Gas-geochemical approaches to earthquakes prediction

Concentrations of a wide range of terrestrial gases in ground water and soil air have commonly been found to be anomalously high along active faults, suggesting that the faults may be paths of least resistance for the outgassing process of the ''solid''earth. Anomalous temporal gas-concentration changes with durations of a few hours to many months have been recorded before many large and some smaller earthquakes at stations mostly located along active faults at epicentral distances of as much as several earthquake-source dimensions where the associated coseismic strain changes are estimated to be as small as 10-8. This result suggests that the earthquakes and the associated anomalies are both incidental results of some small but broad-scale episodic strain changes in the crust. Such strain changes may be amplified at the earthquake and anomaly sites and, together with sufficient pre-existing stresses, may reach some critical levels (above half fracture strengths) for generation of the earthquakes and anomalies. Significant gas-concentration changes have also been observed at times of other known crustal disturbances, such as underground explosions, ground-water pumping, and earth tide, and from stressed rock samples in laboratory experiments. These results suggest several possible mechanisms for the deformation-related gas-emission changes that involve movement of crustal fluids of non-uniform chemical composition or enhanced water/rock reaction at newly created rock surfaces. However, gas-concentration changes may also be caused by various non-tectonic environmental changes, which must be recognized in the search of true earthquake precursors. (author). 30 refs, 32 figs