Geohazards Flashcards
What is a geohazard?
A geological condition that is dangerous or potentially dangerous to the environment and the people that live within it
What types of geohazards can there be? (not examples?
Natural/artificial
Short term/long term
large scale/small scale
Frequent/infrequent
What is the focus of an earthquake?
Point within the Earth at which the earthquake originates as movement occurs along a fault planes.
What direction do seismic waves radiate from the epicentre and focus?
All directions
How is stored elastic strain energy released?
The force (stress) on the rock caused by relative movement on both sides of the fault. This transfers energy and releases elastic strain stored in rock. When fault ruptures suddenly, elastic strain is reduced and energy released as heat and seismic waves
What is amplitude?
Maximum extent of an oscillation, measured from the position of rest
How does energy release at the focus affect amplitude?
The greater the energy released, the greater the amplitude of the earthquake
What happens to amplitude as the waves move away from the focus of earthquakes?
Amplitude can change depending on the rock type.
Some energy is lost as energy is transferred to the surrounding rock.
As energy is lost (attenuation), amplitude decreases
What is attenuation?
The loss of energy experienced by a wave shown as a reduction in amplitude as it propagates through material
In what type of rock do seismic waves travel faster?
Rigid, cold. Because material are transferred easier
What does empirical mean?
Based on observation or experience. Without due regard for system and theory (without numbers). Mercalli scale is empirical
What is the mercalli scale?
An empirical measure for earthquakes.
Intensity changes with distance (subjective).
1-12, based on what is felt. Measures intensity
What are the disadvantages of using the mercalli scale?
Subjective.
Not easily comparable.
Based on buildings, some may have different structures and regulations.
Based on memory
What are the advantages of the mercalli scale?
Don’t need specialist equipment.
Available to everybody
What is the richtor scale?
Better than mercalli, but not outdated.
Logarithmic, 1-10 scale.
Uses a seismogram. Lag time of p and s waves. Further away = greater lag time, richtor scale will not change with distance.
Objective. Mathematical.
Harder to measure top scales.
Underestimated size of large earthquakes
What is moment magnitude?
Technology (new seismometers) allow for greater energy readings.
Waves arrive at seismometer. lag time (distance), amplitude + actual displacement of rock at earthquake site
What is the equation for moment magnitude?
Mw = (2/3) logE - 6.1
Energy (E) is measured in joules
Mw = moment magnitude
What are p waves?
Primary waves.
Longitudinal.
Travel fastest + arrive first.
Body waves.
Travel through liquids (but slower)
What are s waves?
Transverse waves.
Perpendicular to wave direction.
Arrive second
What are L waves?
Shear.
Love waves rotate on horizontal plane. These are flat circles.
More damaging, side to side, more likely to fracture
What are R waves?
Rayleigh waves.
Vertical circles. Parallel to wave direction, like water waves.
Bigger waves, slower.
Damaging, but less than L
How is the built environment affected generally by ground movement?
L waves cause most damage.
Buildings fracture, office blocks pancake.
Bricks and mortar separate.
Bridges, freeways etc collapse.
Utility pipes separate from each other
What is competent rock?
Rigid, incompressible, high strength, e.g. granite. Most igneous and some metamorphic
What is incompetent rock?
Less rigid, particles less close together. Weak and plastic, tend to fold and develop cleavage. e.g. shale
How does rock competency affect the way that energy is transferred and the buildings that are built upon them?
Waves move faster through competent rock. Competent rocks are more able to withstand the waves, but there will be less energy transferred (dissipated) to surroundings and easier for energy to propagate through rocks.
Incompetent rocks more damaged
What is liquefaction?
Saturated or partially saturated unconsolidated material losing strength and rigidity in response to applied stress, usually an earthquake
How does liquefaction affect the built up environment?
Unconsolidated material and incompetent rock hold more water in pore spaces.
Rock compressed due to seismic waves - pore space reduced - water escapes (upwards).
Loss of strength allows buildings to tip and sink
Where may developments be banned altogether?
On the actual fault zone.
Unconsolidated sediments near the fault.
Alluvial sediments (near fault)
What are some examples of earthquake engineering? (8)
Shielding buildings. bending pipes. mass damper. shock bracers. ground base isolation. diagonal cross brace frames. reinforce with steel. deep foundation
What does shielding a building mean when protecting from an earthquake?
Install different layers of rigidity around a building. Waves deflect off less rigid rock, leaving the building safer
What is the priority with earthquake protection?
Allowing people to flee before building collapse, not necessarily protecting the building.
What is ground base isolation?
Wide base (wider than building), building moves independently from the ground.
What is an example of a mass damper?
Taipei 101.
How can building design reduce the effects of vertical and horizontal stress?
Height.
Shape - asymmetrical + irregular more susceptible to twisting. In symmetrical, stressed are distributed evenly
What is it called when older buildings have earthquake resistance added into it later?
Retrofitting
How can buildings be designed to resist shear forces?
Encase building in framework of diagonal steel girders or cables.
Triangles are good at absorbing stress.
Reduces movement, but doesn’t necessarily save it.
Prevents pancaking. gives more time for evacuation
How can buildings be designed to absorb sway?
E.g. Mass dampers.
Use of hydraulic systems like shock absorbers in cars but bigger.
Flexible connections between different parts of the building.
Controlled rocking frame - building is allowed to rock and returns to centre
How are services (pipes) protected from earthquakes?
Most important is to protect gas pipes (causes fires).
Also worry about water pipes, electrical wiring, and communication.
Flexible pipes or flexible sections
What is natural frequency?
All structures vibrate at their own natural frequency. If this is similar to the ground, the outcome will be worse - more movement.
We want the buildings to have a different natural frequency to the ground. We can control/change it
How do you calculate natural frequency?
f = (1/2π)√(k/m)
F - frequency
k - Stiffness of buildings. depends on construction.
m - mass of the building, depends on height
What is seismic risk?
The possibility of suffering harm or social and environmental loss because of seismic activity
What impacts seismic risk? (5)
Can be changed - mainly from retrofitting.
- History of seismicity
- Nature/mag of hazard
- Frequency
- Earthquake resistance (engineering)
- No. of people/buildings
What is PML?
Probable Maximum Loss
What is probability?
Likelihood of an event occurring
Shown as fraction, percentage or decimal.
Always 0-1
What is the return period?
Average length of time for an earthquake of a given magnitude to occur again or to be exceeded
What is the calculation for return period?
We need to learn this (not provided)!!
T = (n+1)/m
n - no. of years on record
m - number of recorded earthquakes
How are earthquakes taught in Japanese schools?
Get under desk, head first and covered, and hold table legs.
Stay in school, outside and travelling may be dangerous.
Children and teachers in upper years taught how to use fire extinguishers.
What is a forecast?
Uses data and calculations.
A statement of probable occurrence of an even, calculated from data.
What is prediction?
A statement about what you think will happen (based on observation, not maths)
What data can be used for forecasting?
Past geological activity, create hazard map.
Mag, freq, years.
Seismic monitoring
What science be used for prediction?
Nearest active plate
Largest earthquake in area.
Estimate return period (no maths).
Assume epicentre
What are the advantages to making earthquake predictions public?
Lives saved.
Brings attention to safety precautions (evac routes, etc)
What are the disadvantages to making earthquake predictions public?
Social panic.
Mass movement of people (injuries)
If wrong, value lost.
Time period may be short - so counterproductive.
What is the Maximum Considered Earthquake (MCE)?
An earthquake that’s expected to occur in a given area once every 2500 years (2% in 50 years).
These are used in building codes to help protect it.
It is calculated.
What is a building code?
A.k.a planning permission or building regulations.
Set of rules which specify the standards for construction objects such as buildings and non buildings
What is an example of a building code in an earthquake zone?
Must have steel reinforcements.
hospitals, schools, etc, made to withstand more.
Rubber joints or pipes withstand.
Different codes for different areas
What is the role of a geologist/seismologist in an active seismic area?
Provide facts
Risk assessments
draw hazard maps
decide if public warnings are necessary
What steps should local authorities take to reduce fatalities in a town?
retrofit older buildings.
Consider evac routes, especially avoiding older buildings
What are 5 ways to predict earthquakes?
Physical properties, stress, animal behaviours, radon emissions, seismic gap model
How can physical properties be used to predict earthquakes?
Sometimes small earthquakes come before big one.
p wave velocity may decrease, then increase again before earthquake.
Coloured lights in sky have been seen - may be caused by changes in electrical properties of quartz other stress
How can stress be used to predict earthquakes?
P wave velocity decrease, then increase before earthquake.
Area around focus may tilt (deformation), tiltmeters and GPS.
Strain gauges and boreholes measure deformation and therefore any increase in stress.
Water is replenished in groundwater stores before earthquake
How is animal behaviour used to predict earthquake?
Research in China - animals show disturbed behaviours before quake:
- pigs squeal, dogs bark, ground birds perch and snakes leave burrows.
Animals feel p waves.
How can radon emissions be used to predict earthquakes?
Radon - a radioactive decay product of uranium on granite.
Short half-life (3.8 days).
Radon levels sensitive to short term fluctuations. Radon gas accumulates in water wells.
Increase in radon gas suggests gas is percolating through microcracks and earthquake is imminent.
What is the seismic gap model?
Usually at transform.
Plates tend to slip past in sections.
We can track these sections to predict where the next will happen
What can cause tsunamis? (5)
Shallow focus earthquake
submarine volcanic eruption
meteorite
release of glacial lake
large landslide - underwater. A.k.a turbidity current
Why do tsunamis get bigger closer to land?
As it moves closer to land, waves slow down and amplitude gets bigger (transfer of energy). Especially when entering an enclosed area e.g. estuary
What kind of early warning systems are in place for tsunamis?
Sirens,
mobile alerts + radios
Seismograms (only is earthquake caused)
evacuation plans
education
What do early warning systems achieve against tsunamis?
Saving lives.
Saving property relies on reducing wave energy.
Maintain coral reefs
Maintain coastal trees and vegetation
What happened with the Tohoku tsunami, Japan in 2011?
After earthquake, pacific warnings were given. Countries were warned to expect several waves over a 12hr period.
People were evacuated, cancelled trains, closed schools and sewage works and patrolled beaches.
Ships put to sea where amplitude is low
What is clay?
A sedimentary rock. Flat, platy clay minerals. Plastic/mouldable
How does clay form?
From chemical weathering of silicate minerals (silicate includes mafics. Silicate not just silicic). e.g. carbonation + hydrolysis
Most comes from the chemical weathering of feldspar.
Why is a large proportion of the British Isles covered in clay?
A result of glacial retreat
What are the 4 types of clay we study?
Kaolinite,
montmorillonite
vermiculite
illite
What is kaolinite?
Non-expanding. Low shrink-swell capacity.
What is kaolinite used for?
Ceramics and porcelain.
Filler for paint, rubber, plastic.
Paper industry to produce glossy finishes
What is montmorillonite?
Expanding structure. High shrink-swell capacity
What is montmorillonite used for?
Drilling mud for oil industry. Productive liners. Facial powder. Cat litter
What is vermiculite?
Limited expansion. Medium shrink-swell capacity.
When heated, it expands to be a low-density medium.
What is vermiculite used for?
Refractory processes/fire proofing.
Insulation (added to cement).
Gardening
Packing material - high absorbency
Cat litter
What is illite?
Non expanding. Low shrink-swell capacity
What is illite used for?
Ceramics and stoneware. fillers
What are the two sheet types that can form clay?
tetrahedral and octahedral
What is a tetrahedral sheet?
A silica sheet.
Represented with a trapezium shape.
Negative charge
With both tetrahedral and octahedral sheets, how do they connect?
Both ionically bond to something positive between the sheets as both sheets have negative charge. Ionic bonds are weaker
What is an octahedral sheet?
Aluminium sheet.
Represented by a rectangle.
AlO6
Multiple bridging oxygen
What is a 1:1 structure of clay?
Units are joined by stronger hydrogen bonds. This prevents hydration, giving low shrink swell.
Hard for water to get between layers.
Tet, oct, tet oct
What is an example of a 1:1 clay?
Kaolinite
What is a 2:1 structure of clay?
Tend to shrink-swell more than 1:1 (excl. illite).
Water can get through the sheets.
Bonds held together by cations.
Oct sandwich.
tet, oct, tet, tet, oct, tet
Why does illite not have a high shrink-swell despite having a 2:1 structure?
Stronger cation bonds
What is an example of a clay with a 2:1 structure?
Montmorillonite
Vermiculite
illite - not always used because no shrink swell
What are the problems associated with building on clay?
Some clays shrink and swell - vertical movements + subsidence.
Foundations damaged and cracking.
Shrinking - damaged pipes.
Soils with clay are also a risk
How can engineers mitigate the problems caused by clay?
Understand water table. And monitor & maintain. Expensive
Deep foundations past clay. Pile foundations.
Add or remove trees. Trees absorb water
Artificial technology for water pumping.
Continuous and reinforced raft foundations
What are raft foundations?
Large (larger than building) concrete slab reinforced with steel bars and mesh. this spreads weight over larger area. tries to avoid tilting or cracking
How can the properties of clays be changed?
Smectite = 2:1 - shrink swell.
It contains Na+ ions. Their ability to shrink swell by 1500% (15x).
If we add lime/limestone slurry. This adds Ca2+ ions. These displace Na ions. Shrink swell 100% (2x). Then you can build foundations.
Can also add bacteria - changes the charge, e.g. Fe3+ to Fe2+. Changes oxidation state and lowers shrink swell
What is subsidence?
The vertical downward movement of ground due to shrinking of clay or clay-rich soils
What are the 4 mining strategies that can cause subsidence?
Longwall mining, shallow mining, deep mining, salt mining
What is longwall mining and how can it cause subsidence?
Extraction method for coal.
Excavate a scene, move forward and allow for back wall to collapse/
Area affected above > area of mining.
May leave small gaps from fractured rock. Some subsidence can happen 10-15 years after this, but it usually happens within weeks
What is shallow mining and how can it cause subsidence?
Old small workings such as bell pits and piller & stall workings.
These left unstable voids underground.
Problematic because records are rarely kept for these, so the collapse in unexpected
What is deep mining and how can it cause subsidence?
A.k.a stope mining
Big voids and voids on each other.
Usually more records for these, so they know there is a subsidence risk and therefore better management
What is salt mining and how can it cause subsidence?
Causes massive voids.
Can remove salt with hot water.
We try to refill voids with pressurised water but water might move through rocks, causing the pressure to drop, thus causing subsidence
What are the similarities between crown holes and sinkholes?
They look the same.
Circular depression of any size.
Caused by subsidence
What are crown holes?
Anthropogenic in source (our fault).
e.g. mines
What are sinkholes?
Natural causes.
e.g. limestone caves - natural dissolution from acidic groundwater
How can subsidence be avoided?
Concrete underpinning
Avoid karst terrain (limestone)
What are the three main causes of mass movement?
Increase in mass
Increase in slope angle
Decreasing friction
Why would there be an increase in mass, which may lead to mass movement events?
Water presence, or buildings added
Why might there be an increase in slope angle which may lead to a mass movement event?
River erosion, roads, tectonic activity (faults)
Why might there be a decrease in friction which may lead to a mass movement event?
Water added. Acts as a lubricant
How are the types of mass movement classified?
Velocity of material on the move.
If mass movement is really slow, it would be a …?
Creep, or solifluction
If mass movement was medium speed, it would be a …?
Landslide or slump
If mass movement was fast, it would be a …?
Rockfall
If clays and water are involved in a mass movement event, it would be a …?
Flow (if water major component)
e.g. mudflow, mudslide
Why could competent rock result in a mass movement event?
More steep slopes. more likely to break.
Makes translational slides (moves all at once). breaks up - talus slopes
Why could incompetent rock result in a mass movement event?
Less steep slope, more isotropic.
Low shear strength, more rotational slide
What are the 5 triggers of a mass movement event?
Addition of water, building on slopes. clearance/deforestation, sudden drop in water table, earthquakes
Why would clearance/deforestation result in a mass movement event?
less trees = less roots = less soil stability
Why would a sudden drop in the water table result in a mass movement event?
Changes pore spaces within rocks
Rock beds facing which direction are more likely to be involved in a mass movement event?
Those facing down a slope
What are 8 methods that can be used to stabilise material and prevent a mass movement event?
Build concrete retaining wall,
gabions
rock bolts
rock drains
wire mesh
slope modification
shotcrete (sprayable concrete)
Vegetation fix
