HAZARDS: Seismic hazards Flashcards
Love surface waves.
Seismic waves that travel along the surface of the earth, which shake the ground at right angles to direction of movement.
Describe the Richter scale:
- Measures magnitude of earthquake.
- No upper limit.
- Logarithmic.
- Major earthquakes are above 7.
Describe the randomness vs. regularity of earthquakes:
- Don’t seem to follow any clear pattern or trend
- Occurance is largely random
Often the first warning of tsunami.
Wave trough in front of the tsunami which results in a reduction in sea level.
Drawdown.
The nature of an earthquake and its magnitude are affected by what three factors?
- Margin type
- Rate of movement
- Depth of focus
What are the three different scales that can be used to measure an earthquake?
- Mercalli scale
- Richter scale
- Moment magnitude scale (MMS)
How does depth of focus affect the nature of an earthquake?
- Deep focus quakes tend to be higher magnitude than shallow focus, but often do less damage.
- This is because deeper shockwaves have to travel further to reach teh surface, which reduces their power.
The point which the pressure release occurs within the crust.
The shockwaves spread out from this.
Focus of an earthquake.
What can trigger a tsunami?
- Underwater earthquakes
- Volcanic eruptions
- Landslides that slide into the sea
What are the two different types of body waves?
Primary or secondary.
What is the primary hazard associated with a seismic event?
An earthquake.
What are the primary hazards of an earthquake?
- Ground shaking
- Ground rupture - major risk for large engineered structures like dams, bridges and nuclear power stations.
Seismic waves that travel along earth’s surface.
Surface waves.
What causes an earthquake?
- Tension builds up at all three types of plate margin.
- When the plates jerk past each pther it sends out shockwaves/vibrations.
- This is the earthquake.
Surface waves.
Seismic waves that travel along earth’s surface.
When a tsunami reaches land, what will its effects depend on?
- Height of the waves and distance they have travelled.
- Length of the event at the source
- Extent to which warnings could be given
- Coastal physical geography, both offshore and on the coastal area
- Coastal land use and population density
In earthquake-prone areas buildings and other structures can be fitted with devices such as shock absorbers and cross-bracing to make them more earthquake proof.
Retrofitting
Body waves.
Seismic waves that travel through the earth.
What are the two types of seismic waves?
Body waves - travel through earth
Surface waves - travel along earth’s surface
What is a tsunami?
Giant sea waves caused by the displacement of large volumes of water.
Ground rupture.
Visible breaking and displacement of the earth’s surface, probably along the line of the fault.
Benioff zone.
Where earthquakes tend to happen as oceanic crust is being subducted underneath continental crust at destructive boundaries.
What broad category of earthquake tends to cause the greatest damage?
Shallow focus.
How does rate of movement affect the nature of an earthquake?
- Tectonic plates move in relation to each other at different rates.
- There is no clear relationship between rate of movement and earthquake magnitude.
Where pressure is released (after the slow build up of stress within the rocks due to the movement of the Earth), and parts of the surface experience, for a short period, an intense shaking motion.
The primary hazard caused by seismic activity.
What is an earthquake?
Tsunami
Giant sea waves generated by shallow-focus underwater earthquakes, violent volcanic eruption, underwater debris slides and landslides into the sea.
Drawdown.
Often the first warning of tsunami.
Wave trough in front of the tsunami which results in a reduction in sea level.
Describe the occurance of earthquakes at plate boundaries and intraplate earthquakes:
Plate boundaries - 90%
Intraplate - 10%
Where earthquakes tend to happen as oceanic crust is being subducted underneath continental crust at destructive boundaries.
Benioff zone.
What are the similarities and differences between the Richter scale and the Mercalli scale?
Use a table.
Why do tsunamis cause a high death toll?
The often come with very little warning, as they travel very fast in deep water.
It is not until they reach shallower water, that they gain height.
Describe the magnitude and frequency of seismic hazards:
- Hundreds of low magnitude quakes happen around the world every day.
- Higher magnitude happen much less frequently.
- Number of quakes varies from year to year.
What is an earthquake?
Where pressure is released (after the slow build up of stress within the rocks due to the movement of the Earth), and parts of the surface experience, for a short period, an intense shaking motion.
The primary hazard caused by seismic activity.
Describe the predictability of earthquakes:
- Movement of plates can be monitored to predict at risk areas.
- But it’s currently impossible to tell when an earthquake will strike a particular place, and what magnitude it is likely to be.
Seismic waves that travel through the earth.
Body waves.
Epicentre
The point of the Earth’s surface directly above the focus.
Where the quake is first felt.
When will a tsunami be most powerful?
Why?
If it starts closer to the coast.
Becuase waves lose energy as they travel towards land, so the closer it starts to the coast, the less energy it will lose.
How is a tsunami triggered by an underwater earthquake?
- Earthquake causes seabed to move, displacing water.
- Waves radiate out from the epicentre.
- The greater the movement of the sea floow, the greater the volume of water displaced and the bigger the wave produced.
How can earthquakes cause landslides and avalanches?
- Shaking of the ground can dislodge rock, soil or snow, causing landslides or avalanches that move downslope quickly.
- Shaking may also loosen ground material, making it easier for water to infiltrate - the weight of the extra water may trigger a landslide even after ground shaking has stopped.
Seismic waves that travel along the surface of the earth, which have a rolling motion.
Rayleigh surface waves.
Describe the moment magnitude scale (MMS):
- Based on total amount of energy released by earthquake.
- Logarithmic.
- No upper limit.
- More accurate than Richter scale so is more widely used.
What are the three broad categories of earthquakes?
- Shallow focus - 0 to 70 km
- Intermediate focus - 70 to 300 km
- Deep focus - 300 to 700 km
Retrofitting
In earthquake-prone areas buildings and other structures can be fitted with devices such as shock absorbers and cross-bracing to make them more earthquake proof.
What is soil liquefaction?
When soil is saturated with water. the vibrations of an earthquake can cause it to act like a liquid.
This makes the soil weaker and easier to deform, so its more likely to subside, especially where it has a heavy weight on top of it.
Rayleigh surface waves.
Seismic waves that travel along the surface of the earth, which have a rolling motion.
List some secondary effecs of earthquakes:
- Soil liquefaction
- Landlsides/avalanches
- Tsunamis
- Fires
- Effects on people and the built environment
How does margin type affect the nature of an earthquake?
- Biggest occur at destructive margins, as subduction of a plate causes massive pressure to build up, causing a huge quake when released.
- At constructive margins quakes tend to be lower magnitude than at destructive or conservative margins.
Focus of an earthquake.
The point which the pressure release occurs within the crust.
The shockwaves spread out from this.
At what plate boundary are the most powerful earthquakes associated with?
Destructive margins.
Seismic waves that travel along the surface of the earth, which shake the ground at right angles to direction of movement.
Love surface waves.
Visible breaking and displacement of the earth’s surface, probably along the line of the fault.
Ground rupture.
What human activity might cause minor earthquakes?
Why?
- Building large reservoirs - puts pressure on underlying rocks .
- Fracking - hydraulic fracturing of rock in order to release gas.
Describe the Mercalli scale:
- Measures impacts of earthquake using observations of event, eg reports and photos.
- Scale is between 1-12
- 1 = earthquake only detected by instruments.
- 12 = earthquake that cause total destruction.
The point of the Earth’s surface directly above the focus.
Where the quake is first felt.
Epicentre
Giant sea waves generated by shallow-focus underwater earthquakes, violent volcanic eruption, underwater debris slides and landslides into the sea.
Tsunami
What are the two different types of surface waves?
Love and Rayleigh
