  The results of the analysis of the subclustering in a sample of ESO Nearby
Abell Cluster Survey (ENACS) galaxy clusters, with data complemented from other
studies, are presented. The analysis is performed by using the S-tree method
enabling one to study the hierarchical properties of clusters, namely by the
detection of the main physical cluster and of its subgroups. The results
indicate: (a) systematically lower genuine cluster velocity dispersions than
known from previous studies; (b) existence of 2-3 subgroups in each cluster.
Due to certain properties of specific dynamical entities, we denote these
subgroups as galaxy associations, which can become an essential challenge for
formation mechanisms of galaxy clusters.
