Tectonics

The crust

The crust

Varies in thickness (5-10km beneath the ocean).

Amounts to less than 1% of the Earth’s total mass.

Made up of serval major plates.

The mantle

Widest layer (2900km thick). The heat and pressure means the rock is in a liquid state that is in a state of convection.

The core (Inner and Outer)

Hottest section (5000 degrees). Mostly made of iron and nickel and is 4x denser than the crust. Inner section is solid whereas outer layer is liquid

What is a Tectonic Plate?

A tectonic plate is a massive, irregularly shaped slab of solid rock, composed of both continental and oceanic lithospheres. These tectonic plates move in various ways against each other on areas know as plate margins.

Theory of Plate Tectonics

In 1912, Alfred Wegener proposed the theory of continental drift. He suggested the existence of Pangaea and that continents drift. Evidence for this includes;

1 Geology- Rock sequences and jigsaw fitting of the world’s continents.

2 Fossil records –Fossil remains of reptiles found in different continents.

3 Living species – Some species found on different continents are similar.

4 Climatology- Glacial deposits on the Equator suggests plate movement.

Vine and Matthews’s theory included the Palaeomagnetism – Record of the Earth’s polarity on erupted lava.

Divergent/destructive plate boundaries

Oceanic-continental (fold mountains and volcanoes). Eg. Andean Mountain range

Oceanic-oceanic (volcanic island arcs)

Continental-continental (plates are similar in density so collide and uplift to fold the crust). Eg. Himilayas

Divergent/constructive plate margins

Continental-continental: Caused by geologically recent mantle plume splitting a continental plate to create a new ocean basin. It can cause Basaltic volcanoes and minor earthquakes.

Oceanic-oceanic: Eg. Mid Atlantic Ridge. New lithosphere forms at constructive margins, where rising plumes of magma stretches the crust to create intense volcanic activity on the ocean floor.

Conservative plate boundary

Can cause powerful earthquakes. Eg. San Andreas Fault, California, USA

Volcanic hotspots

A concentration of radioactive elements inside the mantle may cause a hotspot to develop. From this, a plume of magma rises to melt through into the plate above. Where lava breaks through to the surface, active volcanoes can occur above the hot spot.

Intra-plate earthquake

An intra-plate earthquake refers to an earthquake that occurs within the interior of a tectonic plate.

Asthenosphere

The upper layer of the earth's mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur.

Slab pull

Oceanic plate (more dense) is subducted under continental plate. The oceanic plate pulls itself into the mantle, causing further subduction

Ridge push

The slope created when plates move apart has gravity acting upon it as it has a higher elevation. Gravity pushes the plates further apart, widening the gap

Benioff Zone and Subduction Processes

 The Benioff Zone is an inclined zone in which many deep earthquakes occur, situated beneath a destructive plate boundary where oceanic crust is being subducted.

As the asthenosphere and lithosphere at the ridge are heated, they expand and become elevated above the surrounding sea floor.

At a subduction boundary, one plate is denser and heavier than the other plate. The denser, heavier plate begins to subduct beneath the plate that is less dense.

The subducting plate is much colder and heavier than the mantle, so it continues to sink, pulling the rest of the plate along with it. The force that the sinking edge of the plate exerts on the rest of the plate is called slab pull.

How do earthquakes occur

Earthquakes (shallow focus – less than 70km) happen when two plates become locked causing friction to build up. From this stress, the pressure will eventually be released, triggering the plates to move into a new position.  This movement causes energy in the form of seismic waves, to travel from the focus towards the epicentre. As a result, the crust vibrates triggering an earthquake.

P (seismic) waves

Travel through solids and liquids.

Shakes the Earth in the same direction as the travelling wave

Fastest type of wave.

S (seismic) waves

Travel through solids only.

Shakes the Earth vertically (90°angle to the travelling wave).

Most damaging type of wave.

Surface waves

They can occur closest to the surface. Travel slower than P and S waves but are more destructive.

Love waves travel through solids only. Shakes the Earth in the same direction as the travelling wave.

Rayleigh waves travel through solids and liquids. Shakes the Earth in a rolling motion (like an ocean wave)

Earthquake secondary effects

Liquefaction (Solid material changed into a liquid state. Damage to building foundations, results in them sinking)

Landslides and avalanches

Tsunamis (earthquakes cause the seabeds to rise, leading to the displacement of water, leading to powerful waves spread out from the epicentre)

Formation of tsunamis

Large waves caused by the displacement of water triggered by underwater earthquakes, submarine landslides and volcanic eruptions.

In the open ocean, the wave can travel at 500-950km/h and has a wavelength of 200km and a small amplitude (wave height) of 1m.

Closer to land the water gets shallower, causing the waves to increase in size but slow down.

Just before the tsunami reaches the coast, The water withdraws down the shore (drawback).

In Japan 2011, when the tsunami waves reached inland, in some places the waves were 20 metres high. Overall, the tsunami destroyed 200,000 buildings, and killed 19,000 people

Ash cloud

Small pieces of pulverised rock and glass which are thrown into the atmosphere.

Gas

Sulphur dioxide, water vapour and carbon dioxide come out of the volcano.

Lahar

A volcanic mudflow which usually runs down a valley side on a volcano

Pyroclastic flow

A fast moving current of super-heated gas and ash (1000 degrees). This travels at 450mph

Volcanic bomb

A thick (viscous) lava fragment that is ejected from the volcano

Jokulhlaup

Flood of meltwater underneath an ice cap or glacier caused by a volcano underneath erupting and melting the ice above it. Eventually the warm meltwater will have enough volume to lift the ice away from the bedrock, carrying it till it is deposited in low land areas.

Paleomagnetism

Sea floor spreading at divergent plate margins

Magma is released at the gap creating new oceanic crusts

Symmetrical magnetic stripes are formed either side.

New rock is closest to the ridge and pushes the older rock further away.

The earth’s magnetic field alternates every 400,000 years and the rock contains iron oxide minerals which point to the magnetic north.

This can be seen on the ocean floor by the alternating iron oxide mineral directions which reflect the magnetic field at the time of cooling down and meeting the water

Viscosity

Lava that flows easily has a low viscosity (runny). Shield volcanoes - wide, shallow sides, formed by low viscosity lava which flows far from the source. Shield volcanoes are found at hotspots or along constructive plate margins. Their eruptions are mostly predictable

Higher viscosity makes it harder for gas to escape so they are far more explosive. High viscosity creates composite volcanoes because lava builds up close to the volcano when it solidifies. Found along destructive plate margins and steep sided. Extremely unpredictable.

Basic rocks

Low in silica which makes them low in viscosity

Low in viscosity means they are runny

Runny means that gases escape easily so basic lava is not very explosive

Basalt is a basic rock

Oceanic crust is mainly basalt so it is basic

Acid rocks

High in silica which means it is high in viscosity so they are sticky.

Sticky means that the gases don’t escape easily so acid lava is very explosive

Rhyolite is an acid rock

Continental crust is mainly made of acid rocks

Intermediate rocks

Somewhere in between acid and basic. Eg. andesite

Hazard vs disaster

Hazard: a perceived natural event that has the potential to threaten life and property

Disaster: The reality of a hazard happening: when it causes a significant impact on a vulnerable population

The Degg’s Model

The Degg's Model shows that a natural disaster only occurs if a vulnerable population is exposed to a hazard. For example, if the magnitude of the hazard is large, such as a magnitude 9 earthquake, but there is little infrastructure of population density near the epicentre, then no one will experience the hazard and the disaster is small and weak.

Pressure and release model (PAR)

The Pressure and Release Model (PAR Model) is a model that helps understand risk in terms of vulnerability analysis in specific hazard situations. PAR is a tool that shows how disasters occur when natural hazards affect vulnerable people.

It suggests that the socio-economic context is important. Takes into account the root causes, dynamic pressures and unsafe living conditions

Hazard profiles

Compares the physical processes that all hazards share and helps the decision makers to identify and rank the hazards that should be given the most attention and resources.

Hazard profiles are useful for comparing the same hazard in different locations (for example, the Sichuan Earthquake to the Haiti Earthquake)

However it is difficult to compare different hazards (volcanoes, tsunamis, earthquakes) without a certain degree of accuracy

Only based off physical factors

How can governments use hazard profiles?

Implement land-use zoning to keep danger areas clear.

Use hazard-resistant designs. Improved buildings and infrastructure.

Educating local people about disasters and ensuring community preparedness.

Management strategies to reduce losses; insurance and aid deployment

Park’s Model

Plots the quality of life after the disaster against the time since the disaster has occurred.

The Park model takes into account:

That hazards are inconsistent. Things such as the magnitude, development and aid received change over time.

All hazards have different impacts and responses.

Wealthier countries have different curves as they recover faster. They have well-equipped services with technology

Why do some tectonic hazards develop into disasters?

• Morphology/relief - steep mountains - hazards could cause a landslide/avalanche

• Areas near sea could experience tsunamis as well - 2004 boxing day tsunami

• Degree of urbanisation - distance from population

• Development - Higher income means more prepared, better education, earthquake proof buildings

• Time - links to response

• Higher magnitude

Socio and economic impacts of tectonic hazards

Level development in the region or country.

Insured impacts vs non-insured losses.

Total numbers of people affected and the speed of economic recovery following the event.

Degree of urbanisation and value of land

Absolute versus relative impacts on GDP; higher relative impacts are more devastating.

Tectonic hazards in wealthy locations are often more costly because the infrastructure is more developed and the loss of business is more significant. However, unlike lower developed areas, they have the resources usually to recover quicker and better/reduce impacts overall.

Hazard/risk equation

The hazard-risk equation attempts to capture the various components that influences the amount of risk that a hazard may produce for a community or population.

Tectonic measuring

Earthquakes: Richter scale/Moment magnitude scale, Mercalli scale

Volcanoes: VEI scale (Volcanic Explosivity Index)

Moment Magnitude scale (updated Richter scale)

Measuring energy released during an earthquake related to the amount of slip/movement on the fault plane.

Each magnitude has 10x greater than the previous (logarithmic scale)

Mercalli scale

Measures earthquake intensity and what people feel (subjective). 1 - 12 (12 being catastrophic).

VEI (Volcanic Explosivity Index)

Relative measure of the explosiveness of volcanic eruptions. Ranges from 0-8. No human has experienced a VEI 8 supervolcano.

Understanding Risk

Unpredictability – many hazards are not predictable and people can be caught out by timing or magnitude.

Lack of Alternatives – People stay in hazardous areas for a multitude of reasons.

Dynamic Hazards – the threat from hazards fluctuates and human influence can play a role.

Cost-Benefit – the benefit of staying in a hazardous location may outweigh the risk  (perception of risk plays a role here)

Russian Roulette Reaction – the acceptance of the risk as something that will happen whatever you do, that is, one of fatalism.

Prediction, planning, protect (Earthquakes)

Prediction: Satellite surveying of changes in earth’s surface, radon gas sensor (radon gas is released when plates move), water table level (water levels fluctuate before an earthquake). They can also use seismic records to predict when the next event will occur.

Prepare: Training, practicing drills, emergency kits that include first-aid, blankets and tinned food

Protect: Earthquake-resistant buildings, raising public awareness, improving earthquake prediction

Prediction, planning, protect (tsunamis)

Predict: Early warning systems

Prepare: Evacuation routes on coastlines with sirens and signals, DART (Deep-ocean Assessment and Reporting of Tsunami) buoys moored to sensors on the sea floor can monitor passing tsunamis

Protect: Buildings designed with raised, open foundations and made of strong materials such as concrete. Tsunami walls have been built around settlements to protect them.

Modify the loss

Aid after event have occurred. Eg. insurance, emergency short and long term aid.

Pros: Allows economic recovery, provides help to people after affected

Cons: Insurance doesn’t save lives

Modify the vulnerability

Changing factors which can put an area in a vulnerable position. Eg. public education, hi-tech monitoring, adaptation.

Pros: Can be cheap and available for all communities

Cons: Hard for communities to relocate

Modify the event

Controlling designs of buildings to reduce infrastructure dangers in hazards. Eg. Land use zoning, aseismic buildings, resistant buildings, tsunami defense

Pros: can be cheap and available for all communities

Cons: Can be expensive and not available for lower developed countries

Multiple hazard zone

Area at risk of various different hazards which have the potential to combine and create even riskier and more dangerous events. Even riskier in densely populated areas. Eg. Philippines

Predict and preparation (Volcanoes)

Predict

Seismometers to detect earthquakes.

Thermal imaging can be used to detect heat around a volcano.

Gas samples may be taken and chemical sensors used to measure sulphur levels.

Preparation

An exclusion zone around the volcano.

Emergency kit of key supplies.

Having evacuation routes.

Trained emergency services with good communication systems.

Japan earthquake and tsunami 2011

Magnitude 9 earthquake, Pacific plate subducted under Eurasian Plate.

Before event Japan had: 10m high sea walls along 40% of Japan’s coast. Tsunami prediction just 3 mins after earthquake giving people 20 mins to get to safety. Buildings designed to cope with earthquakes.

Short term effects: Nearly 15,000+ deaths, 300,000 buildings destroyed. Radiation levels 8x normal levels due to nuclear meltdown. Rural areas stranded after transport links destroyed. 40m high tsunami wave, plates slid 50m, liquefaction.

Long term effects: Most expensive natural disaster in history. Economic slowdown, 1 million without running water, many people not allowed to return due to radiation.

Immediate response: 100,000 Japanese soldiers sent out to search and rescue, people evacuated

Long term responses: Reconstruction, tsunami defence system reconsidered and extended

Iceland Volcano 2010

Constructive boundary between the North American and Eurasian plate

10km high ash pume reached stratosphere causing huge problems for air-travel in Europe

Ash made Icelandic soil very fertile

Preparation: Diggers in place to dam rivers, texts sent to locals with a 30 min warning

Management: Long term - Icelandic government rebuilt river banks even higher than before. Immediate - 700 locals evacuated, flights cancelled in Europe

Palu City, Indonesia. 2018 earthquake/tsunami

• Causes: Multi-hazard zone, movement on the Palu-Koro fault. Warning systems failed

• Dynamic pressures: Poor communication links, rapid population growth + urbanisation. However, population is familiar with hazards

• Geographical modification: Located in bay, funnelling tsunami towards city. 7.5M earthquake, tsunami 7m high. Soil liquefaction caused mudflow, submerging buildings

• Effects: 70,000 buildings destroyed (adequate building standards), looting, 4000+ deaths

Sichuan 2008 earthquake, China

Indian and Eurasian plates collided. 7.9M earthquake. 69000 deaths. The Wenchuan county at the epicentre was cut off by landslides. 5 million buildings collapsed. Most areas affected are fertile farming lands and 78% of families engaged in farming

Immediate responses: Troops parachuted in to Wenchuan as landslides made roads inaccessible. 3.3 million tents were needed to house people made homeless.

Long term responses: Banks wrote off debts owed by survivors who didn’t have insurance. 1 million temporary homes were expected to be put up in the next 3 years

2010 Earthquake Haiti

• Root causes:

  • Low quality buildings - lack of steel. Combined with dense population (high birth rate)

  • Weak and corrupt government (corruption due to drug trafficking)

  • Already in more than $1billion debt which had to be cancelled after earthquake

  • Lack of planning for natural events

  • 7M earthquake, loose sediment below amplified shaking (lateral spreading, liquefaction, amplification)

• Unsafe living conditions

  • No aid into port which was destroyed (lateral spreading and liquefaction)

  • debris blocked roads

  • Pancake failure - 50% of buildings demolished due to poor building methods and materials -easily collapsed, killing those inside

  • Poorly educated people with limited resources or training

  • No warning systems, limited planning

  • 200,000 deaths, 1.5million homeless

  • Cholera outbreak

• Responses

  • Aid slow to arrive due to damaged port

  • Bottled water + purification tablets provided

  • Buildings built to a higher standard, although people still living in temporary shelter a year later

  • Heavily relied on international aid ($330million from EU)

Mount Pinatubo 1991 volcano (Philippines)

VEI of 6

Scientists were able to predict it so thousands of lives were saved and millions of dollars of property damage saved

800+ deaths, mainly from collapsing roofs and mud flows as lahars

$700m damages

Dams built to control destructive lahars

Christchurch

Strong local and national government in strong and stable economy. Established disaster planning and response. High life expectancy. High building regulations -wooden frames absorb shock so buildings don’t collapse.

7.1 M earthquake in 2011, lateral and verticle movement, some buildings still collapsed, liquefaction, 185 deaths

Schools closed, people couldn’t get to work. Many chose to sleep outside for a few days after. $40 billion worth of damage needed to repair infrastructure.

Economic governance

How decisions affect economic activities and relationships with other economies. Affects equity, poverty and quality of life

Administrative governance

How policy is implemented. Requires good building codes, land use planning, environmental risk and vulnerability monitoring

Political governance

Process of making policy including disaster risk planning. Brings together state, non-state and private-sector players and stakeholders

Trends and patterns in global hazards

Trends since about 1960

• Number of recorded hazards has increased

• Number of deaths falling, but spikes with mega-events

• Economic costs have increased significantly

• Number of people affected rising

• Number of tectonic hazards has remained fairly stable

Reasons behind patterns and trends

• Improvements in monitoring and recording events

• Improvements in technology

• global population has increased by about 4.3 billion since 1960

Hazard management cycle

Continuous 4 stage cycle

Mitigation - Preventing/minimising effects. Identifying potential hazards and making steps to rescue impacts

Preparedness - Minimising loss of life and property while also facilitating response and recovery

Response - Aims to save lives, protect property, make affected areas safe and reduce economic impact

Recovery - Getting back to normal. Immediate needs and long term focuses to improve systems for the next time

Key players in modifying disaster losses

Communities

Insurers

Governments

NGOs