It is intended for use in removing aftershocks or "declustering" the catalog. The methods used are described in "Second-Order Moment of Central California Seismicity, 1969-1982" (P. Reasenberg, JGR v.90., pp.5479-5495, 1985). The current version (CLUSTER2000x) reads catalogs in a variety of standard formats. However, it is not fully Y2000 compatible. While it reads the Y2K formats for HYPOINVERSE and HYPO71, CLUSTER2000x requires all data to belong to one century.
Identify clusters (e.g., aftershocks) in an earthquake catalog