Tectonics

• Specialist concepts in tectonic processes and hazards include inequality linked to vulnerability and responses, interdependence linked to aid mitigation and adaptation, and resilience linked to strengthening strategies.
• Tectonic hazards such as earthquakes, volcanic eruptions and secondary hazards like tsunamis represent a significant risk in some parts of the world, especially where active tectonic plate boundaries interact with areas of high population density and low levels of development.
• Understanding the causes of tectonic hazards is key to increasing the degree to which they can be managed and putting in place successful responses that can mitigate social and economic impacts and allow humans to adapt to hazard occurrence.
• Distribution of earthquakes, volcanoes and tsunamis are factors contributing to the risk from tectonic hazards.
• Plate boundaries are important because they are often associated with earthquakes and volcanoes, not all earthquakes and volcanoes form and develop on a plate boundary though.
• Intraplate earthquakes are earthquakes that occur inside plate margins due to stresses and pressures that can occur in the crust, these are usually weaker earthquakes and can happen in places such as the UK.
• Intraplate volcanoes are volcanic eruptions that take place inside plate boundaries, they can happen due to a fracture in the crust where a magma plume in the mantle creates the eruption, this is created in areas such as Hawaii.
• The total number of people affected by hazards and disasters is increasing, especially for meteorological and hydrological hazards.
• The economic costs associated with both hazards and disasters have significantly increased since 1960.
• Key features of earthquakes since 1980 include a number of disasters between 15 and 40 per year, variable deaths, and mega-disasters in 2004 (Banda Aceh) and 2010 (Haiti) resulting in over 200,000 deaths each.
• Key features of volcanoes since 1980 include a lower number of disasters, much lower deaths, and only seven eruptions that have killed more than a hundred people.
• Globalisation of production and supply chains allows international businesses to reduce costs and become more efficient, but mega-disasters significantly damage globally-sourced businesses.
• Multiple hazard zones are places where two or more natural hazards occur, and in some cases interact to produce complex disasters, examples include California, Indonesia, and Japan.
• Prediction in the context of tectonic hazards means knowing when and where a natural hazard will strike on a spatial and temporal scale that can be acted on meaningfully in terms of evacuation.
• Forecasting, much less precise than prediction, provides a percentage change of a hazard occurring, for example, a 25% chance of a magnitude 7.0 earthquake occurring in the next 20 years.
• Prediction of tsunami and eruptions depends upon technology, which has to be in place, operational and linked to warning dissemination and evacuation systems.
• Hot spot is a similar process operating at hot spots where mantle plumes rise from deep in the mantle and partial melting of ultramafic material results, convection in the mantle slowly transport heat from the core to the Earth’s surface, mantle plumes carry the heat upwards in narrow, rising columns of hot material, which spreads out when the plume head meets the base of the rigid lithosphere.
• Lower pressures allow decompression and partial melting of mantle peridotite, in a concentrated zone of the asthenosphere, forming enormous volumes of basaltic magma, this basalt may erupt onto the surface over very short time scales (less than 1 million years) to form flood basalts.
• Tsunami characteristics: wave heights are typically less than 1 m, wavelengths are usually more than 100 km, speeds are 500-950 km/h
• P waves: the fastest kind of seismic wave that can move through solid rock and fluids like water
• S waves: slower than P waves and can only move through solid rock, moves rock up and down or side-to-side
• L waves: the fastest surface wave that moves the ground from side-to-side
• Primary hazards of earthquakes: crustal fracturing, ground shaking and displacement
• Secondary hazards of earthquakes: liquefaction, landslides
• Primary hazards of a volcano: lava flows, pyroclastic flows, ash falls, gas eruptions
• Secondary hazards of a volcano: lahar, jökulhlaups
• If the plume provides a continuous supply of magma in a fixed location, it forms a hot spot, as the lithosphere moves over this fixed hot spot due to plate tectonics, the eruption of magma onto the surface forms a chain of volcanoes parallel to the movement of the plate.
• Disaster threshold: 10 or more deaths, 100 or more people affected, US $1 million in economic losses
• Hazard risk equation: hazards + vulnerable population = disaster
• Some communities have a high resilience to hazards
• Earthquakes at subduction zones occur at a range of focal depths from 10 km to 400 km.
• Slab pull is the mechanism where new oceanic lithosphere is forced under the continental plate and sinks into the mantle.
• The theory of mantle convection is no longer accepted as the driving force for plate movement.
• There is a correlation between the edge of the subducting plate and the overall velocity of the plate, suggesting slab pull is an important mechanism for plate movement.
• Types of volcanoes include stratovolcanoes, cinder cone volcanoes, and shield volcanoes.
• Constructive plate margins involve plates moving apart from each other, resulting in mid-ocean ridges and continental rift valleys.
• Decompressional melting of peridotite causes partial melting and produces mafic magma.
• Effusive (non-explosive) eruptions are expected at constructive plate margins.
• Earthquakes at divergent plate boundaries are shallow focus earthquakes.
• Earthquakes at divergent plate boundaries can be generated by magma rising or the differential spreading of magma.