Earthquakes represent the release of stress built up in the Earth’s crust due to tension,
compression or shearing of rocks. A series of shockwaves originate from the earthquake
focus and this point on the Earth’s surface is the epicentre.
95% energy released in first 5 seconds.
Seismic Waves:
Depth of focus:
Can be used to categorise earthquakes.
Shallow focus = surface down to 70km. Occurs in cold brittle rock due to fracturing from
stress within the core. Common but normally low energy.
Deep focus = 70-700km deep. Occurs with high temperature and pressure as minerals
change type and volume which contributes to energy release. Less frequent but often
powerful.
Measuring earthquake magnitude:
Seismometers measure seismic wave strength, displayed on seismograph.
• Richter Scale
Developed in 1935. Measures seismic wave amplitude and earthquake magnitude (energy
released at source). Logarithmic scale starting at 1, largest recorded 9 in Sendai, Japan,
2011.
• Modified Mercalli Scale
Measures intensity (strength of shaking felt) form I to XII (total structural damage).
Qualitative, observation and description.
• Moment Magnitude Scale (Mw)
Measure magnitude of large earthquakes more accurately. Uses physical movement as a
direct function of energy.