Tectonics:

• Tectonic plates


  Earth's tectonic plates move at a speed of 2-5 cm per year
• Types of tectonic plates

  • Seven very large major plates (African, Pacific)
  • Smaller minor plates (Nazca, Philippine Sea)
  • Dozens of small microplates
• Tectonic plates


  • All fit together into a constantly moving jigsaw of rigid lithosphere
  • Each plate is about 100 km thick
  • Its lower part consists of upper mantle material
  • Its upper part is either oceanic or continental crust
• Theory of plate tectonics


  • Developed because of a number of key discoveries
• Key discoveries


  • Alfred Wegener's Continental Drift hypothesis in 1912
  • Arthur Holmes' ideas in the 1930s that Earth's internal radioactive heat was the driving force causing mantle convection
  • Discovery in 1960 of the asthenosphere, a weak, deformable layer beneath the rigid lithosphere
  • Discovery in the 1960s of magnetic stripes in the oceanic crust
  • Recognition of transform faults by Tuzo Wilson in 1965
• It remains a theory because scientists have not yet directly observed the interior of the Earth
• Constructive margins


  • Mantle convection forces plates apart
  • Tensional forces open cracks and faults between the two plates
  • These create pathways for magma to move towards the surface and erupt, creating new oceanic plate
  • Eruptions are small and effusive in character, as the erupted basalt lava has a low gas content and low viscosity
  • Earthquakes are shallow, less than 60 km deep, and have low magnitudes, usually under 5.0
• Destructive margins and subduction zones

  • Mantle convection pulls oceanic plates apart, creating the fracture zones at constructive margins, and convection also pulls plates towards subduction zones
  • Constructive margins have elevated altitudes because of the rising heat beneath them, which creates a 'slope down which oceanic plates slide (gravitational sliding or 'ridge push')
  • Cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the oceanic plate pulls itself into the mantle (slab pull)
• Subducting plate


  1. Begins to melt at depth by a process called wet partial melting

  2. Generates magma with a high gas and silica content

  3. Erupts with explosive force
• Collision zones


  • The Himalaya mountains is a location where two continental plates are in collision (the Indo-Australian and Eurasian plates)
  • Collision began about 52 million years ago
  • As both continental plates have the same low density, subduction is not possible
  • Plates have crumpled, creating enormous tectonic uplift in the form of the Himalayan and Tibetan Plateau
  • Magma is being generated at depth, but it cools and solidifies beneath the surface so eruptions are rare
  • Cut by huge thrust faults that generate shallow, high-magnitude earthquakes
• Transform zones


  • Conservative plate boundaries consist of transform faults
  • These faults 'join up' sections of constructive plate boundary as they traverse the Earth's surface in a zig-zag pattern
  • In some locations, long transform faults act like a boundary in their own right, most famously in California where a fault zone - including the San Andreas fault — creates an area of frequent earthquake activity
  • Earthquakes along conservative boundaries often have shallow focal depths, meaning high-magnitude earthquakes can be very destructive
  • Volcanic activity is absent
• Earthquakes


  1. Sudden release of stored energy

  2. Tectonic plates attempt to move past each other along fault lines, they inevitably stick

  3. Allows strain to build up over time and the plates are placed under increasing stress

  4. Earthquakes are generated because of sudden stress release — so-called stick-slip' behaviour

  5. A pulse of energy radiates out in all directions from the earthquake focus (point of origin, sometimes called the hypocentre)
• Earthquake motion


  • Displaces the surface, so a fault scarp is formed
• Earthquake seismic waves


  • P-waves (primary waves) - fastest, arrive first, cause the least damage
  • S-waves (secondary waves) - arrive next, shake the ground violently, causing damage
  • L-waves (Love waves) - arrive last, travel only across the surface, have a large amplitude and cause significant damage, including fracturing the ground surface
• Earthquakes can rupture a fault line for up to 1000 km
• Earthquakes can cause ground shaking that lasts up to 5 minutes, as well as dozens of powerful aftershocks
• Earthquakes frequently generate large landslides as secondary hazards, especially in areas of geologically young (and therefore unstable) mountains such as the Himalayas
• Liquefaction is a particular hazard in areas where the ground consists of loose sediment such as silt, sand or gravel that is also waterlogged - often found in areas close to the sea or lakes
• Intense earthquake shaking compacts the loose sediment together, forcing water between the sediment out and upward, undermining foundations and causing buildings to sink, tilt and often collapse
• Volcanoes at destructive plate margins

  • Major volcanic eruptions frequently have more than one hazard associated with them
  • These are often secondary hazards, which are an indirect consequence of the eruption (lahar, jökulhlaup)
  • Large composite volcanoes found at destructive plate margins represent a significant tectonic hazard
  • These eruptions often have lava flows, pyroclastic flows, lahars and extensive ash and tephra fall that can affect areas up to 30 km from the volcanic vent
• Tsunami


  • Can be generated by landslides and even eruptions of volcanic islands
  • Most are generated by sub-marine earthquakes at subduction zones
  • Occur when a sub-marine earthquake displaces the sea bed vertically (either up or down) as a result of movement along a fault line at a subduction zone
  • The violent motion displaces a large volume of water in the ocean water column, which then moves outward in all directions from the point of displacement
  • The water moves as a vast 'bulge' in open water, rather than as a distinct wave
• Lava flows - Extensive areas of solidified lava, which can extend several kilometres from volcanic vents if the lava is basaltic and low viscosity, It can flow at up to 40 kmh. They occur in composite and shield type of volcanoes.
• Tsunami characteristics


  • Wave heights are typically less than 1 m
  • Wavelengths are usually more than 100 km
  • Speeds are 500-950 km/h
• In the open ocean tsunami waves are barely noticeable
• As the waves approach shore

  They slow dramatically, wavelength decreases but wave height increases
• Tsunami


  Hit coastlines as a series of waves (a wave-train'), the effect of which is more like a flood than a breaking wave
• Sub-marine earthquakes that occur close to shorelines


  Can generate intense ground-shaking damage, followed by damage from the subsequent tsunami
• Land-use zoning: preventing building on low-lying coasts (tsunami), close to volcanoes and areas of high ground-shaking and liquefaction risk

  Advantages:

  Low cost

  Removes people from high-risk areas

  Drawback:

  • Prevents economic development on some high-value land, e.g. coastal tourism

  • Requires strict, and enforced, planning rules
• Aseismic buildings: cross-bracing, counter-weights and deep foundations prevent earthquake damage

  Advantages:

  • Widely used technology can prevent collapse property

  protects both people and propert.

  Drawback:

  • High costs for tall/large structures

  • Older buildings and low-

  income homes are rarely protected (Figure 13)
• Vent modification


  The most desirable type of management
• Vent modification is not always possible
• Loss modification


  Implies that a disaster has occurred and caused damage to people and property, the least desirable form of management
• Modify event (before the hazard strikes, long term)

  1. Mitigate the impacts of the hazard, by reducing its areal extent and/or effective magnitude

  2. Modify vulnerability
• Modify event (before the hazard strikes, short term)

  Get people out of the way of the hazard, or help them cope with its impacts by building resilience
• Modify loss (after the hazard strikes, short and long term)

  Reduce the short- and long-term losses by acting to aid recovery and reconstruction
• Event modification


  • Relies on technology and planning systems and can be high cost
  • Less likely to be used in developing and emerging countries, although low-cost examples of hazard resistant design are possible to build
• Tectonic hazards


  Natural events that have the potential to harm people and their property
• Disaster


  The realisation of a hazard, i.e. harm has occurred
• Disasters have to involve people and they occur at the intersection of people and hazards, as shown by the Degg disaster model (Figure 5)
• Threshold level to determine if an event is a disaster


  • 10 or more deaths
  • 100 or more people affected
  • US$1 million in economic losses