We compare several modem (1977-present) worldwide earthquake catalogs to infer their completeness, earthquake origin time and hypocenter location accuracy, magnitude/scalar seismic moment errors, and difference between individual focal mechanism/moment tensor solutions. The Harvard centroid moment tensor (CMT), US Geological Survey (USGS) MT, USGS first-motion (FM) focal mechanism, PDE and ISC catalogs have been analyzed and compared. The catalogs' completeness and accuracy vary in time and depend on earthquake depth and tectonic environment. We propose a new statistical method for evaluating catalog completeness and show the results for the CMT dataset. A difference in frequency range of seismic waves used in earthquake processing leads to varying degrees of catalog completeness for foreshocks and aftershocks close in time. Earthquake origin time versus centroid time as well as hypocenter location versus centroid location can be explained well by earthquake scaling relations. Comparing moment magnitudes and regular earthquake magnitudes yields estimated magnitude uncertainties and shows that latter magnitudes poorly estimate earthquake size for large events. Moment errors reported in the CMT solutions are well correlated with the CMT/GS-MT magnitude difference, and hence indicate magnitude uncertainty well. A normalized seismic moment tensor has 4 d.f. and its accuracy can be represented as the non-double-couple (non-DC) component-value, the 3-D angle (Phi) of DC source rotation, and a position of the rotation pole. Our results suggest that a routinely determined non-DC component is in most cases only an artifact. The distribution of the Phi-value varies over catalog time, earthquake depth, focal mechanism, and magnitude. The seismic moment errors and the value of the non-DC component are indicative of the Phi-value; for the best solutions, the 3-D angle in the CMT catalog is on the order of 5-7degrees. The CMT catalog is obviously the best dataset in completeness and accuracy of its detailed solutions. Our results specifying uncertainties and completeness of global earthquake catalogs, can be used in studies of geodynamic processes, tectonic deformation associated with earthquakes, earthquake stress analysis and in many other applications of earthquake catalog data. Seismogram interpretation techniques can be reviewed and possibly revised in light of these results. (C) 2002 Elsevier Science B.V. All rights reserved.