🌋3.1.5.4 - Seismic Hazards Flashcards
What are seismic hazards?
Earthquakes, tsunamis, landslides and liquefaction
What is seismicity?
The Earth’s shaking, can be human induced or natural
How are seismic waves formed?
Pressure builds up at a point where plates meet
Sudden release due to rock failure creates waves
The waves cause the ground to shake
Intensity of the shaking is dictated by the depth of focus and energy release
What are tsunamis caused by?
Earthquakes under the sea, moving a column of water above it, displacing it
What are the characteristics of tsunamis?
Waves less than 3ft tall, but are compress towards land, gaining height. A wave train which propagates form the earthquake.
What is liquefaction?
The mixing of sand or soil with groundwater during the shaking of a moderate or strong earthquake. Ground becomes very soft and similar to quicksand.
When is land susceptible to landslides?
It has experienced heavy rain
Fractured or unconsolidated rock
Fold mountains, unstable plate boundary
Steep relief
How can earthquakes be predicted?
Past seismic events
Remote sensing
Radon gas emissions
How can past seismic events be used to predict earthquakes?
Looking into past frequencies and magnitudes to infer future events can be used for prediction
How can remote sensing be used to predict earthquakes?
GPS can be used to monitor the smallest of movements by the plates, stationary plates suggest build up of pressure
How can radon gas emissions be used to predict earthquakes?
Radon escapes form cracks in the earth’s crust and a sudden increase may suggest that an earthquake may be imminent
Are earthquakes predictable?
No
What type of boundary to most earthquakes originate from?
Destructive - at subduction zones
Where is liquefaction a particularly dangerous hazard?
In areas where groundwater is near the surface and the soil is sandy
What is subsidence?
When the ground surface is lowered often during earthquakes
What are the characteristics of P waves?
Fastest waves
Earth moves backwards and forwards but surface stays flat
Moves through solids and liquids
What are the characteristics of S waves?
A ripple, up and down movement
Water wave, surface doesn’t stay flat
Shakes earth at right angles
What are the characteristics of Rayleigh waves?
Up and down movement and side to side, moves like a snake
Low frequency, rolling motion
Radiate along the surface
What are the characteristics of Love waves?
Slowest waves, surface
Causes sideways motion and most damage
One pulse, up and down and some side to side
What is a seismograph used for?
To measure ground movement
How does a seismograph work?
Pen attached to weight to track movements
What is a seismometer used for?
Recording ground movement data digitally
What are the issues with a seismometer?
So small that waves must be amplified
What does the Mercalli scale range from and to?
1-12
What is a 1 on the Mercalli scale?
Instrumental - not felt by many people unless in favourable conditions
What is a 12 on the Mercalli scale?
Cataclysmic - total destruction
What are the positives of the Mercalli scale?
Can be used for damage comparison Relates to epicentre location No equipment required Idea of damage done, builds a picture Perfect for response planning Allows links for human need
What are the negatives of the Mercalli scale?
Subjective, qualitative measure
Uncertainty and bias
Location of observer alters results
Compares effects, not the event
What does the Richter scale measure?
Ground deformation and energy release
What type of scale does the Richter follow?
Logarithmic
What is the highest recorded value on the Richter scale?
8.9
Seismicity
Earth shaking
Can be human induced through fracking, mining and reservoir construction
Earthquake Formation
. Friction along margins = stress in lithosphere
. Stress suddenly overcome
. Plates fracture along faults
. Sends a series of shock waves to the surface
. Tremors can be followed be several weeks of after shocks whilst crust settles
What factors make the damage done by an earthquake worse?
Time of day
Depth of focus
Population density
Level of development
Building quality
P Waves
Fast moving, first to reach the surface
Travel through solids and liquids (crust, mantle, core)
High frequency
S Waves
Second waves to reach the surface
Travel through liquids (mantle)
Sideways movement, shaking earth at 90° to direction of travel
Do more damage than P waves
Love Waves
Slowest moving
Do most damage because of sideways movement
Rayleigh Waves
Radiate from epicentre in complicated, low frequency rolling motions
Tsunami
Large sea wave generated by earthquakes on the ocean floor, large landslides and submarine volcanic eruptions
Tsunami Formation
. Pressure between 2 plates suddenly released
. Large waves of energy displace water
. Water train created, spreads in concentric circles to shore
. Upon reaching shore, wave shoaling occurs (friction = highly destructive wave)
. Wave retreats taking debris, forming the next wave
What factors make the damage done by a tsunami worse?
Distance travelled
Focus depth (shallower = larger)
Population density
Level of development
Warning given
Landslide
Movement of rock, debris or earth down a slope
Landslide Formation
Ground shaking due to earthquake destablishes cliffs and steep slopes
Causes material to quickly move down slope
What factors make landslides worse?
Steep gradient
High relief rainfall (saturates ground = unstable)
Shallow soil
Unstable rock
Liquefaction
Jelly like state of silts and clays resulting from intense ground shaking
Liquefaction Formation
Sand or soil mixes with ground water
Becomes very soft, causing subsidence and building collapse
What factors make liquefaction worse?
High ground water
Reclaimed soil
Sandy soil (loosely packed grain = more pore spaces to fill with water)
Richter Scale
Logarithmic
1 to 10+
Easy comparison between disasters
Quantitative
Modified Mercalli Scale
Qualitative
0 to XII
Different levels for different areas
Only useful with buildings around
Preparedness: Name some of the events that occur in the minutes before an earthquake
Microquakes before main tremor
Magnetic changes in local rock
Unusual animal behaviour
Prevention: Although almost impossible to prevent, what have some scientists suggested?
Putting water and oil in margins
Lubricates movement
Plates will slide not stick
Adaptation: What form does protection mainly take?
Preparation by modifying the human and built environments to decrease vulnerability and reduce loss
Adaptation: 2 features of hazard resistant structures
Concrete weight on roof- moves in opposite direction to force of earthquake, counteracting stress
Rubber shock absorbers in foundations- allows some movement
Adaptation: What can people in old buildings do?
Retrofit them to make them more aseismic
Mitigation: How does education help?
Minimises loss of life, through earthquake drills
Adaptation: 2 ways fires can be prevented
Smart meters- cut off gas in an earthquake over a certain magnitude
Tokyo gas companies- switch off pipelines, reducing the number of fires
Adaptation: 2 ways of protecting against tsunamis
Tsunami warning systems
Build 12m high tsunami walls (ineffective)
Protection: How can the emergency services be ready?
Heavy lifting gear available
Carefully organised and planned response
People in communities given first aid training to treat injured before help arrives
Mitigation: Why are people urged to take out insurance in richer countries?
To cover their losses- expensive
Adaptation: Land use planning
Schools and hospitals built in low risk areas
Green space put into high risk areas, forming a safe area away from fires and aftershock debris + distribution areas
Prevention: How can any impacts from liquefaction be prevented?
Authorities preventing building on land prone to liquefaction
Mitigation: How to earthquake and tsunami warning systems reduce damage caused?
Detect weak shockwaves
TV / SMS warnings given
Aids evacuation
Predicting earthquakes: past seismic events
. Seismic gap theory
. Regular pattern can help predict the next earthquake
. The seismic gap states that earthquake hazard increases with time since the last large earthquake on certain faults or plate boundaries
Animals in prediction
. They may detect seismic activity and notice smaller vibrations and exhibit unusual behaviour
. They may sense ionisation of air caused by the large rock pressure in earthquake zones in their fur
. May smell gases
Lazar beams
Can detect plate movement by directing the beam across the fault line
Pros of mercalli scale
. Useful in determining damage caused to an area and responses can be planned
. Structural engineers contribute information for assigning values of VIII which is useful for rebuilding phases
Cons of mercalli scale
. Can only measure earthquakes in inhabited ares
. Not very scientific
. Damage caused may not accurately reflect an earthquake’s strength
. Only measure effects not energy released
. Witnesses that scale is based on may not be accurate
Mercalli scale vs Richter scale
. Unlike the mercalli scale which looks at damaged caused the Richter scale measures ground deformation and energy released
. In richter as you move up the magnitude frequency decreases
. Large earthquakes are rare as it takes many years for the pressure and energy to build up
Do scientist prefer the Richter scale or Mercalli scale?
Scientists prefer the Richter scale as it is more scientifically based, logarithmic scale, rather than human observation
Steps to seismic waves
1) Pressure builds at the point where 2 plates meet
2) Sudden release (due to rock failure creates waves)
3) The waves cause the ground to shake
4) The intensity of the shaking is dictated by the depth of the focus and the energy release
5) The result of this is the earthquake hazard
P-waves (primary)
Fastest waves, moving through solid and liquids causing the earth to move backwards and forwards
S-waves (secondary)
Slower waves which move through a sideways motion, shaking the Earth at right angles to the direction of travel. Can’t move through liquids but do much more damage then P-Waves.
Love waves
Surface waves which are the slowest. Cause a sideways motion and the most damage.
Rayleigh waves
Radiate along the sirface in a low-frequency rolling motion.
