Hi Don,

yes, sagging is causing the accumulation of energy which is relaxed by main shock.

The coal seam is no so weak as you supposed. It is able to accumulate about 1E6 J (1 million J) of energy, but in comparison with sandstone is weaker and more elastic.

The energy 1E5 - 6 J is energy between 100000 and one mil. J of seismic energy, that is about magnitudo 2. It is not destressed energy or accumulated energy. Accumulated energy is about 2 or 3 order higher.

Pavel