Hi Petra. I think I may have at least one answer to your question about why the water in some wells fluctuates before an earthquake. The following is in part from the Fujita Research.

Fujita Research was founded in 1989 in California and now has branch offices in Bangkok, Tokyo and Brighton, England.

Earthquake Prediction (Report 018)

Changing water levels in deep wells is recognized by the IASPEI as a significant precursor to earthquakes. Perhaps part of the reason for this is explained by the discoveries of German scientists working at “KTB”, which is planned to be the deepest hole ever drilled in the earth’s crust (to 10 km). At a depth of 3 900 m the researchers struck water. A heavy brine with a salinity twice that of sea water, it was at a temperature of 118°C and contained 80% by volume of gases in solution, principally N2(70%) and CH4 (29%). The discovery of this brine led the researchers to postulate a “crustal ocean”, with tides, currents, and flows, all of which could conceivably react to seismic activity.

No simple model exists to connect pre- or co-seismic fluctuation of ground water levels to this crustal ocean. Lomnitz (3) considers that, ultimately, the mechanism will be found to be related to pressure changes, rather than changes in volume in the focal region (as most geophysicists currently believe). Such regional pressure changes can be detected at deep wells.

The sensitivity of deep wells to seismic activity is remarkably varied. A number of deep wells in China are reported (4) to be extremely sensitive to changes in pressures, and can reliably detect earthquakes occurring halfway round the world. This observed sensitivity is probably due to their being quite protected from surface noise (rainfall, seasonal effects, etc.). As a result, China relies a great deal on deep wells for earthquake prediction. Indeed, over 100 research wells in excess of 1000 m deep have been drilled solely for earthquake prediction purposes. In these wells, water levels are continually monitored to ±0.5 cm and temperatures to ±0.01°C. Japan also relies to some extent on such wells - some 93 wells are monitored for earthquakes. In general a pre-seismic variations at observation wells follows this sequence:

1) A gradual lowering of water levels of a period of months or years
2) An accelerated lowering of water levels (rate often exponential) in the final few months or weeks preceding the earthquake.
3) A “rebound” where water levels begin to increase rapidly in the last few days or hours before the main shock.

In the monitoring of water levels in deep wells, care must be taken to correct the data for “earth tides”. This is due either to volume changes caused in fractured aquifers by tidal strain, or perhaps by changes in gravitational acceleration alone. In either case, it is important that data is corrected for this phenomenon. In addition water extraction from the aquifer must also be considered. In many parts of the planet the water table is falling due to water abstraction for drinking and irrigation. It is quite possible that such drops could be mistaken for a long-term seismic precursor.

• Fujita Research 018