea floor spreading
Mid ocean ridges at divergent plate boundaries
- magma is forced up through the ridge
- hardens as it cools, creating new crust and pushes plates apart
- this forms new crust
- confirmed by palaeomagnetism
Palaeomagnetism
- confirmed the sea floor is spreading
- the earth’s magnetic field changes direction (magnetic N&S swap) every 400,000 years or so
- The iron particles in rising magma align themselves with the earth’s polarity as they harden to form new crust.
- scientists found symmetrical magnetic stripes along both sides of mid ocean ridges
Wegener’s continental drift hypothesis (1912)
- theory that our now separate continents were once joined together in supercontinents (Pangea)
Holme’s hypothesis
- the earth’s radioactive heat was the driving forces of convection currents in the mantle that could move tectonic plates
Benioff zone
a seismically active underground area within a subduction zone, where the slab is being thrust downwards. The different speeds ad movements of rock causes many intermediate/deep focused earthquakes
Why is the Benioff zone an important theoretical framework?
It is an important factor in determining earthquake magnitude, as the position and depth of the hypocentre can be determined
Locked fault
A fault that is not slipping because the frictional force is greater than the shear stress across the fault. Locked faults can be ‘stuck’ for hundreds of years, building up enormous stress before it releases, generating large magnitude earthquakes. ‘elastic rebound theory’
What release of a locked fault resulted in a disaster?
The ‘mega-thrust locked fault’ subducting Indian plate resulting in the 2004 Indian Ocean tsunami
What is an earthquake
The shaking of the ground caused by sudden motions along faults or fractures in the Earth’s crust
What is the sequence of events leading up to the generation of an earthquake?
- A gradual build up of tectonic strain, stored as elastic energy in crustal rocks
- The rock fractures when the pressure exceeds the strength of the fault
- this produces a sudden release of energy, creating seismic waves that radiate away from the point of fracture
- the brittle crust then rebounds either side of the fracture, creating ground shaking felt on the surface
Hypocentre
The ‘focus’ point within the ground where the strain energy of the earthquake stored in the rock is first released
The focal length
The distance between the hypocentre and the epicentre on he surface
Primary (P) waves
vibrations caused by compression (+dilation). Spread quickly from the fault (8km/sec)
Secondary (S) waves
Vibrate at right angles to the direction of travel and cannot move through liquids. Move more slowly (4km/sec)
Love (L/Q) waves
surface waves with the vibration occurring in the horizontal plain. High amplitude
Intensity
A measure if the ground shaking. This ground shaking causes the most building damage and loss of life.
Soil liquefaction
Water saturated material temporarily losing strength under the pressure of strong shaking. The water pressure increases to a point where the soil moves easily.
How does soil liquefaction impact infrastructure?
Water is forced to the surface, undermining foundations and causing buildings to settle, tilt and eventually collapse
Lateral spreading
land adjacent to rivers and sloping ground slides across a liquefied area of soil, sometimes creating large fissures and cracks in the surface
Magnitude of an earthquake
Related to the amount of displacement in the fault, which is a measure of energy release. It is measured at the epicentre of the earthquake.
Tsunami characteristics
Characterised by:
- long wavelengths (150-1000km)
- low amplitude (0.5-5m)
- fast velocities (up to 600km/h in deep water)
Tsunamis retreat after they have reached the shore, dragging debris into the water
Tsunamis: wave shoaling
The waves are barely detectable in deep water as the energy moves underwater. The tsunami only seems to appear in shallow water as the energy has less water depth to move through, so waves increase in height.
How do tsunamis start?
Initiated by undersea earthquakes (uplift of subducting plate), volcanic eruptions, landslides or slumps. The event displaces the water column upwards, gravity pulls it back down and waves ripple out.
Factors that influence the impact of a tsunami
- duration
- wave amplitude, water column displacement
- distance travelled
- physical geography of coast (water depth and gradient of shoreline)
- degree of coastal system buffer (mangroves, coral reefs)
- timing of event (night vs day), quality of early warning systems
- degree of coastal development and proximity from coast
Primary volcanic hazards
- pyroclastic flows
- tephra
- lava flows
- volcanic gases
Pyroclastic flows
- responsible for most primary volcanic related deaths
- hot gases and pyroclastic material (ash, pumice, glass shards) ejected explosively out the vent of the volcano
- clouds can be up to 1000 degrees
- moves at 100km/h
- most hazardous when ejected sideways, close to the ground
Tephra
- volcanic eruptions sometimes eject rock fragments into the atmosphere
- ‘bombs’ (>32mm diameter) to ‘dust’ (<4mm)
- can cause building rooves to collapse
- starts fires on ground
- reduce visibility, affect air travel (clogs up engines)
- large rocks don’t travel far but can cause injury and death
Laval flows
- pose a big threat to human life if fast moving
- viscosity is dependent on silicone content
- can reach up to 15m/s on steep slopes
- can be up to 10170 degrees, can take years to cool fully
Volcanic gases
- associated with explosive eruptions and lava flows
- water vapour, sulfur dioxide, hydrogen, carbon dioxide
- sulfur dioxide can cause acid rain
- carbon dioxide is colourless and odourless, can accumulate in valleys and kill people
Secondary impacts of volcanoes
- lahars
- jokulhlaups
Lahars
- volcanic mudflows
- composed of relatively fine sand or silt material
- degree of hazard depends of steepness of slope, volume of material, particle size
- triggered by heavy rainfall during or after eruption, or eruption melting snow or ice on the slope of the volcano
Other secondary impacts of volcanic eruptions
- landslides
- tsunamis
- fires
- crop damage = food shortage
- CO2 suffocation
- shortage of clean water
- damage to businesses = unemployment
- emergency services can’t access some places
Other primary impacts
-building collapse
- roads blocked
- deaths and injuries
Ways of monitoring and predicting volcanic eruptions
- magma rising causes small earthquakes that can be detected using seismometers
- magma builds pressure as it rises, causing a change in shape as the volcano swells
- volcano may slope or tilt as magma moves, can be monitored using a tiltmetre
- sulphur may be emitted from a volcano before it erupts, as magma is close to the surface, causing it to smell of egg
- water and rock can be tested for temperature. A higher temperature may indicate agma is closer to the surface
Measuring volcanic eruptions
Volcanic Explosivity Index (VEI):
- 0 - 8 on a logarithmic scale (non-explosive to extremely large)
- amount and height of volcanic material ejected
- duration of eruption
- qualitative descriptive terms
Early detection of tsunamis
- water pressure
- seismic frequency
- water retreating
Risk:
Why are people affected by hazards
- unpredictability of hazards
- lack of (living) alternatives
- threats from hazards fluctuate
- perception of risk, cost-benefit from staying in a hazardous area
- Russian roulette reaction (fatalism, risks will happen whatever you do)
Disaster risk and age index
Highlights
- aging population
- acceleration of risk in a world that is increasingly exposed to a range of hazard types
>7 = higher risk
4-6.9 = moderate risk
<3.9 = lower risk
Risk
the exposure of people to a hazardous event. The probability of a hazard occurring that leads to a disaster
Why are a high percentage of disaster victims elderly?
- lack of mobility and independence when it comes to evacuation
- less likely to leave their homes
- less likely to hear about early warnings (less access to/ knowledge of technology
- higher rates of disability
- disaster usually greatly impacts heating electricity, access to healthcare etc
- less likely to recover from injuries / secondary illnesses
Pressure and Release model (PAR)
A disaster is the intersection of two processes:
- processes generating vulnerability on one side
- the natural hazard event on the other
Resembles a ‘nutcracker’ with increasing pressure on people arising from both sides (vulnerability and impact/severity of hazard)
To relieve the pressure, vulnerability has to be reduced
What processes generate the vulnerability of a community to a tectonic hazard?
Root causes:
- limited access to power, structures, resources,
- ideologies, political and economic systems
Dynamic pressures:
- Lack of appropriate skills, training, investment, press freedom, ethical standards
- macro-forces, rapid population change, urbanisation, deforestation, debt repayment, arms expenditure
Unsafe conditions:
- fragile physical environment and local economy
- vulnerable society
- public actions
Why are impacts of earthquakes usually much greater than the impacts of volcanic eruptions?
A relatively small proportion of land and a relatively small proportion of the human population have direct exposure to volcanic activity. Secondary impacts of earthquakes also increases the area that are affected by earthquakes (landslides, tsunamis etc)
How levels of development affect risk and vulnerability to natural disasters
Low levels of development are closely associated with high levels or risk and vulnerability to natural disasters. Development enables people to achieve their aspirations and increases the resilience of the community
What should economic impacts take into account as well as the land area exposed to the hazard?
- level of development in the region
- insured impacts vs non-insured losses
- total number of people affected and the speed of economic recovery following the event
- degree of urbanisation and value of land
- absolute vs relative impacts on GDP
What are the differences on the impacts of hazards happening in developed and non-developed countries?
- Hazards that happen in richer, more developed places often are more costly to rebuild because the infrastructure in more developed and the cost of business is more significant. However, richer countries have a higher GDP and higher resilience, so are able to rebuild the economy quicker.
- Less-developed countries tend to have other, more pressing problems such as poverty and disease, so they aren’t able to spend much money on preparing for hazard events
Tectonic hazard profile
A technique that compares the physical processes that all hazards share (e.g. frequency, magnitude) to help decision makers to identify and rank the hazards that should be given the most attention and resources
3 different scales/measurements of earthquakes
Richter scale (0-9):
- measurement of height of waves produced by earthquake
- logarithmic measure of magnitude
- absolute scale
Mercalli Scale (I-XII):
- Measures the experienced impacts of the earthquake
- a relative intensity scale
- based on a series of key responses (e.g. people awakening, damage to structures)
Moment Magnitude Scale (MMS), (0-9):
- modern measure, describes earthquakes in terms of energy released
- magnitude is based on seismic movement of the earthquake (calculates by amount of slip on the fault, area affected and Earth-rigidity factor)
-
Common social impacts of hazards
- loss of life
- stress on emergency and healthcare services
-damage to houses - infrastructure damage
- not able to
communicate - water and electricity
shortage
- not able to
Common economic impacts of hazards
- damage to transport
(people cannot get to work, communities stranded without aid) - building damage
- lack of trade
- loss of jobs and businesses
- damage to farms and factories
Strato-volcanoes/ composite volcanoes
Steep-sided cones formed from layers of ash and acidic lava flows. The eruptions from these volcanoes may be a pyroclastic flow rather than a flow of lava. When composite volcanoes erupt they are explosive due to the thick, highly viscous lava that is produced. The thick lava cannot travel far down the slope of the volcano before it cools. Composite volcanoes are usually found at destructive plate margins. (Mount Fuji Japan, Mount St Helens USA)
Shield volcanoes
Flatter, dome-shaped volcanoes formed from layers of lava. Eruptions are typically non-explosive. and produce fast flowing lava that can flow for many miles. Eruptions tend to be frequent but relatively gentle. Usually found at divergent (constructive) boundaries and sometimes at volcanic hotspots
Calderas
A big volcanic crater formed when an explosive eruption blows the top off, the magma chamber is emptied and the sides of the volcano collapse. e.g. Yellowstone
Mid-ocean ridges
Magma rising up from the sea floor, either from a hotspot volcano or a constructive plate boundary
Volcanic hotspots/ island chains
An area in the mantle where heat rises as a hot thermal plume, high heat and lower pressure in the lithosphere enables the melting of the rock. Magma rises through cracks and erupts as active volcanoes on the sea floor. The plates move over the stationary hotspot. Volcanoes are formed and then are moved away from the magma plume to cool and form a chain of extinct volcanic islands
WorldRiskIndex (DRI)
Assesses the risk of disaster. Is calculated on a country by country basis through the multiplication of exposure and vulnerability. Those with the highest vulnerabilities include countries in West Africa such as Mali and Nigeria
The Risk-Poverty Nexus model
Low income households suffer a disproportionate share of disaster loss and impacts:
- asset inequality
Don’t have a ‘fall back’ to help them recover. More likely to live in areas with poor infrastructure that is more at risk of hazards
- inequality of entitlement
e.g. USA have unequal application of the law. Poor and Black people have suffered more after Hurricane Katrina
- political inequality
If government are able and willing to spend money on rebuilding and combatting disaster. May to more to protect voters and wealthy donors.
- social status inequality
High social status = people can access jobs and healthcare more easily
Resilience
The ability to protect lives, livelihoods and infrastructure from destruction, and to restore areas after a natural hazard has occured
How does governance and political conditions affect vulnerability and resilience
- existence and enforcement of building codes
- the quality of the existing infrastructure
- the existence of disaster preparedness plans
- The efficiency of emergency services
- the quality of communication systems
- the existence of public education and practiced hazard response
- the level of corruption of government officials and businesses
How do economic and social conditions affect vulnerability and resilience
- levels of wealth
- access to education
- quality of housing
- quality of healthcare
- amount of income opportunities
What physical and environmental conditions affect vulnerability and resilience
- population density
- rapid urbanisation
- accessibility of an area
Multiple Hazard Zones/ disaster hotspots
Places where multiple physical hazards combine to create an increased level of risk for the country and its population. Repeated events means there is never an extended recovery period
What is the hazard management cycle
A theoretical model of hazard management as a continuous 4-stage cycle
Different activities occur in each stage but there is a large amount of overlap.
What stages are in the hazard management cycle
- Mitigation (prevention)
- before and after hazard event
- identifying potential natural hazards and taking steps to reduce their impact
- reconstruction, risk assessment and planning
Developing building codes, zoning and land use planning, building protective structures - Preparation
-before hazard event
- preparing to deal with a hazard event
- risk assessment and planning and pre event activities
preparedness plans, early warning systems, evacuation routes, stockpiling supplies, public awareness - response
- during hazard event
- coping with disaster
- emergency operations, restoration
search and rescue, evacuation, restoring critical infrastructure, ensuring critical services continue - recovery
-after hazard event
- short and long term recovery (including decreasing future vulnerability), getting back to normal
reestablishing permanent infrastructure, providing and setting up aid and financial assistance, repairing and reopening businesses and schools
The Park model (hazard response curve)
A model that shows how a country or region might respond after a hazard event. Can be used to compare how areas at different levels of development might recover from a hazard event. It is a curve to show how much the hazard event has decreased the ‘normal’ quality of life, and how long it takes for the communities to return to normal function or ‘Build Back Better’
Hazard mitigation
Strategies meant to avoid, delay or prevent hazard events.
Hazard adaptation
Strategies designed to reduce the impacts of hazard events
Examples of hazard mitigation
Land use zoning
Diverting lava flows
GIS mapping
Hazard-resistant design and engineering defences
Land use zoning
A process by which local government planners regulate how land in a community may be used.
Common in wealthy countries. Areas at risk from volcanic eruptions or tsunamis are divided into zones based on the likely type and level of damage from hazard event. Land use planners use hazard maps.
In areas at high risk:
- any settlements tend to be limited
- certain types of structures will be prohibited such as critical buildings (hospitals) and buildings that pose a risk if damages (nuclear power stations)
- some communities ay be resettled
- areas which provide as natural protection (reefs, mangroves) will be protected.
Diverting lava flows
Include digging channels or building barriers to try and divert the flow of lava. In general it is fairly ineffective as the path taken by is hard to predict, the terrain has to be suitable and diverting lava may cause it to damage other communities
GIS mapping
Can be used in all stages of the hazard management cycle. For example, to identify where evacuation routes should be placed or to help with rescue and recovery. GIS maps can use information from the hazard such as the epicentre of an earthquake and information such as the location of airports and airstrips to help aid agencies identify areas most affected by the hazard and to find the nearest location where emergency supplies and relief workers can land
Hazard-resistant design and engineering defences
Collapsing buildings are one of the main causes of death from tectonic hazards.
New building structures can be built to withstand ground shaking by building:
- rubber shock absorbers in the foundations
- reinforced latticework foundations deep in bedrock
- rolling weights on roof to counteract shock waves
- steel frames
Roofs of houses built by volcanoes can be sloped to reduce ash build up. Buildings at rick from tsunamis can be elevated and anchored at foundations. Protective structures such as sea walls can stop or slow tsunamis or landslides
Hazard adaptation strategies
High-tech monitoring
Crisis mapping
Modelling hazard impact
Public education
High-tech monitoring and early warning systems
Technological monitoring systems for tectonic hazards allow scientists to learn more about these natural processes in hope of eventually being able to predict them more accurately further in advance. They also sometimes allow an early warning system for tsunamis and volcanic eruptions, so the relevant authorities can be informed and rapid alerts issued to communities at risk through satellite-communication technology and mobile-phone technology (e.g. Japan’s government 2011 sent out rapid warnings and coordinated preparation activities through texts)
Crisis mapping
Used in the 2010 Haitian earthquake. Uses crowd-sourced information as well as satellite imagery, other maps and statistical models. A free resource that allows people to create live interactive maps set up a map site for Haiti. Local people could provide information on the app so rescue and aid workers could see where people were trapped in the rubble or needed food or water.
Modelling hazard impact
Computer models allows scientists to predict the impacts of hazard events on communities. Also allow scientists to compare the effects of different scenarios (e.g. effects on tsunami if a sea wall is built). Can be used by decision makers to develop effective plans and strategies
Public education
Helps people understand what people can do to protect themselves before, during and after a hazard event.
- regularly practicing emergency procedures (e.g. earthquake drills in Japanese schools)
- encouraging households and workplaces to create emergency preparedness kits
- provide education materials such as education on how to build buildings to withstand earthquakes
Community preparedness and adaptation
People living in a community at risk from natural hazards developing suitable preparedness plans and educating local residents. Efficient in low income countries where governments may not have the resources.
- creating list of vulnerable people who may need special assistance
- organising practice evacuation drills
- providing first aid courses
In some developed countries, the government supplies communities with resources to help them do this
Key players in managing loss
Aid donors:
- emergency, short term or long term
- NGOs, intergovernmental organisations, local or foreign governments
- money, personnel, services, equipment
NGOs:
- often helps in all stages of hazard management cycle to build community’s resilience
- vital for when local governments are struggling to respond
Insurance:
- mostly in developed countries
- provides individuals and businesses with money to help repair and rebuild
Communities:
- immediate search and rescue efforts, waiting for aid to arrive in remote communities
- long term resilience
