Flashcards in ✅18.104.22.168 - Seismic Hazards Deck (31)
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
How can earthquakes be predicted?
Past seismic events
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?
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?
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?
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