Hazards Flashcards
(41 cards)
What is a natural hazard?
a natural process that could disrupt humans
e.g. could cause death, injury, damage property
What is a natural disaster?
a natural hazard that has happened
How do we class something as a hazard?
To be classed as a hazard, it needs to pose a threat to humans
What is a seismic hazard?
the hazard associated with potential earthquakes in particular areas
What are the different hazards/consequences associated with earthquakes? (seismic hazards)
Tsunamis
Subsidence
Liquefaction
Ground shaking
Landslides
Describe the process of an earthquake?
- Movement of bedrock either side of a fault line applies stress to the rock in the fault zone
- This movement transfers energy into the rock and increases the elastic strain energy stores in the rock, causing the rock to deform
- When this deformation exceeds the frictional force holding the rocks together, a sudden slip (movement) occurs along the fault (an earthquake)
- This releases the accumulated stress and the rocks on either side of the fault return to their original shape, this is known as elastic rebound
- The energy from this sudden slip sends seismic waves through the crust and Earth to the surface
What is an active fault?
a fault where there is a potential source of earthquakes
- some suggestions say where there has been movement in the past 100,000 years
Give an example of an active fault?
Anatolian Fault, Türkiye
In what ways do we measure earthquakes?
Magnitude
Intensity
What do we measure for magnitude?
What are we actually measuring?
seismometer, seismograph
actually measuring amplitude of waves
What is magnitude?
- a number that characterises the relative size of an EQ
- it is based on measurement of the maximum motion recorded by a seismograph (amplitude)
What issues arise when measuring earthquakes?
different levels of movement in different rocks
What ways can we measure magnitude?
- Local or Richter (ML)
- Moment (MW)
- Surface wave (MS)
- P-wave
What is the Local or Richter (ML) scale?
The original magnitude relationship defined in 1935 by Richter and Gutenberg
- based on the maximum amplitude of S-waves recorded on a Wood-Anderson torsion seismograph
- ML values can be calculated using data from modern instruments
- L stands for Local because it only applies to EQ relatively close to the seismic station
How does moment (MW) measure magnitude?
Based on the seismic moment of the EQ, which is equal to the average amount of displacement on the fault multiplied by the fault area that slipped.
- it can also be estimated from seismic data if the seismometer is tuned to detect long-period body waves
How can Surface Waves (MS) be used to measure magnitude?
A magnitude for distant earthquakes based on the amplitude of surface waves measured at a period near 20s
How can P-waves be used to measure magnitude?
- based on P-wave AMPLITUDE
Technique is being increasingly used to provide very rapid magnitude estimates so that early warnings can be sent to utility and transportation operators to shut down equipment before the larger (but slower) S-waves arrive
What are the similarities between all the magnitude measuring systems?
All are logarithmic
- amplitude of ground motion increases by x10 each time
- energy increases by x32 each time
Why do earthquakes happen at transform plate boundaries?
as the jagged sides catch on each other/where a transform boundary meets a divergent boundary (fault) it creates a huge build up of pressure, then as it slips it will all be released at once, causing a big EQ
- two conflicting movement types mean more movement is going on so it is more likely for pressure and impact
Where do the very large earthquakes occur?
Why?
Convergent subduction boundaries
(often oceanic-oceanic)
because there is the potential for a greater width of rupture zone on a gently dipping boundary than on a steep transform boundary
What happens to the depth of earthquakes with distance from the ocean trench?
further distance = deeper earthquakes because of subduction, but there are not many earthquakes at extreme depths due to melting during subduction. Deep = hot
What scale is used to measure Earthquake intensity?
What is the range
Modified Mercalli Scale
goes from I-XII where I is ‘not felt’ and XII is ‘extreme’
What are the factors affecting the intensity of an earthquake?
- proximity to epicentre
- depth of focus (distance)
- geology = subsurface rocks and sediment (local ground conditions)
- standard and design of buildings
- magnitude/size of EQ at focus
Why does an earthquake lose its power with distance from the focus? (depth and distance)
amplitude and frequency attenuate due to energy disspiating into the rock as it travels
