1.3A Hazards Caused by Earthquakes
Earthquake waves (P, S and L waves) cause crustal fracturing, ground shaking and secondary hazards, (liquefaction and landslides).
Earthquakes in General
Earthquakes are a sudden release of stored energy:
Waves
Earthquakes generate three type of seismic wave:
Crustal Fracturing
Secondary Hazards
Earthquakes frequently generate large landslides as secondary hazards. This is especially true in area of geologically young (and therefore unstable) mountains such as the Himalaya. Landslides accounted for up to 30% of the deaths in the 2008 Sichuan and 2005 Kashmir earthquakes.
Liquefaction is a particular hazard in areas where the ground consists of loose sediment such as sand, silt or gravel that is also waterlogged - often found in areas close to the sea or lakes.
Intense earthquake shaking compacts the loose sediment together, forcing water between the sediment out and upwards. This undermines foundations, and causes buildings to sink, tilt and often collapse.
In some earthquakes (e.g. Japan (2011?)) tilts of up to 60 degrees have been recorded.
It causes water-saturated material to temporarily lose normal strength and behave like a liquid under the pressure of strong shaking. Water pressure increases to the point where the soil particles can move easily.
The effects of this are:
Earthquakes in General
Earthquakes are a sudden release of stored energy:
- A pulse of energy radiates out in all directions from the earthquake focus (point of origin).
- The epicentre is the point on the Earth's surface directly above the focus.
- In some cases, the earthquake motion displaces the surface, so a fault scarp can be seen.
Waves
Earthquakes generate three type of seismic wave:
- P-Waves
- Primary waves
- The fastest, (rate of about 8 km/sec) so they arrive first
- Cause the least damage
- S and L waves have a larger amplitude and energy force
- They are vibrations causes by compression (like a shunt through a line of connected train carriages)
- S-Waves
- Secondary waves
- Arrive next (about 4 km / sec)
- Shake the ground violently, causing damage
- L-Waves (or Q waves?)
- Love waves (gotta love waves)
- Arrive last, as they travel only on the surface (gotta love the surface) - 'vibration occurring in the horizontal plane'
- They have a large amplitude (gotta love large amplitudes) and cause significant damage, including fracturing the ground surface.
Crustal Fracturing
- Earthquakes cause crustal fracturing within the Earth, but also buckle and fracture the ground surface.
- Some very large earthquakes, such as the one that generated the 2004 Indian Ocean tsunami, rupture a fault line for up to 1000 km.
- This can be though of as 'unzipping' a fault, with energy pulses being generated along the entire fault length.
- Such earthquakes can cause ground shaking that lasts up to 5 minutes, as well as dozens of powerful aftershocks.
Secondary Hazards
Earthquakes frequently generate large landslides as secondary hazards. This is especially true in area of geologically young (and therefore unstable) mountains such as the Himalaya. Landslides accounted for up to 30% of the deaths in the 2008 Sichuan and 2005 Kashmir earthquakes.
Liquefaction is a particular hazard in areas where the ground consists of loose sediment such as sand, silt or gravel that is also waterlogged - often found in areas close to the sea or lakes.
Intense earthquake shaking compacts the loose sediment together, forcing water between the sediment out and upwards. This undermines foundations, and causes buildings to sink, tilt and often collapse.
In some earthquakes (e.g. Japan (2011?)) tilts of up to 60 degrees have been recorded.
It causes water-saturated material to temporarily lose normal strength and behave like a liquid under the pressure of strong shaking. Water pressure increases to the point where the soil particles can move easily.
The effects of this are:
- damage to roads and bridges, as well as telecommunications