Seismic hazards Flashcards
How does seismic activity cause earthquakes?
-Tension and pressure builds up in the plate margin due to FRICTIONAL DRAG
- When plates jerk past eachother seismic shockwaves are released in the form of an earthquake
- SHOCKWAVES are spread out from and are strongest closest to the focus
- Focus doesn’t have to be a single point and can be a fault line
- Epicentre is above the focus and is the point on Earth’s surface where earthquakes are first felt
- Can cause ground to shake or rupture along the fault
- Aftershocks occur for several weeks afterwards
What are the 3 scales to measure earthquakes
- Richter scale: measures magnitude of how powerful shaking is. No upper limit.Logarithmic scale. Major earthquakes are above 7
- Moment magnitude scale: measures energy released by earthquake. No upper limit. Logarithmic. More accurate than Richter scale
- Mercalli scale: measures impacts of earthquake using observations. Between 1-12.
Give features of seismic waves
Contain P (primary) and S (secondary) waves which combined are called body waves which travel through the body of the Earth
- P waves can travel through solid and liquid so through mantle and core and reach Earths surface first
- S waves can only travel through solid so through the mantle and reach the surface half as fast as P waves
Surface waves are Love and Rayleigh waves which travel through the surface of the Earth
- Love waves are the slowest and cause the most damage
- Rayleigh waves are elliptical and radiate from the epicenter in low frequencies
Explain tsunamis as a secondary effect of earthquakes
- Large waves caused by the displacement of large volumes of water
- Triggered by underwater earthquakes causing sea bed to move
- More powerful if starts closer to coast as waves lose energy as they travel towards land
Explain landslides and avalanches as a secondary
- Ground shaking dislodges rock, soil or snow causing these features which move downslope quickly
- Shaking can also make it easier for water infiltration as it loosens ground material causing extra weight which can trigger a landslide after the shaking
Explain liquefaction as a secondary hazard of earthquakes
- Saturated soil acts as a liquid when i experiences the vibrations from an earthquake
- Soil is therefore weaker and easier to deform so it subsides when it has a heavy weight such as a building on top of it
Explain the spatial distribution of seismic hazards (give an exception)
- Most occur around destructive and conservative margins ( However UK experience 300 a year)
- Some occur around constructive margins
- Most powerful associated with destructive margins
- 75% earthquakes activity occurs in Pacific Ring of Fire
- 23% of annual global seismic energy release in Alpine- Himalayan- Collision Zone between continental plates
Explain the randomness of seismic hazards
They don’t follow any pattern or trend
Explain the magnitude and frequency of seismic hazards
Number of earthquakes varies globally each year. Low magnitude earthquakes happen more frequently than high magnitude ones
Explain the regularity of seismic hazards
Some countries are more prone to seismic hazards due to the fact that a number of plate margins converge underneath the countries surface and it is located within the Pacific Ring of Fire
Explain the predictability of seismic events
Scientists can monitor the movement of tectonic plates to predict areas of risk. However its impossible to determine magnitude and time and earthquake will strike a particular place
Explain 2 prevention strategies as a long term response to seismic hazards
- Authorities prevent land being built on which is prone to liquefaction
- Build a giant sea wall to prevent tsunamis hitting land
Explain 2 preparedness strategies as long term responses to seismic hazards
- Authorities install earthquake warning systems to detect weak seismic activity that may build and broadcasts this information nationally
- Individuals and businesses create plans for responses and escape routes for if an earthquake struck
Explain 2 adaptation strategies as long term responses to seismic hazards
- Buildings designed to withstand earthquakes with flexible materials and shatter proof glass
- Tall strong building designs to reduce vulnerability so people can escape tsunamis quickly
Give human causes of earthquakes
- Deep mining or fracking e.g. Oklahoma 2009
- Building reservoirs which puts pressure on underlying rock
How can geology impact the severity of seismic events ?
Low frequency waves are amplified by soft sediment causing energy to disperse out from epicenter quicker
How is climate change influencing seismic activity?
- Link between rainfall and frequency of earthquakes. During summer monsoon season in the Himalayas the weight of rainfall compresses the crust vertically and horizontally stabilising it but when there is less water it is destabilised
- Isostatic rebound ( evidenced by raised beaches in Scotland) can generate earthquakes as glacial ice melts
