The recent Italian earthquake in Ischia near Naples and specially Pompeii shows an interesting point. Pompeii has a volcanic history. According to archeological research in Pompeii, many bodies were conserved intact due to ashes covering the bodies. It means there were, maybe, some carbon over the volcanic layers. This volcano might be active for the year or semi- active and burning the organic matters. Then, the empty spaces under the soil of course enough deep. You can have some Schlumberger techniques to see what is under the soil and fill with some adequate material according to your engineers like some foams, or diluted concrete and etc.
It is good for other regions to have such examination before it happens.