The compilers of the different data bases sometimes had different conventions about the size or locations of the earthquakes listed. There are a number of different types of earthquake magnitude, corresponding to seismic energy in different frequency bands, and the different types of magnitude calculated for a single earthquake commonly differ slightly, and under some circumstances, may differ greatly. Hypocenters of earthquakes are listed to different levels of precision in the different data bases. Estimates of the hypocenters and magnitudes of earthquakes before about 1900 must commonly be based on scant evidence, and the compilers of different catalogs sometimes interpreted the scant evidence differently. Catalog Source provides information on the different catalogs that may explain some inconsistencies.
References gives a bibliography for the sources of the different catalogs.