2008 M6.6 and M6.4 Andreanof Islands Earthquakes

Mainshock and Aftershocks: 

On April 15, 2008, at 22:59 UTC (2:59 pm ADT) a strong earthquake, magnitude 6.4, occurred in the Andreanof Islands region of the Aleutian Islands. It was followed by a larger earthquake of magnitude 6.6 on April 16 at 5:54 UTC (April 15, 9:54 pm ADT) (red stars on the above map). The events were situated about 124 km (77 miles) ENE of Amchitka and 179 km (112 miles) W of Adak. The AEIC located over 400 aftershocks in the first 5 days of the sequence (crosses). The largest aftershock of magnitude 5.0 occurred on April 18 at 20:51 UTC (12:51 pm ADT). These earthquakes are the largest to occur in this region since the magnitude 7.2 event on December 19, 2007 (large white star on the map).

Tectonic Setting: 

The April, 2008 earthquakes occurred in the area separating rupture zones of the 1965 M8.7 Rat Islands earthquake to the west and the 1957 M8.6 Andreanof Islands earthquake to the east (approximate extent of the rupture zones is shown in red). In the last five years, sixteen M6-7 and three M7+ earthquake occurred within the area shown on the above map. Most of these events occurred on the plate interface between the subducting Pacific and overriding North American plates.

Source Mechanism: 

The current sequence of events is of a different origin than the above mentioned underthrusting events. The faulting parameters estimated from the waveform inversion indicate strike-slip type of motion for both 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, similar sequence of earthquakes occurred about 200 km to the west of the April 2008 events (smaller white stars on the map). In western Aleutians, the crust is partitioned into rotating blocks, such as Buldir and Near blocks. In addition in this region, 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.