In order to pare down the list of potential precursors, we applied some criteria.  

If a particular phenomena had been observed and documented in a research paper, it makes the first cut.

A precursory signal which, in general, has been observed to occur about 30 days or so prior to a larger earthquake makes the list.  Examples often cited in research are well water level anomalies, and radon content in well water.

 We wanted to narrow down the potential precursor list to only those that are likely to be within finanancial reach of the amateur, and to those signals that had, with some reliability, been seen in other work.

Example possibilities include:

        Earth current anomalies
        Resistivity changes
        Earth outgassing:
                Radon (abrupt, spike-like increase, then sharp decrease)    
                Hydrogen
                Helium    
                Methane
                Carbon Dioxide
                Mercury vapor
        Gas well head pressure fluctuations
        Enormous increase in microseismicity, then sudden decrease.  
        Well/stream turbidity
        Electrostatic phenomena
        Ion concentration
        Atmospheric Field anomalies
        Surface temperature increases (as observed from space imagery)
        Ionospheric anomalies
        Night Airglow changes
        Ground tilt/uplift