2015 M6.8 Sutwik Island Earthquake

Mainshock and Aftershocks: 

A magnitude 6.8 earthquake occurred on Thursday, May 28 at 11:00 pm AKDT (May 29 7:00 UTC) in the Alaska Peninsula region (red star on the map). It occurred at a depth of 73 km (45 miles) and was located 125 km (78 miles) ENE of Chignik. The Alaska Earthquake Center reported only about 50 aftershocks through end of the month (open circles on the map). The aftershock depths range between 52 and 84 km (32-52 miles). Due to the sparse network coverage, only aftershocks with magnitudes 2 and above can be located. The largest aftershock, of magnitude 4.9, ocurred on May 29 at 8:40 pm AKDT (May 30 4:40 UTC). It is common for deeper events to produce only a small number of aftershocks.

Felt Reports: 

This earthquake was felt strongly in Alaska Peninsula and Kodiak communities and as far away as Anchorage and Valdez. Maximum reported shaking intensity IV (light) was reported in King Salmon and South Naknek. No damage or injuries have been reported. Rock falls were observed in the Chignik area by residents.

Tectonic Setting: 

This earthquake occurred in the downgoing portion of the subducting Pacific Plate. The Aleutian trench marks the boundary between the subducting Pacific and overriding North American plates in southern Alaska and Aleutian islands. While the largest earthquakes occur on the interface between these two plates (like the 1964 magnitude 9.2 earthquake), the downgoing portion of the Pacific Plate keeps producing earthquakes down to 250 km (155 miles) in Aleutians and southern Alaska. The faulting type of this earthquake is consistent with down dip extension of the Pacific Plate as it is pulled into the mantle under its own weight. This is the largest intermediate-depth earthquake in this region since December 6, 1999 M7.0 earthquake (orange star).