Seismicity associated with a cluster of earthquakes of mb ≧ 4.5 near Adak, Alaska: Evidence for an asperity?

Abstract

Abstract 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 m b ≧ 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 m b ≧ 4.5 have occurred in one of the cluster areas in the past 3 yr in contrast to two earthquakes of m b ≧ 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 m b ≧ 4.5 may indicate a change of focal mechanism reflecting a reorientation of stress around the asperity prior to larger earthquakes.