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Mining accidents are sometimes preceded by high levels of crackling noise, which follow universal rules for the collapse of minerals. The archetypal test cases are sandstone and coal. Their collapse mechanism is almost identical to earthquakes: the crackling noise in large, porous samples follows a power law (Gutenberg-Richter) distribution P ~ E -ɛ  with energy exponents ɛ for near critical stresses of ɛ = 1.55 for dry and wet sandstone, and ɛ = 1.32 for coal. The exponents of early stages are slightly increased, 1.7 (sandstone) and 1.5 (coal), and appear to represent the collapse of isolated, uncorrelated cavities. A significant increase of the acoustic emission, AE, activity was observed close to the final failure event, which acts as “warning signal” for the impending major collapse. Waiting times between events also follow power law distributions with exponents 2+ξ between 2 and 2.4. Aftershocks occur with probabilities described by Omori coefficients ρ between 0.84 (sandstone) and 1 (coal). The “Båth’s law” predicts that the ratio between the magnitude of the main event and the largest aftershock is 1.2.

Collapsing minerals: Crackling noise of sandstone and coal, and the predictability of mining accidents