Heterogeneities of the instrumental seismicity catalog (1904-1980) for strong shallow earthquakes

For earthquakes with magnitude M< or =7, we used the catalog given by Abe (1981), which lists the surface-wave magnitude (M (sub S) ) of every reported large event that occurred between 1904 and 1980, according to the original formulation of Gutenberg (1945). We find that, for instrumental-related reasons, the M (sub S) of large events prior to 1908 have been systemically overestimated by at least 0.5 magnitude unit, and the M (sub S) of events in the period 1908-1948 have been consistently overestimated by 0.2 unit, relative to the M (sub S) assigned to shocks occurring after 1948. When these corrections are taken into account, the catalog of events with M (sub S) > or =7.0 is shown to be largely complete (i.e., nearly all the large shallow shocks which occurred in the Earth are listed) for most of the world since early in the century. For earthquakes at the magnitude 6 level, we used the International Seismological Center (ISC) tape and Regional Catalogues of Earthquakes. We find a highly incomplete seismicity catalog at this magnitude level for the first half of the century, mainly due to numerous increases and decreases in the earthquake detection and reporting capabilities; a more complete but extremely inhomogeneous catalog for the period 1950 to 1963, due to the use of different formulations and criteria to calculate and report the parameter "magnitude" by the various seismological agencies; and a largely complete and homogeneous (in M (sub S) ) record since 1964.--Modified journal abstract.