Topic 1 - Tectonics

Intra-plate earthquakes

• Can occur anywhere

• The cause is not fully understood but is thought to be:

  • Tectonic stresses causing ancient fault lines to reactivate

  • The plates are moving over a spherical surface and this causes zones of weakness

Hotspot volcanoes

• These occur over stationary magma plumes (columns of rising magma) in the asthenosphere

• The tectonic plate moves over the plume leading to the formation of a chain of volcanic islands (Hawaii)

• The oldest island is the one furthest away from the plume

Plate boundary types

• Divergent

• Convergent

• Transform

Main layers of Earth

• The crust 

• The mantle 

• The core

Crust Characteristics

• There are two types of crust:

  • Continental - a thicker (45-50km), less dense layer (mostly granite)

  • Oceanic - a thinner (6-10km), denser layer (mostly basalt)

• The crust consists of seven major and several minor tectonic plates

Mantel Characteristics

• The mantle is between the crust and core and is the widest layer

  • The upper mantle has two layers:

    • The rigid layer above the asthenosphere, which together with the crust, makes up the lithosphere

    • The asthenosphere is a semi-molten, plastic type layer, which moves under high pressure

  • The lower mantle is hotter and denser than the upper mantle

  • The intense pressure, at depth, keeps the lower mantle solid

Core Characteristics

• The core is made up of two parts:

  • Inner Core - solid centre, mostly composed of iron

  • Outer core - semi-molten, mostly liquid iron and nickel

Convection Currents

• The heat from radioactive decay in the core moves upwards into the mantle

• It creates convection currents, which push up into the spreading mid-ocean ridges, forcing them further apart (ridge push)

Ridge Push

Magma rises as the plates move apart. The magma cools to form new plate material. As it cools It becomes denser and slides down away from the ridge. This causes other plates to move away from each other.

Slab Pull

The denser plate sinks back into the mantle under the influence of gravity. It pulls the rest of the plate along behind it.

Seafloor spreading

• Palaeomagnetism provides evidence that the sea floor has gradually moved apart at a mid-ocean ridge

• Lava cools and solidifies with the minerals lining up with the magnetic field

• The direction of the minerals on either side is a mirror image

Subduction and slab pull

• Convection currents in the mantle drag the overlying lithosphere towards each other

• A subduction zone is formed when two plates meet

  • The heavier, denser plate subducts under the lighter, less dense plate

• As oceanic crust cools, it becomes denser and thicker, and gravity forces the lithosphere down into the subduction zone 

• As it sinks, it drags or pulls the plate with it

• This is known as slab pull

Convergent plate boundary (Oceanic/Continental)

• The denser, heavier oceanic plate subducts under the lighter, less dense continental plate

• This forms deep ocean trenches in the subduction zone

Found at convergent plate boundaries (Oceanic/Continental)

Volcanic eruptions, fold mountains, and earthquakes (at Benioff Zone)

Convergent plate boundary (Oceanic/Oceanic)

The heavier of the two oceanic plates subduct

Found at convergent plate boundary (Oceanic/Oceanic)

deep ocean trenches and island arcs 

Island arc formation

• A series of volcanic islands, formed in an arc shape, e.g. the Caribbean

• Submarine volcanic eruptions, lead to crust building up and rising above sea level

Constructive/divergent plate boundary (Oceanic/Oceanic)

plates are moving apart, volcanic eruptions and earthquakes

Collision plate boundary (Continental/Continental)

• Two plates of similar density move towards each other

• As neither plate can sink into the denser rocks below, they are crushed, crumpled and forced upwards, usually folding in the process

Found at Collision plate boundary (Continental/Continental)

collision fold mountains, earthquakes

Transform/conservative plate boundary

Plates move slowly past each other, they become stuck and pressure builds, the plates eventually 'snap' past each other

• These can be called ‘strike-slip’ faults as they strike/stick and then slip/release past each other

Magnitude/magma at divergent boundaries

• Earthquakes tend to be mild and shallow

• Eruptions tend to be small and effusive

• The eruptions are usually of basalt lava:

  • Low gas content

  • Low viscosity

  • Higher temperature

Magnitude/magma at convergent boundaries

• Friction and pressure build up in the Benioff zone (the area within the subduction zone where most friction and pressure build up occurs) causes strong earthquakes

• Volcanic eruptions tend to be explosive as the magma is forcing its way to the surface

• These eruptions are often rhyolite lava:

  • High gas content

  • High viscosity

  • Lower temperature

Magnitude at transform boundaries

• Plates can stick causing a significant build up of pressure and powerful earthquakes

Characteristics of basalt magma

• Black/dark grey

• 45-55% silica content

• 1000-1200°C

• Low viscosity

• Easy gas escape

• Gentle eruption

Characteristics of andesite magma

• medium/dark grey

• 55-65% silica content

• 800-1000°C

• Medium viscosity

• Medium gas escape

• Medium eruption

Characteristics of rhyolite magma

• Light colour

• 65-75% silica content

• 600-900°C

• High viscosity

• Difficult gas escape

• Effusive eruption

Primary (P) Wave Characteristics

• Body wave

• Fastest

• Reach the surface first

• Travel through liquids and solids

• Cause backwards and forwards shaking

• Least damaging

Secondary (S) wave characteristics

• Body wave

• Slower than P waves

• Only travel through solids

• Cause a sideways motion

• More damaging

Love (L) wave characteristics

• Surface wave

• Slowest

• Cause a side to side motion

• Larger and energy is focussed on the surface

• Most damaging

Primary hazards of earthquakes

• Direct results of earthquakes

• Ground shaking

• Crustal Fracturing

Secondary hazards of earthquakes

• Result of primary hazards

• Landslides

• Avalanches

• Liquefaction

• Flooding

Primary hazards of volcanic eruption

• Direct result of eruption

• Pyroclastic flow

• Lave flow

• Ash fall

• Gas eruption

Secondary hazards of volcanic eruption

• result of the primary hazards

• Lahars

• Jokulhlaups

Jokulhlaups

Floods caused by sudden release of water and rock when glacial ice is melted in an eruption

Causes of Tsunamis

• As the sea bed jolts from an earthquake, water is displaced and forced upwards to create a wave

• As it approaches land wave length becomes compressed and waves get taller

• As it reaches shore a vacuum is created and pulls back water, leaving the sea bed exposed

• Can also be caused by landslides and volcanic eruption underwater

Natural hazard

An even with the potential to cause harm to the environment, people, or economy, caused by nature

Disaster

When a hazard causes actual harm to the environment, people, or economy

UN definition of a disaster

‘A serious disruption of the functioning of a community or a society involving widespread human, material, economic or environmental losses and impacts, which exceeds the ability of the affected community or society to cope using its own resources’

Vulnerability

How susceptible an area/population is to damage from a hazard event

Factors affecting vulnerability

• Level of development

• Population density

• Size of hazard

• Preparation and planning

Hazard risk equation

Allows judgment to be made regarding resilience

Factors affecting resilience

• Building construction

• Population density

• Level of urbanisation

• Infrastructure

• `Wealth

• Healthcare system

• Emergency services

• Education

• Level of corruption

Pressure model

Demonstrates how there is a range of factors that increase vulnerability and why some areas lack resilience. Its made up of 3 sections:

• Root causes

• Dynamic pressures

• Unsafe conditions

  It is the combined with the hazard

Release model

Demonstrates how vulnerability can be reduced and resilience increased by addressing 4 things:

• Safety

• Reducing the pressures

• Addressing root causes

• Hazard mitigation

Moment Magnitude Scales (MMS)

• Measured with seismographs

• Measures energy released from the focus

• Goes from 1 (Nothing felt) to 10

• Logarithmic scales (Factor 10)

Modified Mercalli Intensity Scale (MMIS)

• Measures impact on people and environment

• Goes from I to XII

Volcanic Explosivity Index (VEI)

• Measures size of eruption based on observations and measurements, including height of material ejected, volume of material, and eruption duration

• Logarithmic scale (0-8)

Hazard profiles

Used to compare tectonic events, usually. include the same factors

• Magnitude

• Speed of onset

• Areal extent (Area affected)

• Duration

• Frequency

• Spatial predictability (Is the a pattern to where)

Advantages of hazard profiles

• Can compare multiple hazards

• Can help plan for future events

Disadvantages of hazard profiles

• Doesn’t account for all factors

• Focuses on physical not human

• Multi-hazard events are hard to represent

• Subjective

Reasons for an increase in global disasters

• Population increases (more people to be affected)

• Increased monitoring and reporting

• Global warming

Where are some multi-hazard zones?

Phillipines (earthquakes, typhoons, tsunamis, volcanoes, landslides)

Hazard prediction

Knowing when (temporal scale) and where (spatial scale) a hazard will occur, earthquakes cannot be predicted

Hazard forecasting

Gives a percentage chance of a hazard occurring over a set period

Earthquakes and prediction/forecasting

• Cannot be predicted, just known on plate boundaries

• Can be forecast by studying seismic gap theory, radon emissions, and animal behaviour

Seismic gap theory

Highlights areas most at risk of an earthquake due to time since the last one and historical frequency

Volcanoes and prediction/forecasting

• Warning signs beforehand (magma rising, surface level changes, increased gas emission, increased seismic activity)

• Changes monitored with GPS, tilt meters, satellites, seismometer and gas detectors

Tsunamis and prediction//forecasting

• Can’t predict earthquake induced

• Ocean monitoring tech can be used post earthquake

Hazard Management Cycle

Shows how the events of one hazard inform planning and prep for the next one

Stages of hazard management cycle

Advantages of hazard management cycle

• Can be used by organisations and individuals

• Allows for preparation and response to hazardous events

• Identifies potential hazards

• Reduces risk/saves lives

• Improves level of preparation

Disadvantages of hazard management cycle

• Might not be possible for smaller/poorer communities to implement

• Some hazard are less predictable, management can happen for every eventuality

• Community may oppose aspects implemented

Park’s model

Shows the impact of a hazard event on peoples quality of life over time

• shows where different management strategies are implemented before, during, and after the event

• Varies for each event depending on levels of prep and planning, development, and aid (national and international)

Advantages of Park’s model

• Applicable to range of hazards

• Can be used for risk assessment and preparation framework

• Level of economic activity and social stability take into account

• Useful to analyse response

Disadvantages of Park’s model

• Only shows single event

• Quantitative data not shown (Eg. death count)

• Preventative measures not shown

• Resources required may not be an option to implement for smaller/poorer communities

3 aspects of disaster modification

• Event

• Vulnerability (increasing resilience)

• Loss

Modification of event (earthquakes)

Happens prior to event, challenging as prediction isn’t possible but can build resistant buildings

Modification of event (volcanoes)

• Draining crater lakes to reduce lahars

• Barrier and channel construction to divert lava flow

• Hazard risk mapping and land use zoning to prevent development of high risk areas

Modification of event (tsunamis)

• Land use zoning

• Building offshore barriers/sea walls

• Replanting mangroves

Modification of vulnerability (increasing resistance)

Happens before event occurs

• Land use zoning - ensuring that people are not living in high-risk areas

• Hazard resistant buildings 

• Improved services and infrastructure

• Hazard risk mapping to identify areas at highest risk

• Planning of evacuation routes

• Education of the population to ensure that they know the actions to take when a hazard event occurs - earthquake drills

• Improved storage of food, water and medical supplies so

• Monitoring and warning systems to allow people time to evacuate

Modification of loss

Happens after event occurs

• Evacuation

• Search and rescue teams

• Emergency aid

• Short-term aid - shelter, reconnecting of water and electricity supplies

• Development aid - long-term aid to help with reconstruction and recovery

• Insurance

• Local communities - supporting each other, providing shelter and helping with the search and rescue effort