Tectonics Topic 2.1 (tectonic processes affecting the magnitude of earthquakes)

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6 Terms

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earthquakes

refer to the shaking of the earth’s ground due to sudden release of energy in the earth’s lithosphere

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earthquakes occur when :

  1. rock masses on either side of a fault are pushed by tectonic forces

  2. friction causes them to get locked, and stress builds up

  3. when the stress exceeds the strength of the fault, the rocks snap or suddenly move to a new posiiton

  4. this sudden movement causes seismic waves to be released, resulting in ground shaking


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tectonic processes that result in earthquakes

Focus → point in the earth’s crust where seismic waves are released

Epicentre → point on the earth’s surface directly above focus (shaking generally felt most strongly near epicentre)

<p><span style="background-color: transparent;"><strong><em>Focus</em></strong> → point in the earth’s crust where seismic waves are released</span></p><p><span style="background-color: transparent;"><strong><em>Epicentre</em></strong> → point on the earth’s surface directly above focus (shaking generally felt most strongly near epicentre)</span></p>
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measurement of earthquakes

measured using seismometers (sensitive instruments that detect ground vibrations and determine the magnitude of an earthquake) 

  • Greater seismic energy released during an earthquake, greater the magnitude 


scales used to measure rate of magnitude :

  1. Richter scale (ML)

  2. Moment magnitude scale (Mw)


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Richter scale

  • Richter scale calculates magnitude using the height of the largest wave recorded on seismometers

  • Magnitude is measured based on max seismic intensity reached, rather than the total seismic energy released 

  • Scale numbered from 1-10, with 10 being the highest 

  • Each whole number increase in scale represents roughly 32 times increase in energy

  • Limitations : 

    • Richter scale would rate an earthquake with a single drastic spike in wave energy as having a higher magnitude than a long earthquake with many large intense waves 

    • Underestimating longer earthquakes which release more overall energy by rating them lower even though likely to do more damage

    • No longer commonly used


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Moment Magnitude scale

  • Rates earthquake magnitude based on total energy released during earthquake 

  • Generally more accurate, especially in measuring earthquakes of magnitude 8 and above

  • Scientists have adjusted magnitudes of past earthquakes that were initially measured using richter scale 

  • example : prince william sound earthquake in alaska, usa in 1964 was initially rated 8.6 on richter scale but later adjusted to 9.2 based on Mw scale