Tectonic Summary

The July 16, 2023, M 7.2 earthquake south of Sand Point, Alaska (south of the Alaska Peninsula), occurred as the result of thrust faulting on or near the subduction zone interface between the Pacific and North America plates. The preliminary focal mechanism solution indicates rupture occurred on a fault dipping either shallowly to the northwest, or steeply to the southeast. The location, mechanism and depth – and the large size of the event – are all consistent with slip occurring on the subduction zone interface between the two plates. At the location of this event, the Pacific plate converges with North America to the northwest at a rate of about 70 mm/yr, subducting at the Alaska-Aleutians trench 125 km to the southeast of the earthquake.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust faulting events of the size of the July 16, 2023 earthquake are typically about 55x30 km (length x width).

Large earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, nine other earthquakes M7 and larger have occurred within 250 km of the July 16, 2023 event. The largest of these was a M8.6 earthquake on April 01, 1946, which occurred about 150km to the west-southwest of the July 16, 2023 earthquake. The 1946 M8.6 generated a tsunami that devasted the lighthouse on Unimak Island and swept away its five occupants. Tsunami damage also occurred at Dutch Harbor and Ikatan Island in the Aleutian Islands, on the west coasts of North and South America, and in Hawaii. At Hilo, Hawaii, the tsunami took 159 lives and caused one death in California. The Alaska-Aleutian Trench also hosted the second largest earthquake recorded by modern seismic instrumentation, the M9.2 March 27 1964 earthquake in the Prince William Sound region of Alaska

The stretch of the Alaska-Aleutian subduction zone beneath the Shumagin Islands has not ruptured in a historical great (M>8) earthquake. This observation led to definition of the ‘Shumagin Gap’ in the context of seismic gap theory, which proposes that fault sections that have not slipped for the longest elapsed time will be the site of future earthquakes. The interpretation that the Shumagin Gap can host extremely large earthquakes is clouded by 1) geodetic observations, which show that the Shumagin Islands and neighboring Sanak Island are above a section of the subduction interface that is very poorly coupled and storing very little elastic strain; and 2) geologic observations, which have documented little land level change and tsunami inundation since ~3,000 years ago on Simeonof Island in the Shumagins.

Event Details

Event Map

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)