Abstract: The characteristics of long period w aves, reco rded at several representative wells before large earthquakes, are analy zed.The reasonable explanation on these long period events occurred before earthquakes is given in theory.It is thought that the long period w ater level events, occurred at some sensitive wells before earthquakes, may be related to the process such as fault creeping, silent and slow earthquake, fault pre-ex tending and seismic source nucleation.The system consisting of well and w ater layers behaves like a long period seismograph. The system responds very well to the valuable precursor low frequency w aves.

Key words: Water level of well, Long period seismic wave, Seismic source nucleation, Mechanism Discussion