Hi,

In Bruce Bolts book Earthquakes he includes a chapter on "Clues to recognizing impending earthquakes". He outlines five stages to this process. I cannot of course quote this entire section, so excuse me if I paraphrase.

1. p-wave velocities change and these can detected by seismometers.
2. change in ground level/tilt. (inconclusive results so far.)
3. release of radon, an inert gas, into the atmosphere along fault zones particularly from deep wells. (earthquake lights?) piezioelectrics.
4. Change in electrical conductivity in rocks. Rocks which are saturated with water, such as granite, change drastically just before rocks fracture in a high pressure device.
5. Increase in regional seismicity.

The earthquake process through these stages,
Slow build up of elastic strain due to underlying tectonic forces; cracks may develop in the crust above fault zones causing the volume of water to diffuse into the rock, causing dilatant volumes to fall in the rocks, then the ground swells and tilt begins, as the water fills the microcracks it creates instability increasing micro-earthquake activity, emission of radon gas tapers off from the fresh cracks and electrical resistivity diminishes further and then the earthquake of moderate to large size initiates. The final stage is the reduction of aftershocks and ground healing. END QUOTE.

Incorporated into this process we can see two interesting stages; gas which can create a vapor seen by humans above faults and rocks which change due to increased absorbtion of water. The electrical qualities change under these conditions and are higher than normal.

I hope this helps a little. My suggestion is that you may wish to look at - Dilatantcy Diffusion Theory.

Petra