🟠✅Natural Hazards - Seismic Hazards Flashcards
Earthquake definition
Shaking of the grounds surface (violently). There’s a sudden release of energy in crustal rocks - plate boundary - movement (sticking) can lead to build up in pressure and when that’s released shock waves release large amounts of energy.
Seismicity meaning
Refers to frequency type and size of earthquakes expericed by an area over a period of time
Seismograph / seismometer
Apparatus that measures/records vibrations within the earth and of the ground eg. Force/duration.
System of monitoring - the degree of dispersion of weight is whats measured. The greater the degree of dispersion (movement) the greater the lines.
Measured the type of energy / shock waves.
Focus vs epicentre
Focus - point where the rocks break
Epicentre - point on the surface of the earth above an earthquake
Shock waves
When two plates move side by side, friction builds up and pressure increases; this pressure is stored as potential energy, it cannot move so it just builds up. When the pressure becomes too much, the plates eventually move.
All of the energy that has been built up must go somewhere, so it is transferred into kinetic energy, which is released and vibrates throughout the ground. The further away from the focus, the weaker the shockwaves, as the energy is transferred into the surroundings.
Body waves
Primary = 4-8km/sec in earths crust / shake the ground propagating direction / compressional wave / can travel through liquid and solids / longitudinal
Secondary = 2.5-4km/sec / shale the ground perpendicular / can only travel through solid / transverse
Surface waves (love waves)
Transverse / Perpendicular to the direction of energy
Very damaging to buildings
Can only travel though solids
2km/sec
Surface waves (Rayleigh)
Elliptical / rolling motion or circular motion
Can travel through solids and liquids
1km/sec
Ridge push
Gravitational sliding away from a spreading ocean ridge.
As fresh magma wells up at mid-ocean ridges to form new young, oceanic lithosphere a higher elevation is formed at spreading ridges.
The new oceanic crust gradually cools down and thickens and is pushed downhill (gravity) as new magma emerges from the active zone of divergence behind it.
Slab pull
The weight of the denser oceanic plates material sinking into the mantle at the deep ocean trenches and pulling the rest of the plate slab with them as gravity causes them to slide down.
Each plate moves at its own rate.
Convection
Heat from the core generates thermal convection currents in the asthenosphere. Lighter / less dense molten rock rises towards the crust and spreads before cooling and shrinking.
This circulation of magma moves plates, new crust if formed at constructive zones and old crust id destroyed at subduction zones
Destructive C+O
Denser oceanic plate subducts below the continental.
The plate subducting leaves a deep ocean trench.
Fold mountains occur when sediment is pushed upwards during subduction.
The oceanic crust is melted as it subducts into the asthenosphere.
The extra magma created causes pressure to build up.
Pressurised magma forces through weak areas in the continental plate
Explosive, high pressure volcanoes erupt through the continental plate, known as composite volcanoes.
Destructive O+O
Heavier plate subducts leaving an ocean trench. Fold mountains will also occur.
Built up pressure causes underwater volcanoes bursting through oceanic plate.
Lava cools and creates new land called island arcs.
Destructive C+C
Both plates are not as dense as oceanic so lots of pressure builds.
Ancient oceanic crust is subducted
slightly, but there is no subduction of
continental crust.
Pile up of continental crust on top of
lithosphere due to pressure between
plates.
Fold mountains formed from piled of continental crust
Constructive O+O
Magma rises in between the gap left by the two plates separating, forming new land when it cools.
Less explosive underwater volcanoes formed as magma rises.
New land forming on the ocean floor by lava filling the gaps is known as sea floor spreading (as the floor spreads and gets wider).
Constructive C+C
Any land in the middle of the separation is forced apart, causing a rift valley.
Volcanoes form where the magma rises.
Eventually the gap will most likely fill with water and separate completely from the main island
Conservative
Between any crust, the parallel plates move in different directions or at different speeds. No plates are destroyed so no landforms are created. When these plates move, a lot of pressure is built up. On oceanic crust, this movement can displace a lot of water. On continental crust, fault lines can occur where the ground is cracked by the movement.
Intra Plate earthquake
Caused by stresses within a plate.
Since plates move over a spherical surface, zones of weakness are created. Intraplate earthquakes happen along these zones of weakness. The earthquakes may take place along ancient faults or rift zones.
Earthquake hazards - ground shaking
Primary
tructures like buildings, bridges, dams can be severely damaged.
Cliffs and sloping ground can get demolishes.
Debris from these may fall and injure people.
Earthquake hazards - tsunamis
Secondary
Long wavelength oceanic waves generated by the sudden displacement of seawater by a shallow earthquake, volcanic eruption or submarine landslide.
A number of waves may be produced and they can travel long distances at high speeds to flood far-off shores.
The height of a tsunami varies and may be affected by the sea floor depth and shape, and other factors.
Eg. New Zealand is susceptible to tsunamis originating from distance sources around the Pacific Ring of Fire as well as from very close to our coastline. Near source tsunamis will allow for very little warning.
Earthquake hazards - landslides and rockfalls
Secondary
Ground shaking due to earthquakes destabilises cliffs and steep slopes, causing landslides and rockfalls as a significant side-effect. Heavy rain and unconsolidated or fractured rock are exacerbating factors.
Earthquake hazards - liquefaction
Secondary
Liquefaction occurs when waterlogged sediments are agitated by seismic shaking. This separates the grains from each other, reducing their load bearing capacity.
Buildings and other structures can sink down into the ground or tilt over, whilst underground pipes and tanks may rise up to the surface.
Spatial distribution
Along all plate boundaries.
The Ring of Fire accounts for 90% of the world’s Earthquakes (shown in the diagram as the Circum-Pacific belt).
The Alpine-Himalayan belt accounts for 5-6% of the world’s earthquakes.
Magnitude - logarithmic Richter Scale
Measure of the strength of seismic waves
Magnitude - modifies mercalli intensity scale
Rate of destruction caused.
Unlike the Richter scale, the Mercalli scale has a definite end at 12 (XII as it is in roman numerals). The Mercalli scale is subjective, meaning sometimes it is disputed as it is dependent on human development being present rather than the strength of the seismic waves.
What’s the magnitude of an earthquake also dependant on?
Depth of focus. Conservative boundaries have the shallowest boundaries, meaning they are closer to the epicentre and the seismic waves are stronger.
Destructive boundaries usually have deeper focuses, meaning the seismic waves are spread over a larger area before they reach the epicentre. This is dependent on the earthquake.
