Abstract: Using the observed water level data recorded from 1 July 1995 to 30 June 1997 a t Su20 well, the correlation dimension D₂ and o rder-2 Renyi entropy K₂ as well as the largest Lya-punov exponent λ₁ of the chaotic attractors of the water level system before and after South Yellow Sea earthquake (M_S 6. 1) in 9 Nov ember 1996 are calculated and studied with the time-delay phase space reconstruction algorithm. Besides, the generalized fractal dimen-sion spectrum D_q before and after the earthquake is also calculated and studied with the generalized Grassberger-Procaccia algorithm. The research result indicates that the water level system is a chaotic system, the freedom degree of which is within the range of 3~16, and the dimension and entropy of the system declined before the earthquake. It also indi-cates that the left half of the generalized fractal dimension spectrum D_q curve became steep before the earthquake. The conclusion of this research can be of great applicable signifi-cance in recognizing the nonlinear dynamic features of the earthquake precursor observa-tion and in earthquake prediction.

Key words: Earthquake prediction, Water level, Chaos, Multifractal, Generalized fractal dimension spectrum