The 5000 km long Queen Charlotte-Alaska-Aleutian seismic zone is subdivided into 17 unequally sized segments. Their boundaries are delineated based on the prior distribution of large and great earthquakes. The 17 segments are chosen to represent areas likely to be ruptured by ''characteristic'' earthquakes. This term usually implies repeated breakage of a plate boundary segment by either a large or great earthquake, whose source dimensions remain consistent from cycle to cycle. This definition does not exclude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single ''giant'' event that is larger than the characteristic size outlined. Conversely, a segment can also sometimes break in a series of smaller ruptures. Formal computations of the conditional probabilities for future large and great earthquakes in the 17 segments of the Queen Charlotte-Alaska-Aleutian seismic zone are based on the following data sets and findings: (1) recurrence intervals from historic and geologic data; (2) direct recurrence time estimates based on rates of relative plate motion and the size or displacement of the most recent characteristic event in each segment; and (3) the application of a lognormal distribution of recurrence times for large and great earthquakes.

Results of these computations indicate seven areas that have high (i.e., ≥60%) conditional probabilities for the recurrence of either large or great earthquakes within the next 20 years (1988--2008). These areas include Cape St. James, Yakataga, the Shumagin Islands, Unimak Island, and the Fox, Delarof, and Near Islands segments of the Aleutian arc. When a shorter time interval is considered (1988--1998), those segments more likely to rupture in large (MS 7--7.7) rather than great earthquakes have a high conditional probability. These areas include the Unimak, Fox, and Delarof Islands segments. The largest uncertainties in these forecasts stem from the short historic record (providing a single recurrence time estimate for some segments, or widely varying estimates for others); from the unknown importance of aseismic slip; and from a vague definition of ''characteristic'' earthquake size. In fact, characteristic earthquake size may be a time-invariant quantity. ¿ American Geophysical Union 1990