Application of a new method of searching for geochemical changes related to seismic activity

222Rn in groundwater at three out of four New Zealand sites changed in a statistically significant way in response to increased monthly coseismic events M2.0–M5, 14–100 km away, particularly events normal to a nearby large faultline. δ18O(H2O) at one site showed a coseismic negative relationship to seismicity the same day. Of other parameters at the four sites (CO2, CH4/N2, H2/N2, δ18O(H2O), δ13C(CO2), δ13C(CH4), D/H, Ar/N2), only the latter showed some consistent sensitivity to seismicity, mainly as after-effects. There was no observable threshold strain value. The 9000 events studied gave considerable statistical power, but correlations useful for seismicity prediction were few and doubtful. The frequent negative relationships may indicate the closing of rock cracks by strain, hence reduced gas flow.