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

Rn-222 in groundwater at three out of four New Zealand sites changed in a s tatistically significant way in response to increased monthly coseismic eve nts M2.0-M5, 14-100 km away, particularly events normal to a nearby large f aultline. delta(18)O(H2O) at one site showed a coseismic negative relations hip to seismicity the same day. Of other parameters at the four sites (CO2, CH4/N-2, H-2/N-2, delta O-18(H2O), delta C-13(CO2), delta(13)C(CH4), D/H, Ar/N-2), only the latter showed some consistent sensitivity to seismicity, mainly as after-effects. There was no observable threshold strain value. Th e 9000 events studied gave considerable statistical power, but correlations useful for seismicity prediction were few and doubtful. The frequent negat ive relationships may indicate the closing of rock cracks by strain, hence reduced gas flow. (C) 1999 Elsevier Science Ltd. All rights reserved.