The energy released from a Volcano is 10^13 Joules. The chemical bond energy between carbon molecules to form 12 grams of carbon compound is typically 10^5 Joules. With the 10^13 Joules one can vaporize 10^3 Tonnes of Carbon.
In the case of an earthquake of magnitude 6, about 10^15 Joules of energy is released, some of these are released before the main shock. This amount is necessary to vaporize 10^5 Tonnes of carbon. Although most of the energy is released in the thermal form, but anyhow should it be unrealistic that one studies the formation of an earthquake cloud before an earthquake on top of the region gaining this energy.
P.S.> The above picture is taken from here.
