Flashcards in Seismic Hazards Deck (11)
How do Earthquakes occur?
Earthquakes are caused by tension that builds up at all three types of plate margin, when this tension is released they jerk past one another causing shockwaves.
+ The earthquake spreads out from the FOCUS, the nearer the focus the stronger the shocks.
+ The EPICENTRE is the point on the earths surface that is directly above the focus.
How can we measure earthquakes?
> The Richter scale measures the magnitude of an earthquake, it is logarithmic meaning with each increase of one on the scale the magnitude is 10 times larger. Major earthquakes happen above a 7.
> The moment magnitude scale measures the totally energy released during an earthquake, it is also logarithmic and more accurate.
> The Mercalli scale measures the impact of an earthquake based upon observations e.g reports and photos, it’s measured between 1 and 12 based on severity.
What other seismic hazards are there?
> Tsunamis are large waves caused by the displacement of large amounts of water, their waves move very fast and often give little warning, due to friction with the sea-bed they loose energy with distance to the coastline.
> Landslides and Avalanches can occur when the shockwaves dislodge rock, soil or snow, earthquakes also loosen ground material making them more susceptible to infiltration, resulting in landslides well after the earthquake happened.
> Soul Liquefaction, this is when soil becomes saturated with water, making the soil weaker and more likely to deform which can result in the destruction of infrastructure.
What factors can influence the magnitude of an eruption?
> Margin type - The biggest earthquakes occur at destructive plate margins and more frequent but less severe eruptions occur at consecutive plate boundaries.
> Rate of movement, this could emit more pressure as some plates move at around 15cm per year and others at around 1cm, however there is no clear correlation between rate of movement and magnitude.
> Depth of focus, Deep focus earthquakes tend to have higher magnitude but are less destructive as the shockwaves loose more energy traveling to the epicentre.
When did the Indian Ocean Tsunami take place?
26th of December (Boxing Day) 2004
What was the cause of the Tsunami?
The tsunami was caused by a shallow focus underwater earthquake, this was a result of the Indian plate being subducted under the Burmese plate.
Where was the epicentre and the magnitude of the Earthquake?
The epicentre was 240km South West of Sumatra Indonesia, the magnitude was a 9.3 on the Richter Scale, it produced waves as high as 30 meters tall which travelled at up to 500 mph.
What were some primary impacts?
> Over 300,000 people, the population equivalent of Northumberland, died.
> Top soils were removed up to 800 meters inland.
> Infrastructure was destroyed in Indonesia 🇮🇩 , Thailand 🇹🇭 , India 🇮🇳 and as far away as South Africa 🇿🇦.
> Large coastal settlements including the city of Banda Aceh were completely destroyed.
What were some of the secondary impacts?
> Up to 500,000 refugees were created with many more homeless.
> Aceh province Indonesia lost 44% of its fishing industry, crippling the local economy.
> Water and soils were permanently contaminated with salt water.
> Crippling wealth inequalities grew as the rich largely protected their properties whilst the poor lost everything.
What did the immediate responses include?
> Huge relief efforts like never before lead by the UN.
> Australian troops were deployed rapidly and rescued hundreds of thousands of people and provided lots of aid.