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I know that they occur when energy that was previously stored is released in seismic waves. But how is the energy stored in the earth in the first place, and what can trigger the release of such energy?

Put another way, the above preconditions for earthquakes can be brought about by natural phenomena such as fault lines in the earth, and also by man-made phenomena such as mining. What are the ways to measure/estimate the "potential" energy of a fault line, and the likely resulting earthquake, and what are the ways to measure/estimate the energy released by mining or other human activities, and how large an earthquake is likely to result?

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  • $\begingroup$ I narrowed the question to focus on measuring seismic forces, and wonder if it can be reopened in its current form. $\endgroup$ – Tom Au Apr 20 '14 at 18:21
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    $\begingroup$ I think it is now a different question which would have been better asked as a separate question - neither of the answers are relevant now. $\endgroup$ – winwaed Apr 20 '14 at 21:54
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There are a number of causes (this list is probably not complete):

  • Elastic Rebound of tectonic faults. This is the most common and well known. Here the energy is elastic energy stored as two blocks of rocks move against each other. There is a point where elastic deformation cannot take up the entire movement, and the rock breaks releasing the elastic energy. This is better explained here: Reid's Elastic Rebound Theory (USGS)
  • Landslides/etc. These typically appear as extremely shallow earthquakes and can often be determined from seismic records.

  • Cavity collapses. E.g. mines, volcanic systems, caves.

  • Subterranean explosions / volume expansion. Eg. mining activity, atom bomb tests, volcanic activity (eg. phreatic involving water; or magma injection)

  • Crystalline phase changes. Fast, deep subduction zones (eg. Tonga) experience very deep earthquakes where the rocks should be too plastic for classic elastic rebound. These are usually interpreted as 'catastrophic' phase changes of olivine -> spinel -> perovskite, and the depths generally match. Eg. Kirby 1991.

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Short answer: A build up of stress between tectonic boundaries caused by a slow, but massive convecting mantle. The stress is released when the mechanical properties of the rock (moving from ductile to brittle) are overcome. The failure of the rocks between the two lithospheric plates cause a drastic slip; a conversion between potential and kinetic energy. This energy is often viewed in the form of seismic waves, but I prefer to think of the wave forms as just a way of mathematically representing the release in energy. The correct dictation, in my opinion, is to say that energy is released by tectonic slip which is quantified and analyzed in seismic waveforms.

There are other methods of relieving stress in tectonic plates such as isostatic rebound, large impacts. Two texts one might read to get a deeper understanding of earthquake mechanics are Turcotte and Shubert and Stein and Wyessesion. Both are upper division/lower graduate level texts. Fowler is great if you do not have a strong mathematical background (multivariate calculus).

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