An area of the main thrust zone south of Adak, Alaska, in the central Aleutian island arc is characterized by clustering of hypocenters of earthquakes of mb ≧ 4.5 and by low b-value relative to the rest of the thrust zone. The location and dimension of two of these clusters has been constant during 15 yr of teleseismic and 9 yr of local monitoring. Six earthquakes of mb ≧ 4.5 have occurred in one of the cluster areas in the past 3 yr in contrast to two earthquakes of mb ≧ 4.1 in the previous 6 yr. The concentration of moderate-sized earthquakes and low b-value may reflect a stress concentration at an asperity on the interface between the Pacific and North American plates. The recent activation of this asperity may indicate preparation for a larger earthquake. A change of local (SV/P)z amplitude ratios of small earthquakes before two earthquakes of mb ≧ 4.5 may indicate a change of focal mechanism reflecting a reorientation of stress around the asperity prior to larger earthquakes.