How complete is the ISC-GEM global earthquake catalog?

The International Seismological Centre, in collaboration with the Global Earthquake Model effort, has released a new global earthquake catalog, covering the time period from 1900 through the end of 2009. In order to use this catalog for global earthquake studies, I determined the magnitude of completeness (M (sub c) ) as a function of time by dividing the earthquakes shallower than 60 km into seven time periods based on major changes in catalog processing and data availability and applying four objective methods to determine M (sub c) , with uncertainties determined by nonparametric bootstrapping. Deeper events were divided into two time periods. Because of differences between the four methods, the final M (sub c) was determined subjectively by examining the main features of each method in both the cumulative and binned magnitude-frequency distributions. The time periods and M (sub c) values for shallow events are 1900-1917, M (sub c) =7.7; 1918-1939, M (sub c) =7.0; 1940-1954, M (sub c) =6.8; 1955-1963, M (sub c) =6.5; 1964-1975, M (sub c) =6.0; 1976-2003, M (sub c) =5.8; and 2004-2009, M (sub c) =5.7. Using these M (sub c) values for the longest time periods for which they are valid (e.g., 1918-2009, 1940-2009, ...), the shallow data fits a Gutenberg-Richter distribution with b=1.05 and a=8.3, within 1 standard deviation and with no declustering. The exception is for time periods that include 1900-1917, during which there are only 33 events with M> or =M (sub c) and, for those few data, b=2.15+ or -0.46. That result calls for further investigations for this time period, ideally having a larger number of earthquakes. For deep events, the results are M (sub c) =7.1 for 1900-1963 (although the early data are problematic) and M (sub c) =5.7 for 1964-2009. For the later time period, b=0.99 and a=7.3.