Tectonic summary of the magnitude 8.2 earthquake in the Gulf of Alaska on 29 July 2021

The July 29, 2021, (July 28, 2021 local time) M8.2 earthquake southeast of Perryville, 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 64 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. Reverse faulting events of the size of the July 29 earthquake are typically about 200×80 km in size (length x width).

Large earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, 8 other earthquakes M7 and larger have occurred within 250 km of the July 29, 2021 event, including earthquakes of M7.8 on July 22, 2020 (located 62 km west of the July 29, 2021 event) and M7.6 on October 19, 2020 (located 145 west of the July 29, 2021 event). Given the temporal and spatial proximity of the July 29, 2021 earthquake to the two previous large earthquakes in July and October 2020, those previous events were foreshocks of the July 2021 earthquake. An earthquake of M8.2 also occurred on November 10, 1938 within 40 km of the July 29, 2021 event. While initial information is incomplete, the July 29 earthquake may have ruptured partially or completely into the ruptured region of the 1938 earthquake.

The Alaska-Aleutian Trench also hosted the second largest earthquake recorded by modern seismic instrumentation, the M9.2 March 27 1964 earthquake, which ruptured to within about 250 km of this event, at the eastern end of the 1938 rupture area. The 1938 event produced a small tsunami that was recorded both locally and in Hilo, Hawaii. The remote location of the earthquake resulted in little impact to people and infrastructure.

Source: USGS