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Owing to Iraq's unique geographic location (29-38.5 N;39-49 E) the eastern and northeastern parts of the country are directly influenced by the seismic activity of the Tauros-Zagros tectonic zones. An earthquake data file containing nearly 550 entries covering the period 1905-1984 is compiled and an analysis of completeness is performed using Stepp's (1973) model. It is observed that the file is complete for 4.8 Ms and greater over the whole 80-year sample of data while during the past 25 years this threshold drops to about 4.1 Ms. The “completed” data file is then employed in re-evaluating the magnitude-frequency formula for the country. To further investigate the problem of earthquake occurrence a simple statistical treatment is applied to a “constant-b” seismicity model whereby an increase in the frequency of occurrence of approximately 35% is identified in the latter half of the seismic history of the country.

Extreme value theory is used to estimate maximum magnitude earthquakes in Iraq (29–38°N, 39–48°E). The seismicity file for the period 1905–1982 is used to determine the recurrence relationship as well as the parameters of Gumbel types I and III asymptotic distributions. The statistical parameters are estimated by both least squares and maximum likelihood techniques. Results show that although the least squares upper magnitude is closer in value to actual earthquake magnitude, the maximum likelihood approximation to the third distribution appears to show a better overall fit to the data sample. This result is reflected in the calculated probability of occurrence of earthquake risk over various design periods.

Summary. Gumbel's theory of extreme value has been employed in the statistical forecasting of maximum-magnitude earthquake occurrence. The basic working hypothesis behind this method assumes that observations follow either Gumbel type I or type III asymptotic distributions. In certain cases, however, it is found that neither type of distribution fits the data well enough to produce accurate parameter-estimates, particularly in the larger earthquake range. This article proposes an alternative approach based on finite-mixture distributions whereby a more realistic prediction of upper earthquake magnitudes (at given return periods) is expected using a combined analysis of both Gumbel types I and III extremal distributions.