TECTONICS - Causes of earthquakes and their seismic waves

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  • Causes of earthquakes and their seismic waves
    • The cause of earthquakes:
      • A sudden release of a store of energy. as tectonic plates move past each other at a boundary, they stick- allowing strain to build up.
        • 'Stick-slip' behaviour occurs when the stress is overcome and a lot of energy is exerted from the 'focus' (place of origin) (HYPOCENTRE).
      • 'Stick-slip' behaviour occurs when the stress is overcome and a lot of energy is exerted from the 'focus' (place of origin) (HYPOCENTRE).
    • Seismic waves:
      • P waves (primary waves) are FAST. can travel through solids, liquids and gases.
        • Caused by compressions travelling parallel to the vibration. they spread quickly from the fault at around (8km/s).
      • S waves (secondary waves) are SLOWER. can only travel through solids.
        • Vibrations travel at right angles to the vibration directional travel. travel around (4km/s). - these cause the DAMAGE (ground shaking).
      • L waves (Love waves) SLOWER still. Only travel across the surface.
        • Waves travel perpendicular to the vibration in a 'side-to-side' motion. these have a high amplitude causing significant damage, fracturing rocks and ground surface.
      • Rayleigh waves - travel along the surface of solids.
        • cause ground-shaking by travelling in circular motions.
    • earthquakes can cause:
      • Crustal fracturing in the earth and buckling on the surface. 2004 Indian Ocean Tsunami.
      • Ground shaking lasting up to 5 minutes and many powerful aftershocks.
      • Landslides can occur in geologically 'young' areas such as the Himalayas. accounted for 30% of deaths in 2008 Sichuan and 2005 Kashmir earthquakes.
      • Liquefaction in grounds with loose sediment. Earthquake shaking compacts loose sediment together, forcing water out. This can invade house foundations, causing collapse.

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