T1 - Geohazards

• Earthquakes at Transform Boundaries:

  • plates slide past each other horizontally
  • shallow focus earthquakes (<70km depth)
  • eg. San Andreas Fault, California (Pacific Plate moving >6mm/yr and North American Plate moving 2mm/yr)
• Earthquakes at Convergent Boundaries:

  • Continental-Oceanic; subduction zone (denser plate subducted beneath more buoyant plate) + Wadati-Benioff Zone (inclined zone of earthquakes beneath more buoyant plate, <600km) eg. Kuril Islands between Japan/Russia.
  • Continental-Continental; subduction largely prevented + no Waditi-Benioff Zone + crustal thickening/deformation over large area + broad zone (>100km) of shallow earthquakes.
• Earthquakes at Divergent Boundaries:

  • shallow earthquakes (<30km)
  • restricted to narrow zone close to spreading ridge
  • often low magnitude
  • developing divergent boundary shallow earthquakes on extensional faults as crust thins
• Magnitude measured on Moment Magnitude Scale (Mw); the intensity of earthquake damage around an event is measured on modified Mercalli scale (related to size of earthquake, depth, distance and local ground conditions).
• Moment Magnitude Scale (MMS/Mw):

  • measures size/magnitude of earthquake (bigger magnitude = more energy released); more accurate than Richter Scale
  • logarithmic scale; increase of 1 step correspones to 10$^1.5$ times increase in amount of energy released + increase of 2 step correspones to 10$^2$ times increase in energy
  • eg. earthquake 7.0 (Mw) releases 32x as much energy at 6.0 and 1000x that of 5.0
• Moment Magnitude (Mw) gives most reliable magnitude for large earthquakes as derived from size of fault rupture, displacement, and energy released (all can be measured).

  • slip along fault can be thought of as a rotation around a pivot on the fault plane of rock moving in opposite directions + represents a lever which is measured by moments + sum of these rotational effects is the seismic moment (Mo)
  • orientation of fault/direction of movement are determined from waveforms recorded at different seismographs
• Mw = $2/3log(Mo) - 10.7$

  Seismic Moment (Mo): Mo = μAd

  • μ = shear strength of rock
  • A = area of rock
  • d = displacement
• Modified Mercalli Scale:

  • measures intensity of earthquakes + qualitative (based on observations; some could exaggerate/reports conflicting)
  • earthquakes classfied by noticeable effects of surface vibrations in area based on earthquake's local acceleration/duration, effects depend on; strength of earthquake at focus, distance from focus, nature of sub-surface rocks and design of buildings

  
• Global Distribution of Earthquakes:

  • 90% occur at tectonic plate boundaries + 10% occur away from plate boundaries (intra-plate quakes); greatest concentration is Pacific 'Ring of Fire'
  • few earthquakes occur >670km depth; intermediate/deep events linked to subduction zones
  • oceanic Nazca plate subducts beneath continental South American plate (convergent boundary); earthquakes progressively deeper

  
• Earthquake - sudden release of strain energy in Earth's crust, resulting in waves that radiate outwards from earthquake source.

  • movements within crust causes stress to build up at points of weakness causing rocks to deform
  • when stress exceeds strength of rock, it fractures along a fault (often at zone of existing weakness in rock)
  • vibrational energy is released, spreading out in all directions; shock waves radiate out from point where rocks first begin to break (focus/hypocentre)
• Distribution:

  • most earthquake activity is concentrated on a number of earthquake belts (eg. Mid-Atlantic Ridge)
  • over 80% large earthquakes occur around edges of Pacific Ocean ('Ring of Fire'); pacific plate subducting beneath surrounding plates (most seismically active zone in world)
• Earthquakes are result of sudden movement along faults within earth; movement releases stored up 'elastic strain' energy in form of seismic waves + movement on faults is response to long-term deformation and build up of stress.
• Plate Tectonics:

  • earth's outermost layer is fragmented into tectonic plates; these from lithosphere (oceanic/continental crust and upper mantle)
  • tectonic plates move slowly relative to each other (few cm/yr) but causes huge amount of deformation resulting in earthquakes at plate boundaries
• Movement of Tectonic Plates:

  • ridge push (buoyant upwelling mantle at mid-ocean ridges); newly formed plates at oceanic ridges are warm (higher elevation at oceanic ridge than colder/denser plate material further away) + gravity causes higher plate at ridge to push away lithosphere
  • slab pull; older/colder plates sink at subduction zones because as they cool they become more dense than underlying mantle + cooling sinking plate pulls rest of warmer plate along behind it
• Elastic Rebound Theory:

  • before earthquakes, buildup of stress in rocks on either side of fault results in gradual deformation
  • this deformation eventually exceeds frictional force holding rocks together and sudden slip occurs along fault
  • releases accumulated stress + rocks return to original shape (elastic rebound) but offset on either side of fault
• • Epicentre - point at surface of earth directly above the focus.
  • Focus - point within earth where an earthquake rupture starts.
  • Seismic Waves - waves that transmit energy released by an earthquake.
• Geohazard - natural geological processes that present a direct risk to people or an indirect risk by impacting development.
• • Earthquake Magnitude - size of the earthquake.
  • Earthquake Intensity - degree of shaking caused by an earthquake at a specific place.
  • Earthquake Frequency - how often a given earthquake with a certain magnitude occurs.
• Earthquakes at Divergent Plate Boundaries: eg. Mid-Atlantic Ridge

  • small/frequent (due to relatively high rock temperatures where spreading happens)
  • restricted to narrow zone close to ridge + shallow (<30k depth) as crust not thick
• Earthquakes at Convergent Plate Boundaries: eg. New Zealand and Himalayas

  • very large earthquakes (M9+) at subduction boundaries + produce most of earthquakes M6+
  • subduction zones have deepest earthquakes; deep-focus earthquakes (300km+) + range of depths shallow-700km
  • powerful/deep earthquakes; abundant/increasingly deep on landward side of subduction zone
• Earthquakes at Passive Plate Boundaries: eg. San Andreas Fault

  • transform faults; large/shallow-focus (<30km) earthquakes (earthquakes in centre regions of these plates not usually large)
  • San Andreas Fault produces around 10,000 earthquakes/yr + New Zealand transform fault around 20,000/yr (most can't be felt)
• Factors Affecting Impact of Earthquake:

  • size/magnitude
  • distance from epicentre
  • depth of focus
  • local ground conditions
  • building standards
• Local Ground Conditions

  • if buildings constructed on reclaimed land from sea, vulnerable to earthquakes + buildings on old lake beds vulnerable to liquefaction + structures with foundations that extend into solid bedrock less likely to be damaged in seismic events
  • eg. Mexico City built on reclaimed lake bed (silty soil); M8.1 event September 1985, large areas of silty soil liquefied (buildings partially sunk into ground)
• Building Standards

  • interlocking triangular components have low centre of gravity + pendulem counter-acts shaking - unreinforced masonry vulnerable to earthquakes
  • HICs have more building standards to protect against earthquakes (old buildings can be retro-fitted) eg. shatter-proof windows
• Ground Movement/Shaking:

  • cracks open at wave crests/close at wave troughs adding highly destructive vertical components
  • upward acceleration of ground may be greater than that of gravity (loose objects thrown upwards)
• Ground Movement/Shaking:

  • movement of ground separates parts of building structure (bricks/stonework seperate along mortar + walls collaspe) + floors separate from supporting walls to 'pancake' each other - oil, gas, drainage, water, sewage pipes may separate and break
  • buildings sway when their foundations move sideways by top storeys lag behind + if dimensions mean its natural frequency matches vibrations, amount of sway increases
• Liquefaction of Ground:

  • vibrations in bedrock, transmitted to overlying superficial deposits at surface + in wet sand/silt, water separates from solid particles and rises to surface
  • houses built on alluvial deposits/floodplains, reclaimed land from sea or old lake beds at most risk (eg. Christchurch NZ/Mexico City)
  • soils loses strength during shaking and behaves like a liquid; buildings sink/subside/topple as ground liquefies - once shaking stops, sand grains compact together again
• Tsunami:

  • uplifted fault block, displacement of large area of seafloor, ocean displaced and waves radiate from source in all directions
  • water shallows and waves slow down (increase in height as wave energy crowded into smaller volume of water)
  • sea floor irregularities reflect some wave energy so less energy reaches coast
  • tsunami event has sequence of waves; 4/5th waves largest + 40% wave energy scattered back to sea

  A) Wave Length: 150-250km

  B) Wave Period: 10-60mins

  C) Wave Height: 0.5-5m

  D) Waves 1m in height in open ocean may reach 20m
• Mass Movement - a sudden slope failure or collaspe, involving the downhill transfer of material, both consolidated and unconsolidated, by one or more of three main mechanisms (fall, slide and flow).
• Mechanism and triggering of mass movement events linked to angle of slope, lithology, weathering, load, groundwater regime, rainfall, ground vibration and vegetation cover.
• Falls:

  • landslides that involve collaspe of material from a cliff/steep slope; usually involve a mixture of freefall through the air, bouncing or rolling + fall-type landslides result in collection of rocks/debris near base of slope

  
• Slides:

  • downslope movement of material that occurs along a distinctive rupture or slip surface (deeper than other landslide types/not structurally controlled)
  • prominent main scrap and back-tilted block at top with limited internal deformation + failure of material at depth with movement rotational about an axis
  • two types; rotational (slump) and translational (planar) slides

  
• Flows:

  • landslides that involve movement of material down a slope in form of fluid + often leave behind an upside-down funnel shaped deposit where material stopped moving
  • types; mud, debris, rock (avalanches)

  
• Factors influencing Mass Movement:

  • Triggers - rainfall in large amounts (intense/frequent/heavy) + ground vibrations from earthquakes or influence from trains/large vehicles.
  • Weathering; rates of weathering differ at locations + certain rock types more susceptible to weathering - Load; the weight (driving force) Vs the friction (resistant force) - Groundwater Regime
• Factors influencing Mass Movement: (Mechanisms)

  • Slope Angle; steeper slope = greater effect of gravity/higher landslide potential.
  • Lithology; clays/sands slip with heavy rain + hard rock in steep cliffs vulnerable to weathering along joints/beds + consolidated Vs unconsolidated.
  • Human Factors; construction increases loads (more vulnerable to landslides) +drainage and mines/quarries affects slope stability.
  • Vegetation; roots bind soil/rock particles and intercept water (removal of vegetation makes slopes vulnerable).
• • Geohazard - the probability of a change in the geological environment of a given magnitude within a specific time period in a given area.
  • Risk - the consequent threat of loss of life/damage to property and infrastructure or the probability of a hazard occuring and creating loss of lives/livehihoods.
  • Exposure - an assessment of which elements are at risk (eg. people, buildings or agriculture).
• • Vulnerability - implies a high risk of exposure to hazard, combined with an inability to cope; dependent upon resilience of people, the reliability of management systems and quality of governance.
  • Disaster - the realisation of a hazard which causes a significant impact on a vulnerable population (suggested it becomes a hazard when 10+ killed, 100+ affected, £1million loses or state of emergency declared).
• Degg's Model of Natural Disasters:

  • the more severe the geophysical event and/or more vulnerable the human population, the more the two overlap and the larger the disaster

  
• Risk/Vulnerability - Kobe, Japan:

  • increasing urbanisation (poor squatter settlements in megacities) creates high hazard risk/exposure; vulnerable to fires post quake
  • developed countries invest in technology for disaster reduction/aid after hazard event (eg. building design/monitoring) + high frequency of earthquakes so well-prepared
  • ageing population increases vulnerability (problem of emergency evacuation/survival); strong central government leads to efficient rescue/phone alerts informs citizens/preparation days
• Kobe, Japan Earthquake:

  • January 17th, 1995; M7.2 earthquake with epicentre 20km off coast of Kobe(northern part of Awaji Island) with focus 16km below surface + Kobe less seismically active area of Japan with last major earthquake in 1916 (M6.1)
  • Japan a multi-hazard hotspot where impact of geohazards amplified by other hazards