In this paper

the phase space is constructed by various features extracted from seismic signalsin time domain. The results of calculation for four sets of samples indicate that there are two"attractors" in thephase space: one is the attractor for underground nuclear explosion

another is for natural earthquake. The yieldedcorrelation dimension are all the fortal dimensions

and less than the dimension of phase space. When the dimensions of phase space increased ho low to high

the dimensions of the attractors become apparently saturated

and different attractors have different fractal dimensions. A new concept

namely

the "attraction center" isproposed in this paper. Moreover

the attractor

the attractor dimension

and the attraction center with their applications to seismic pattern recognition of undegrund nuclear explosion are also explored.