T1 Hazards

• What causes seismic activity?

  • Sudden formation of a fault
  • Sudden slip of an existing fault
  • Movement of magma or volcanic eruption
  • Landslide
  • Meteorite impact
  • Human activity e.g. fracking
• Elastic Rebound Theory

  • Rocks on each side of the fault are forced to move. Rocks at fault are locked together
  • Rocks locked together bend, storing energy
  • Rocks suddenly let go, springing back, causing an earthquake
• Factors affecting earthquake intensity

  • Magnitude of the earthquake
  • Type of seismic waves
  • Underlying geology or unconsolidated sediment - clays tend to amplify earthquakes
  • Liquefaction
  • Resonance
  • Building quality
• Magnitude

  • Richter scale - assigns a number to quantify energy released in an earthquake
  • It is a logarithmic scale.
  • Moment Magnitude Scale - measures energy release more accurately than the Richter Scale
  • Takes into account:
  • Size of rupture area
  • Amount of displacement
  • Rigidity of the local rock
  • Most accurate measurement of large earthquakes. Not used to measure magnitude of smaller events
• Intensity

  • Modified Mercalli Scale
  • Measures earthquake intensity AND impact
  • Relates ground movement to impacts that can be felt and seen by anyone in the affected location
  • Qualitative assessment based on observation and description
• Ground Shaking (Hazard)

  • Ground motion - can damage or destroy buildings
  • Surface rupture occurs when fault actually breaks the ground surface, splitting buildings, disrupting roads etc
• Liquefaction

  • Loose sand and silt that is saturated with water can behave like a liquid when shaken by an earthquake
  • Earthquake waves cause water pressures to increase in the sediment. Sand grains lose contact with each other
  • Sediment loses strength and behaves like a liquid
• Tsunami

  • A large ocean wave that is caused by a sudden displacement of water
  • Two principal driving mechanisms are displacement of water via movement of sea floor and underwater landslide
• Human Factors

  • Population density - if high, results in much larger casualties in cases such as widespread building collapse.
  • Hazard awareness and preparation - Hazard mapping and planning, as well as early warning systems
  • Building type and density - can buildings withstand earthquakes? Buildings made with wood are more likely to result in widespread fires and closely spaced buildings can result in many issues
  • Level of economic development - economic development of a country is important - advanced countries are more likely to be able to afford higher quality building standards.
• Geological evidence for tsunamis

  • Beach deposits located farther inland
  • Shows largescale movement of water
  • Rip-up clasts
  • Show erosive current
  • Abrupt erosional lower contact
  • Sudden change indicated by an erosive base
  • Grain size
  • Shingle or sand // graded bedding
  • Mud caps
  • Mud at the top of a sequence - suspension from standing water.
  • Fossil assemblage
  • Mixed fauna from a variety of environments
  • Twig orientation
  • Indicates seaward return during deposition
• Storegga Slides

  • 3 mass movement events occurring between 30Ka and 6Ka
  • Shetland islands contain evidence of these:
  • Waves of 20m
  • Rip up clasts
  • Graded bedding
  • Sand layers and mud caps
  • Diatoms from different environments
• • Most waves are generated by wind - wavelengths and wave heights are low
  • In tsunamis, more water is displaced and energy extends throughout the entire depth of the ocean
• In open ocean tsunami waves have wavelengths of over 100km but low amplitudes. As they approach the shore they slow down and wavelengths decrease and amplitudes increase.
• • Tsunamis reduce in height and energy with distance
  • Small inlets and shallower shores worsen the effects of tsunamis.
  • Small inlets result in a funnelling effect increasing tsunami heights and shallower shores cause waves to slow more and rise higher
• • Reducing the effects of tsunamis
  • Early warning system by siren, radio and loudspeaker. Education that a retreating sea indicates an approaching tsunami.
  • Reducing energy of wave by:
  • Maintaining coral reefs in good condition
  • Maintaining coastal trees and vegetation
  • Prohibiting building along the coast
  • Designing buildings with no permanent accommodation on the ground floor
  • Walls/ embankments to reduce energy of the wave
• • Bell pits and adits
  • Up to 18th century, coal only mined near surface. This was known as bell pits and adit mines
• • Pillar and stall
  • 3m wide sections of coal seams are left in place to support the root. Coal around the pillar is worked leaving an empty space. Tunnels and workings may also have pit props to prevent smaller rockfalls.
• • Longwall
  • Roof held up by closely spaced, mobile hydraulic steel supports called chocks. Once a slice of coal is removed, chocks are moved forwards and the mined-out area is allowed to collapse. System of deliberate collapse can cause subsidence on the surface.
• • Stope mining
  • Most productive to extract the ore from the ore body itself without the need to construct additional shafts - leaves a cavity, or stope, which may not need support if surrounded by strong rock. Common to drill shafts vertically downwards and then drive horizontal levels through to the ore body. Stope may be backfilled or have a controlled collapse.
• Deep cast mining hazards

  • Flooding
  • Gas and coal dust explosion
  • Collapse of roof and rockfalls
  • Subsidence
  • Waste tips
  • Spoil heaps that can trigger a mass movement
• Open cast mining hazards

  • Sides of a coal quarry can be unstable and collapse
  • Flooding, it extends below water table
• Deep metal mines

  • Often very deep, high geothermal gradient so working conditions are hot
  • Dust and fines
  • Release of toxic metals from the ore
  • Mine waste leading to acid mine drainage
• ABERFAN 1966

  • Tip developed on a hill slope made of permeable sandstone
  • Beneath sandstones there is coal measure shale. Water table was very high.
  • Tip partly built on material left by a landslide from another tip
• ABERFAN 1966

  • Rotational landslip occurred
  • Tip was saturated by water from springs as well as rain and surface fracturing due to mining subsidence increased rate of water flow into the tip
  • Rotational landslip developed a mudflow
• Mineral processing

  • In situ leaching at depth
  • Boreholes drilled into the ore deposit, opened up by explosive or hydraulic fracturing. Leaching solution is pumped down to the ore. Solution carrying dissolved ore is pumped back to the surface via a second borehole and processed.
• • Heap leaching at the surface
  • Ore is crushed and heaped into an impermeable clay or plastic liner. Leaching solution is applied and percolates through crushed ore. Solution of dissolved minerals accumulates and is taken for processing
  • Method is cheap but only recovers 60-70% of the ore. Takes between 2 months and 2 years
• • Froth flotation
  • Separates hydrophobic and hydrophilic material. Yields significant amounts of metal from lower grade ores and can separate a wide range of sulphides, carbonates and oxidises from each other
• • Crushing & tailings disposal
  • Crushing produces fine grained waste called tailings or slimes, which contain harmful and toxic metals. Problems may continue for many years after mines are closed.
• • Smelting
  • Extraction of elemental metal from the ore. Iron extracted from haematite in a blast furnace. Dead zones occur around some smelters where soils are contaminated and vegetation has died
• ACID MINE DRAINAGE MANAGEMENT

  • Source control
  • Migration control
  • Active treatment
  • Passive treatment
• • Source control
  • Prevents oxygen and water reaching the ores
  • Dewatering a mine is expensive but an effective method although water removed will need treating
  • Conversely, abandoned mine could be flooded and sealed with concrete/clay
• • Migration control
  • Aims to neutralise acidity and precipitate the metals as non toxic salts which can be disposed of correctly
• • Active treatment
  • Involves adding bases such as lime, calcium carbonate, sodium carbonate or sodium hydroxide to neutralise AMD after a water body is polluted. Requires constant resupply of chemicals and disposal of salts, expensive but more reliable and effective when dealing with high rates of flow of mine water.
• • Passive treatment
  • Uses natural and constructed wetland ecosystems. Expensive to set up and need more land but require only minimal maintenance and no electrical power or hazardous chemicals.
• • Types of waste
  • Inert waste - no chemical or biological hazards
  • Special waste - hazardous waste e.g. carcinogenic or mutagenic
• • Leachate
  • Solution formed by rainwater percolating through landfill site dissolving any soluble chemicals
  • Can be acidic which increases weathering of underlying limestone
• • Choosing a landfill site - factors to consider
  • Rock type e.g. permeability, stability and resistance to weathering
  • Geological structures
  • Height of water table
  • Size of the hole
• • Choosing a landfill site - geological structures
  • No dips
  • No faults
  • Few/no joints
  • Massive - few bedding planes
• • Choosing a landfill site - Height of water table
  • If water table is high, less distance for the leachate to travel to reach underlying groundwater
• • Ideal landfill location
  • Impermeable
  • Resistant to weathering
  • Few and/or widely spaced joints
  • No faults
  • No dip OR a synform