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The strongest known earthquakes in this region were the 1965 M8.7 Rat Islands earthquake to the west and the 1957 M8.6 Andreanof Islands earthquake to the east. In the previous five years (2004-2008), eighteen M6-7 and three M7+ earthquakes occurred in this region. Most of these events occurred on the plate interface between the subducting Pacific and overriding North American plates.

The April and May, 2008 M6+ events are of a different origin than the above mentioned underthrusting events. Faulting parameters estimated from the waveform inversion indicate a strike-slip type of motion for these mainshocks. Their locations, shallow, and above the down-dip end of the locked interface, are consistent with the events occurring within the crust of overriding North American plate.

In June 2006, a similar sequence of earthquakes occurred about 300 km to the west of the May 2008 sequence. In the western Aleutians, the crust is partitioned into rotating blocks, such as Buldir and Near blocks. In addition, in this region the direction of convergence between the two tectonic plates is oblique to the plate interface. The 2008 and 2006 strike-slip events could indicate relative motions of the crustal blocks or slip partitioning between the plate subduction and strike-slip faults in the region.

Last Modified: May 14, 2020