There have been various potential earthquake precursor events, Radon gas anomalies being one of them along with GPS anomalies i.e. measurement of the GPS error due to the wide variability in Total Electron Content (ionisation) in the upper troposphere. WAAS system of GPS ground stations can map the TEC.

http://www.ias.ac.in/currsci/may102004/1288.pdf

http://ursigass2011.org/abstracts/ursi/GHE3-5.pdf

There has also been visual indications of atmospheric anomalies as well such as rainbow effects in clear sky and strange cloud formations reported as well.

I also remember that even before the recent Japanese Earthquake there were atmospheric IR anomalies recorded as well.

http://www.technologyreview.com/blog/arxiv/26773/