Earthquakes

Earthquakes

1. Facts

   • result of the release of pressure built up in tn the rocks of the Earth's crust
     • Landwaves may trigger mass movements and avalanches
   • energy release as seismic waves
     • lose energy as the radiate outwards from the focus
   • The epicentre suffers the greatest intensity of seismic waves
   • Modified Mercalli scale - measures intensity, the effects
     • semi-quantitative linear scale
   • Richter scale - measures magnitude
     • Quantitative logarithmic scale
     • Improved as moment magnitude scale as was hard to differentiate between intensity
       • E.g. 1964 - Prince William Sound in Alaska - amplitude magnitude of increased to 9.2

2. Shockwave types (3)

   • Primary - Longitudinal
     • Fastest waves average 5 km per second
     • go through solids and liquids
     • expand and contract
   • The nature of wave determines how it travels through the Earth
   • Secondary - Transverse
     • slower 3 km per second
     • cannot travel through liquids
     • side to side
   • Surface/long - Transverse
     • Slowest - greatest wave length
     • carry most of the eq's energy
     • travel around the surface of the Earth
       • E.g. 1960 - Chilean Eq surface waves travelled around the Earth 20 times and still registering on seismometers 60 hours after the main shock
     • up and down

3. Features (3)

   • Foreshock
     • Initial shattering of obstructions or bonds along the failure planes
   • Principal shock
     • most severe
   • Aftershocks
     • recur as shockwaves travel around the Earth
       • decrease in frequency and intensity overtime