Seismic Waves and Earthquakes - Tectonics

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  • Seismic waves and earthquakes.
    • Seismic waves
      • Seismic waves are waves of energy that travel through the Earth's layers, and are a result of tectonic hazards.
      • Body waves- primary and secondary waves.
        • P waves- compressional (like a slinky) and can move through liquids or solids. they move faster through denser rock. (speed in water = 1450 m/s which is the speed of sound).
        • S waves- transverse (shear waves). Travel 90 degrees to the direction of the vibration of the waves. move more slowly than P waves (60% of speed) and moves faster through denser rock. can only travel through solids e.g. rock. these waves are more damaging.
      • Rayleigh waves cause the surface of the ground to move up and down, and travel at 90% of an S wave.
      • Love waves involve the motion of the ground moving side to side- perpendicular to the propagation velocity. travel slightly faster than Rayleigh waves and is a surface wave.
      • Seismic gap theory- predicts the relative size and frequency of earthquakes in a given area given the size and frequency of other earthquakes in the same area. E.g. lots of small, frequent earthquakes compared to non-frequent, higher magnitude earthquakes.
      • Waves cause crustal fracturing, ground shaking and secondary hazards (liquefaction and landslides).
      • How they are measured.
        • The Moment Magnitude Scale (MMS) measures in terms of energy released. magnitude baes on moment of earthquake.
        • Mercalli scale- measures damage.
        • Richter scale- a logarithmic scale ranging from 1-10. measures amount of energy released when earthquake occurs.
    • Elastic rebound theory- rocks on opposite sides of the fault are subjected to force and shift, and they gain energy. Once their maximum energy is exceeded, the rocks snap and take back their original position and shape, which is what causes an earthquake.
    • where do earthquakes occur?
      • Convergent/destructive plate boundaries- oceanic/continental convergence is occurring and subduction takes place, where the more dense oceanic plate subducts beneath the less dense continental plate.
        • at a subduction zone, the plates can become locked (locked fault) until they become so stressed they release and produce an earthquake.
          • Here, the benioff zone is present, where the earths rock melt, become less dense, rise through cracks and fissures and produce earthquakes.
        • Sendai, Tohoku- The oceanic Pacific Plate subducts (sinks under) the Eurasian Plate which Japan sits on.
      • transforming plate boundaries- two plates moving past one another. they may get caught on each other and produce a locked fault until they release under such stress and produce an earthquake.
        • The San Andreas Fault is an example of this.

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