seismic hazards Flashcards
how do earthquakes occur
- caused by tension that builds up at all plate margins
- as the plates jerk past each other they send out shockwaves (the earthquake)
- shockwaves spread out from the focus which can be multiple points and is typically at the fault line
- the epicentre is the point on the earths surface directly above the focus where the earthquake is first felt
- earthquakes cause ruptures along the fault
how is the magnitude of earthquakes are measured
- the Richter scale measures the magnitude of earthquakes
- there is not an upper limit
- the scale is logarithmic (as the level increases by one the magnitude increases by 10
- major earthquakes are 7 and above
- 2011 Tohoku earthquake was 9.0
- largest earthquake recorded at 9.5 in 1960, Chile
how is the energy released by an earthquake measured
- the moment magnitude scale is based on the total amount of energy released
- no upper limit
- logarithmic
- more accurate than Richter scale
how are the impacts of earthquakes measured
- the Mercalli scale uses observations to measure the impacts of an earthquake
- the scale is between 1 and 12
- 1 is an earthquake detected by instruments
- 12 is an earthquake that causes total destruction
how do earthquakes cause tsunamis
- large waves caused by the displacement of water.
- they can be triggered by underwater earthquakes which cause the sea bed to move. this causes wave to radiate from the epicentre
- tsunamis by the coast will be more destructive as they lose energy as they move towards land
- the waves travel quickly in deep water and can hit with little warning causing a high death toll
-travel up to 940km/hr
-25-40km in height
long wavelength,
-10 to 60mins between each wave
2011 Tohoku
how do earthquakes cause landslides/avalanches
- shaking if the ground can dislodge rock, soil or snow causing landslides/avalanches
- the loosening of ground material can also make it easier for water to infiltrate
- the extra weight can cause landslides after the shockwaves
- Gorkha 2015
how do earthquakes cause soil liquefaction
- when soil is saturated with water the vibrations cause the soil to act like liquid
- this makes soil easier to deform
what effects the nature and magnitude of an earthquake
-margin type:
destructive margins: biggest earthquakes occur here as the subduction zone causes pressure to build up.
constructive margins: lower magnitude earthquakes
-depth of focus:
the focus can be close to the earths centre or deep below it.
deeper focuses have higher magnitude but they do less damage as they have to travel further which reduces power.
what is the importance of the magnitude and frequency of earthquakes
- lower magnitude earthquakes occur frequently whilst higher magnitude earthquakes occur less frequently
- the number of earthquakes per year varies year to year
what is the regularity of earthquakes
-seismic hazards occur randomly
predictability of earthquakes
- scientists can monitor tectonic movement to predict areas at risk
- it is currently impossible to predict when an earthquake will occur and what the magnitude will be
short term responses to seismic hazards
rescue and evacuation
seismic hazards case study
date
plate margin
Japan Tohoku earthquake
March 2011
destructive pacific and Eurasian plate
5 social impacts of the Tohoku earthquake
- tsunami killed 15,890 people
- 400,000 buildings destroyed
- 25% of foreigners living in Tokyo left
- people left stranded
- reputation and dignity effected which is important in Japanese culture
2 economic impacts of the Tohoku earthquake
- cost japan $199 billion
- livelihoods destroyed and so money isn’t put into the economy
