Abstract: Basedon the theory of seismic precursor “stress-strain” relation and the fact when deformation and destroying of theterrain under force is the changing of its porosity, Δ H, which is the difference between welldy namic level and static level under the constant flowor the difference of dy namic level under two kinds of different constant flow, can relatively measure the changing of saturated aquiferterrain porosity when using the Darcy′s law of hydrology. Furthermore Δ H can be used torelatively measuredy namic changing process of terrain deformation under force, thus it can be used to be a kind of new type of seismic precursory indexes. Basedon the above-mentioned facts, a kind of newseismic precursory observation system, in which the indexis Δ H, isdesig ned. Basedon theoretical analysis, the indexΔ H is just sensitive to changing information of stress-strain indeep-seatedterrain while it is not sensitive to the factor which is not caused by stress-strain changing and the interfere of ground. It has been primarily verified by some experiments that Δ H has the feature of anti-interference, high sensitivity and high sig nal tonoiseratio. The M_S 7. 5 of fshore earthquake, Hualian, Taiw an on May 31, 2002, which is 400km far from ourexperimental point, provides a case for the feasibility of indexΔ H.

Key words: Earthquake precursor, Hydraulic conductivity, Index of wellΔ, H