SOME STATISTICAL FEATURES OF THE LONG-TERM VARIATION OF THE GLOBAL AND REGIONAL SEISMIC ACTIVITY

The temporal variation of the shallow earthquake activity of the entire world and the regional area around Japan is statistically investigated on the basis of two catalogs of earthquakes: one is Abe's catalog for global earthquakes with magnitude M(s) greater-than-or-equal-to 7 in the period 1897 through 1980, and the other is Utsu's catalog for the earthquakes in and around Japan with magnitude M(J) greater-than-or-equal-to 6 in the period 1885 through 1980. The occurrence rate of earthquakes in both the areas is high in the period from 1920's through 1940's and low in the last 30 years. This persistent fluctuation or trend in appearance can be related to a long-range statistical dependence of the time series of earthquake occurrence. Indices measuring the strength of such dependence are calculated for each catalog to compare with statistical models. Although Perez & Scholz (1984) have claimed the inhomogeneity of the earthquake catalogs under the postulate of constant rate of earthquake occurrence for the world through the century, their rationale is not always guaranteed in view of the long-range dependence nature of earthquake occurrence. The synchronous variation of seismic frequency in the high latitude area of the world and in the regional area around Japan obtained from the independent catalogs is suggestive of an external effect such as a large-scale motion of the earth rather than the presupposed inhomogeneity of the catalogs.