3.1. Hazardous environments resulting from tectonic movement Flashcards
Define earthquakes
- the shaking/trembing caused by the sudden release of energy
- usually associated with faulting/breaking of rocks
- continuing adjustments of position results in aftershocsk
Define focus/hypocenter
The point within the earth where faulting begins
Define epicentre
The point directly above the focus on the surface
Why do earthquakes occur?
- Occur along plate margins
- Plates move at a rate between 1.5cm and 7.5cm per year
- When plates move past/towards/away from each other, the movement is not smooth as friction causes the plate to get stuck => causes pressure to build up
- Energy waves then race rapidly from the focus causing commotion on the ground
Elastic Rebound Theory
Explains how energy is stored in rocks:
- Rocks bend until the strength of the rock is exceeded
- Rupture occurs and the rocks quickly rebound to an undeformed shape
- Energy is released in waves that radiate outward from the fault
2 types of body waves (wave that travel within earth’s interior)
- P Wave/Pressure wave/Primary wave
- S wave/Sheer wave/Secondary wave
P Waves
- The fastest body waves
- These are compressional waves where the material movement is in the same direction as wave movement
- They travel through solids and liquids
S Waves
- Slower than P-waves
- Travel with a sideway motion and move material perpendicular to wave movement thus making the ground move horizontally
- Travel through solids only
- Cause a lot of damage
Mohorovicic Discontinuity - Moho
- Seismographs close to epicentre showed slow travelling P-waves and S-waves which contrasted with the faster moving P-waves and S-waes further away from the shock
- If the shock waves pass through the denser rocks, they speed up and vice versa
- S-waves don’t go through solid so stop at outer core
- Whereas, P-waves are refracted
Types of seismic waves/surface waves (travels through crust)
- Slower than body waves
- Long/love waves (cause ground to move sideways)
- Rayleigh waves (cause ground to move up and down
Impact of P-waves
Can turn solid sediments into fluids like quicksand by disrupting sub-surface water conditions. This is known as liquefaction or fluidisation
Different types of fault
- Normal dip-slip faults: results from tension in crustal rocks pulling apart
- Reverse dip-slip faults: results from compression in the crust collision
- Strike-slip faults: results from crustal blocks sliding past each other
Richter scale
- Measures magnitude of a tremor using seismometer which records data on seismograph
- Logarithmic scale (a size 2 s 10 times larger than 1)
- Less than 3: not felt
- 3-3.9: felt, little damage
- 4-4.9: some structural damage
- 5-5.9: losses in populated areas
- 6-6.9: large losses in urban area
- 7-7.9: serious building damage, major loss of life
- >8: total destruction
Mercalli scale
- measures how much damage is caused by the earthquake based on observations (a result of surface shaking)
- measures intensity of earthquake by indicating the violence of the earth motion
- scale between 1 and 12
Differences between Richter and Mercalli
- Richter measures strengths, Mercalli measures damage/effects
- Richter scale is based on scientific data, Mercalli is based on people’s observations
- Richter measured by seismometers, Mercalli measured by people
- Mercalli uses whole number, Richter uses numbers to one decimal place
Physical factors affecting earthquake
- Magnitude of earthquake
- Time of the day - occur during rush hour can be more damaging
- Distance from epicentre - shorter => less energy is disspated
- Depth of focus (same as above)
- Type of rock and sediments - loose materials are more vulnerable to liquefaction
- Coastal locations are more vulnerable to tsunami
Human factors affecting earthquake damage
- Population/population density
- Type of buildings
- Level of economic development for prediction and preparation
Impacts of earthquakes
Primary hazard (ground shaking, surface faulting)
- loss of life
- total/partial destruction of infrastructure
- communications cut off
- economic depression
- blocked transport routes
Secondary hazard (liquefaction, landslides, rockfalls, tsunamis, debris mudflow)
- same as above
Define tsunami
harbour wave in Japanese
Causes of tsunami
- Sudden motion on the ocean floor.
- This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide. The impact of a large meteorite could also cause a tsunami.
Types of volcanic eruptions
- Icelandic lava
- Hawaiian
- Strombolian
- Vulcanian
- Vesuvian
- Plinian
Iceland lava eruptions
- persistent fissure eruptions
- large quantities of basaltic lava build up vast horizontal plains
- formed Deccan Plateau and Columbia Plateau
Hawaiian eruptions
- central activity
- runny,basaltic lava travels down the sides of the volcano in lave flows
- gases escape easily
- occasional pyroclastic activity
Stombolian eruptions
- frequent gas explosions
- blast fragments of runny lava into air to form cones
- large quantities of pyroclastic rock
- white cloud of steam emitted from crater
Vulcanian eruptions
- violent gas explosions blast out plugs of sticky or cooled lava
- fragments build up into cones of ash and pumice
- occurs when there is very viscous lava which solidifies rapidly after an explosion
- clears blocked vent ad spews large quantities of volcanic ash into atmosphere
Versuvian eruptions
- powerful blast of gas pushing ash clouds
- more violent
- lava flows
- ash falls
Plinian eruptions
- gas rushes up through sticky lava
- blasts ash and fragments into sky in a huge explosion
- immense clouds of gas and volcanic debris several km thick
- parts of volcano may be blasted away during the eruption
Define pyroclastic flows
a fast moving current of high-density mix of hot lava blocks, pumice, ash and volcanic gas
Define lava flows/bombs
streams of molten rock that pour or ooze from an erupting vent
Define fire fountains
a continuous spray of disrupting magma through a vent to form a persistent fountain of magma above the vent
Define lahars
A type of mudflow/debris flow composed of a slurry pyroclastic material, rocky debris and water
How is lahar formed?
- Rain brings soot and ash back to ground and this becomes a heavily saturated mudflow
- Heat from volcanoes melts snow and ice - the resulting flow picks up sediment and turns it into a destructive lahar
Earthquake/tsunami/volcanoes prediction
- seismometers to record swarms of tiny earthquakes
- chemical sensoring
- thermal sensoring
- animal behaviour
- lasers to detect swelling of waves
