2006 M6.4 Rat Islands Earthquake
On June 14, 2006, at 4:18 am UTC (June 13, 8:18 pm ADT) a strong earthquake occurred in the Rat Islands region of the Aleutian Islands (larger red star on the above map). The mainshock (M6.4) was situated 87 kilometers (54 miles) southeast of Buldir Island. The epicenter was located at 51.86N and 176.92E at shallow depth. This earthquake is the largest to occur in this region since the magnitude 6.8 event on June 14, 2005 (smaller white star on the map). The Earthquake Center located a little over 1,200 aftershocks of the M6.4 event through the end of June (crosses). The largest aftershock of magnitude 5.9 occurred 30 minutes after the main shock. The June 14 sequence continued with a magnitude 6.2 earthquake that occurred on June 26 at 6:39 pm ADT (June 27, 2:39 am UTC). It was located at 52.178N 176.077E at 10.1 km depth, which is 75 km or 47 miles NE of the M6.4 event (shown by M6.2 red star on the map).
These events were felt strongly in Shemya, but no there were no reports of damage.
The June 14, 2006 earthquake occurred near the margin of the rupture zone of the M7.7 earthquake of November 17, 2003. The M7.7 event ruptured the boundary between the subducting Pacific and overriding North American plates (approximate extent of the rupture zone is shown by the black line). This region, where the two plates are being forced directly into one another, is one of the world's most active seismic zones. Over one hundred earthquakes of magnitude seven or larger have occurred along this boundary in the past hundred years. The 1965 Mw 8.7 Rat Islands earthquake ruptured a ~600 km-long portion of the plate boundary to the west of the Amchitka Island (see map). In November 17, 2003 M7.7 earthquake, the easternmost part of the 1965 zone failed again. The June 2005 sequence of earthquakes was located east of the 2003 rupture zone, at the easternmost end of the 1965 rupture zone. The 1957 M8.6 earthquake ruptured segment of the megathrust east of the 1965 zone.
The 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 the mainshock and the largest aftershock. It's location, shallow, and above the down-dip end of the locked interface, is more consistent with this event occurring within the crust of overriding North American plate. In the western Aleutians, the crust is partitioned into rotating blocks, such as Buldir Block to the east and Near block to the west of the June 14, 2006 event location. If this event is the manifestation of the block rotation processes, then the NE-striking focal plane is the fault plane for this sequence. It's mechanism is consistent with the clockwise rotation of the crustal blocks. The aftershock locations are also consistent with the rupture along the NE-trending fault plane.