Abstract: In this paper, weanalysed the data of Tangshan, Songpan and Gong he earthquakes with M ≥ 7. 0, studied the judging indexes between“ field precursors” and “ focus precur-sors” of hydrochemical parameters before strong earthquakes. It is pointed out that among the long-term anomalies of hydrochemical parameters, the“ focus precursor” emerges earli-er than the“ field precursor”. A large proportion of all earthquake precursors is focus pre-cursor. The precursory index es of hydrochemical parameters for judging strong earth-quakes ( MS ≥ 7. 0) in a year are presented.

Key words: Strong earthquake, Hydrochemistry, Field precursor, Focus precursor, Judging index