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Flashcards in Hazards (Seismic) Deck (54)
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How are earthquakes formed?

1. Plates move due to gravitational sliding, slab pull and convection currents
2. Plates get stuck due to friction, pressure and tension building up
3. When plates jerk past each other, they release this pressure and send shockwaves through the Earth's crust


Distribution of seismic events

- Found particularly along conservative and destructive plate boundaries
- Ring of Fire accounts for 90% of Earth's earthquakes
- Alpine-Himalayan belt accounts for 5-6%


How does depth of focus affect magnitude of earthquakes?

- Deeper focus earthquakes tend to be of a higher magnitude
- Deeper focus tend to do less damage as shock waves have to travel further which reduces the strength


Why do earthquakes have a higher magnitude at destructive plate boundaries?

Higher pressure builds between plates during subduction


Why are earthquakes lower magnitude at constructive plate boundaries?

Fracturing is more frequent so pressure doesn't have as much time to build up


Three factors affecting magnitude of earthquakes

- Type of plate boundary
- Depth of focus
- Rate of movement


Why might earthquakes occur away from a plate boundary?

- Reactivation of old fault lines possibly from deferred stress release
- Large dams or reservoirs cause pressure on underlying rock and reactivate old fault lines
- Hydraulic fracturing
- Subsidence of old mines


Body waves

Travel through the earth. Can be divided into primary and secondary waves.


Surface waves

Travel along the Earth's surface. Cause most damage as they cause more ground movement. Travel slowly


Primary (P) waves

Alternately compress and expand. Particle movement is parallel to wave direction. Fastest wave and can travel through all substances.


Secondary (S) waves

Transverse. Movement of particles is perpendicular to wave movement. Cannot travel through air or water. Slow but causes more damage.


Rayleigh waves

Ground roll. Rocks move in elliptical motions as the wave passes and breaks up the surface.


Love waves

Horizontal shear waves. Move the ground from side to side at right angles to movement direction. Can damage infrastructure and buildings.


What does the Richter scale measure?

Measures the magnitude of an earthquake based on the amplitude of the secondary waves.


What does the Moment Magnitude scale measure?

Measures the magnitude of based on the total energy released. Distance a fault has moved x force taken to move it.


What does the Mercalli scale measure?

Intensity of an event and its impact. 12 point scale, subjective.



Point in the Earth's surface directly above the focus, where the earthquake is first felt



Point in the Earth's crust where the earthquake starts


Relationship between magnitude and frequency in seismic events

Negative relationship - as magnitude increases, frequency decreases


Primary impacts of seismic events

- Ground rupture (displacement of the Earth's surface along the fault line)
- Ground shaking


Factors affecting the severity of ground shaking from seismic events

- Dependent on magnitude
- Depth of focus
- Distance from the epicentre
- Geological conditions (will affect likelihood of liquefaction)


How do earthquakes result in tsunamis?

1. Earthquakes move the seabed up by several meters
2. This displaces water above. The greater the movement, the greater wave produced.
3. Large waves radiate outwards across the ocean away from the epicentre
4. The water becomes shallower as it approaches the coast and the base is slowed by friction
5. This forces the wave in a circular motion into an elliptical form. This heightens until it can't be maintained and eventually breaks. This is called shoaling
6. The tsunami hits the coast as a large wave


How does the height of the wave affect intensity of tsunami impacts?

The greater the movement, the greater volume displaced and the greater the wave produced.


How does the distance travelled by the wave affect the intensity of tsunami impacts?

Waves lose energy as they travel inland. The closer to the land it starts, the less energy lost and the more powerful the tsunami will be.


How does the shape of the coastline affect the intensity of tsunami impacts?

Funnelled coastlines concentrate the energy on the bay. Irregular coastlines and offshore islands can accentuate the waveform.


How does the relief of the coastline affect the intensity of tsunami impacts?

Cliffs present a natural barrier to the tsunami


How does the presence of natural defences affect the intensity of tsunami impacts?

Coral and mangroves act as natural defences by dissipating wave energy through their large surface areas.


How does population density affect the intensity of tsunami impacts?

High population density causes a greater intensity of impacts. Young and old are most vulnerable, may be gender disparity.


What is liquefaction?

- Weakening of water saturated sediment after an earthquake, causing it to act as a liquid.
- May lead to eruptions of pressurised water and sand, called sandblow which could cause localised flooding
- Causes lateral flow movement of the ground, damaging underground pipelines, causing building to collapse


What conditions worsen impacts of liquefaction?

- Geology: loose sand and silt
- Water table close to surface
- Lack of deep foundations to buildings