measuring earthquakes

• The Mercalli scale uses roman numerals from I to XII and estimates intensity, based on eye witness observations of structural damage. 
• mercalli scale:

  • I - not felt by many people 
  • IV - loose objects falling 
  • VII difficulty standing 
  • Total damage. 
• Mercalli limitations:

  • Subjective 
  • Requires eye witnesses 
  • Low resolution 
  • Not measured at focus
  • Cannot determine depth 
  • Local ground conditions 
  • Local building standards 
• Amplitude of a seismic wave depends on energy, measuring max amplitude on a seismograph - energy released can be measured. This energy can be converted to a magnitude. 
• Magnitude:

  Earthquakes release a huge amount of energy so we use a log scales. 
• On the Richter scale each increase in scale of magnitude is 10 times more ground motion and 32 times more energy released. 
• The movement along a fault plane is a turning force. It is related to total energy released by the earthquake. 
• Moment magnitude is more reliable than the Richter scale for larger magnitude earthquakes. 
• Moment and Richter scale are open ended with no upper or lower limits. Negative values are possible due to an increase in sensitivity of equipment. 
• Highest magnitude was a 9.5 in Chile, unlikely magnitudes above 9.5 due to a limited amount of strain energy that can be stored in the lithosphere.