In this paper,based on the abundant seismic facts,the abnormal fea-tures of hydrogen sulfide in ground water immediately before the occuran-ce of moderate and strong earthquakes in the Tianshan seismic zone are described.A preliminary discussion on the seismic precursory mechanism of underground gases is made.It is noted that,before moderate and stro-ng earthquakes,as a general rule,abnormal changes with hydrogen sulfi-de for a short period of time take place.Mutation phenomena immediate-ly before earthquakes are also obviously observed.Characteristics of hyd-rogen sulfide during the period immediately before earthquakes are as fol-lows:the suddeness of dynamic process (pre-release of stress strain ener-gy),the mutation characteristics of gas amount increase and the short duration of the abnormal stage.Besides,here also an analysis for the sta-te of subjection to forces and the abnormal characteristics of the precu-rsory near fields and far from fields in various stages of earthquake preg-nancy is made.It is considered that the accelerated creep slipping and the appearance of microcracks prior to that of main cracks and the propaga-tion of supersonic vibrations and lowfrequency deformation waves produc-ed therefrom is the main reason for the universal increase in the concent-ration of hydrogen sulfide,radon and other gases in ground water shortly before earthqukes.
In recent years,hydrogen sulfide has become one of the main indices used for predicting earthquakes,and good results have been obtained.