2020-2021 Alaska Peninsula Earthquake Sequence
Seismotectonic Framework of Alaska
On 29 July 2021, a magnitude (M) 8.2 earthquake occurred southeast of Perryville, Alaska, (south of the Alaska Peninsula). It occurred as the result of thrust faulting on or near the subduction zone interface between the Pacific and North America plates. The seismic data indicate the seismic event originated at a depth of ~32 km (20 mi), placing it near the subduction zone interface.
This event was preceded in 2020 by two nearby earthquakes. The first was a M7.8 earthquake (22 July 2020), followed three months later by a M7.6 earthquake (19 October 2020). Both of these events occurred within 150 km (93 mi) of the most recent 29 July 2021 M8.2 earthquake. Given the temporal and spatial proximity of the recent earthquake to the two previous large earthquakes in July and October 2020, those earlier events can be considered foreshocks of the 29 July 2021 M8.2 earthquake.
At the location of these three events, the Pacific plate subducts to the northwest beneath North America at a rate of ~63 mm/yr. Large earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, eight M7 and larger earthquakes have occurred within 250 km of the 29 July 2021 event, including the July and October 2020 events. An earthquake of M8.2 also occurred on 10 November 1938 with an epicenter ~40 km (25 mi) from the 29 July 2021 M8.2 earthquake. Although the details of the 10 November 1938 M8.2 rupture are unclear, the 29 July 2021 M8.2 earthquake appears to have ruptured part or all of the rupture area of the 1938 earthquake.
The map shows the location of the 29 July 2021 M8.2 event with a yellow star, as well as relative plate motions with respect to a fixed North America plate.
Subduction Zone Geometry
Globally, s ubduction zone earthquakes can occur from shallow depths to depths of over 600 km (373 mi). The region of seismicity, which at shallow levels lies along the plate interface, is called the Wadati-Benioff Zone . At crustal and upper lithospheric depths, the plate interface can be frictionally “locked,” resulting in the build-up of stresses and the generation of large megathrust earthquakes (e.g., 1964 M9.2 Great Alaska earthquake). Below the locked zone, seismicity may be a result of stresses produced by slab dehydration and other metamorphic processes. Subducting slabs can extend much deeper into the mantle than the earthquakes of the Wadati-Benioff zone might imply, and even sometimes cross into the lower mantle. According to the Slab2 model (Hayes et al., 2018), the subducting Pacific plate's Benioff zone along the Aleutian Arc extends as deep as ~220 km (137 mi).
Slab2 is a subduction zone geometry model for major subduction zones. Click the button below to explore the subduction zone models around the Earth!
Seismicity of Alaska: 1900-2021
The location of the largest earthquakes (M7 or larger) in the vicinity of Alaska and the Aleutians can be viewed on the map. Also provided are the estimated rupture or aftershock zones of all instrumentally recorded M8.3 and larger Alaskan earthquakes.
Recent History of Seismicity of the Alaska Peninsula and Aleutian Islands
The figure below shows the cumulative seismic moment as a function of time for the earthquakes shown on the seismicity map. The seismic moment of an earthquake, which is the best representation of the size of an earthquake, is a measure based on the area of fault rupture, the average amount of slip, and the strength of the rocks adjacent to the fault. The vertical tick marks on the plot show the time of the M8 or larger earthquakes since 1900. Significant steps in the cumulative seismic moment are typically associated with the occurrence of one or more large events (typically M8 or larger). The exceptionally large jump on the graph in 1964 represents the Prince William Sound earthquake, which at M9.2 is the second largest earthquake ever recorded by modern seismic instrumentation.
This figure shows the dramatic effective of seismic moment release for the largest earthquakes in the region. The largest step in moment release in 1964 is due to the M9.2, the second largest earthquake recorded during the 20th century.
Between 1900 and 1946, the seismic moment rate for earthquake activity was relatively constant and at a rate similar to the period between 1966 and the present. However, between 1946 and 1965 there was a period of numerous great earthquakes along this plate boundary. This is seen in the much steeper growth of the moment rate curve during that time period, culminating in the Great Alaskan earthquake of 1964. It was during this period of great earthquakes that much of the subduction zone ruptured. One exception is an area often referred to as the Shumagin Gap that lies between the 1946 M8.6 and 1964 M9.4 earthquakes. This gap includes the 2020-2021 series of large earthquakes.
Alaska Peninsula Earthquake Sequences of 2020-2021
This map shows the seismicity (M3.5 and larger) from 01 January 1900 to 14 August 2021 for a region surrounding the M7.8 (22 July 2020), M7.6 (19 October 2020), and M8.2 (29 July 2021) earthquakes.
Large circles with thick black outlines are the M8 and larger earthquakes since 1900.
Seismicity of the 22 July 2020 M7.8 Earthquake
Shown in blue are earthquakes following the M7.8 earthquake on 22 July 2020 and prior to the M7.6 earthquake on 19 October 2020.
Focal mechanism for the 22 July 2020 M7.8 earthquake. Moment tensor solutions like this indicate the style of faulting (focal mechanism) and the size of an earthquake. Seismologists model moment tensors using seismic wave recordings on seismic stations around the world.
The 22 July 2020 M7.8 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 focal mechanism solution, along with the location of aftershocks, indicates that the rupture occurred on a fault dipping shallowly to the northwest. The location, mechanism, and depth – and the large size of the event – are all consistent with slip occurring along 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 63 mm/yr, subducting at the Alaska-Aleutians trench 125 km (78 mi) to the southeast of the earthquake.
Seismicity and focal mechanisms following the 22 July 2020 M7.8 earthquake (blue). Also shown in red are the aftershock zones of the 10 November 1938 M8.2 (east) and 01 April 1946 M8.6 (west) earthquakes. Figure from Herman and Furlong (2021) with permission from Science Advances.
Seismicity of the 10 October 2020 M7.6 Earthquake
Shown in orange are earthquakes in the vicinity of the M7.6 mainshock between 19 October 2020 and 28 July 2021.
Focal mechanism for the 19 October 2020 M7.6 earthquake. Moment tensor solutions like this indicate the style of faulting (focal mechanism) and the size of an earthquake. Seismologists model moment tensors using seismic wave recordings on seismic stations around the world.
The 19 October 2020 M7.6 earthquake southeast of Sand Point, Alaska, (south of the Alaska Peninsula) occurred as the result of strike-slip faulting within the subducting slab near the subduction zone interface between the Pacific and North America plates. The focal mechanism solution, in combination with aftershocks, indicates rupture occurred on a moderately east-ward dipping right-lateral strike-slip fault striking towards the north-northwest. Of primary note for this earthquake is that it was not a thrust faulting earthquake on the plate interface itself, but rather a large strike-slip faulting earthquake within the subducting slab. This event can be considered an aftershock of the 22 July 2020 M7.8 earthquake.
Seismicity and focal mechanisms following the 22 July 2020 M7.8 earthquake (blue) and the 19 October 2020 M7.6 earthquake (orange). Also shown in red are the aftershock zones of the 10 November 1938 M8.2 (east) and 01 April 1946 M8.6 (west) earthquakes. Figure from Herman and Furlong (2021) with permission from Science Advances.
This figure shows a simplified view of the plate boundary setting and conditions associated with the 2020 earthquakes. The region of the Shumagin Gap (at shallow levels that produce earthquakes) is considered to be uncoupled and does not produce large megathrust earthquakes. It is bounded by regions that have hosted great earthquakes in 1938 and 1946. The 22 July 2020 thrust faulting event ruptured on the edge of the region of the 10 November 1938 rupture, and it caused aftershocks and afterslip in the region down dip of the uncoupled Shumagin Gap. [Note: these figures are rotated 180 degrees - the view is looking from the North]. Figure from Herman and Furlong (2021) with permission from Science Advances.
Seismicity of the 20 July 2021 M8.2 Earthquake
Shown in green are earthquakes in the vicinity of the M8.2 earthquake between 20 July 2021 and 14 August 2021.
Focal mechanism for the 29 July 2021 M8.2 earthquake. Moment tensor solutions like this indicate the style of faulting (focal mechanism) and the size of an earthquake. Seismologists model moment tensors using seismic wave recordings from seismic stations around the world.
The 29 July 2021 (28 July 2021, local time) M8.2 earthquake southeast of Perryville, Alaska, (south of the Alaska Peninsula) was a thrust faulting earthquake on or near the subduction zone interface between the Pacific and North America plates. The focal mechanism indicates rupture occurred on a fault dipping shallowly to the northwest. The location, mechanism, and depth – and the large size of the event – are all consistent with slip occurring along 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 63 mm/yr, subducting at the Alaska-Aleutians trench ~125 km (78 mi) to the southeast of the earthquake.
This figure shows the seismic activity in the vicinity of the Shumagin Gap over time from January 2019 to August 2021. The upper plot shows the number of M3.0 and larger events per day. The lower plot shows the magnitude of events over time. The colors correspond to the timing of events, changing at the 22 July 2020 M7.8, 19 October 2020 M7.6, and 29 July 2021 M8.2 earthquakes. Prior to July 2020, there were never more than 5 earthquakes in the region per day, and most were smaller than M5.0. Starting with the 22 July 2020 M7.8 event, each of the three M7.6+ earthquakes in the 2020-2021 sequence was followed immediately by a large increase in the number of events per day, as well as numerous aftershocks M5 and larger. Both the number of events per day and the magnitudes of these aftershocks then decreased over time.
Summary
The map above displays the locations of the three large earthquakes discussed here and their corresponding aftershocks. The green contours outline the rupture area of the 29 July 2021 earthquake and are contours of the amount of slip. Larger earthquakes are represented by their focal mechanisms (sized according to magnitude), and the triangle on the right shows the style of faulting observed with each rupture. Note that the 22 July 2020 and 29 July 2021 earthquakes and aftershocks are almost entirely thrust faulting, and the 19 October 2020 earthquake and aftershocks are almost entirely strike-slip faulting. The cross section beneath the map shows the seismicity of the region along the A to B profile (shown on the map). It shows that most of the earthquake activity is along the plate interface. It is notable that the seismicity associated with the 19 October 2020 earthquake is generally at shallower depths along the plate interface than the earthquakes associated with the two main thrust faulting earthquakes.
The sequence of three significant earthquakes within and adjacent to the Shumagin seismic gap along the Alaska-Aleutian subduction zone provides insight into a range of processes that can act on the subduction megathrust. The 22 July 2020 M7.8 subduction thrust faulting earthquake initiated the sequence and ruptured a section of the subduction zone interface along the western edge of the 10 November 1938 M 8.2 earthquake. This event generated additional slip (largely aseismic, meaning a lack a significant seismicity) to the west of the main rupture near the base of the seismogenic zone along the Shumagin Gap.
The 22 July 2020 event was followed on 19 October 2020 by a M7.6 strike-slip faulting earthquake within the downgoing Pacific plate in the Shumagin Gap. This event and its unexpected strike-slip faulting can be produced by the transition from coupled-to-uncoupled mechanical behavior on the plate interface, moving from the site of the 1938 asperity to the Shumagin Gap.
Most recently, this region experienced a M8.2 thrust faulting earthquake northeast of the 22 July 2020 earthquake. The rupture area of the 29 July 2021 event occupied most of the inferred rupture area of the 10 November 1938 earthquake. Stress modeling indicates that the 22 July 2020 event did increase the relevant stress levels in the epicentral region of the 2021 earthquake, thus favoring its occurrence in this location. The 19 October 2020 earthquake, however, had little effect on stress conditions in the region of the 29 July 2021 M8.2 earthquake. These results imply that both the 19 October 2020 M7.6 and 29 July 2021 M8.2 events were promoted by the occurrence of the 22 July 2020 M7.8 earthquake.
Links to earthquake sequence event pages:
Acknowledgements
Earthquake source parameter data (locations, magnitude and focal mechanisms) used here came from USGS Recent Earthquakes of which the USGS National Earthquake Information Center and the University of Alaska-Fairbanks Alaska Earthquake Center are contributors.
References
Hayes, G.P., Moore, G.L., Portner, D.E., Hearne, M., Flamme, H., Furtney, M. and Smoczyk, G.M. (2018). Slab2, a comprehensive subduction zone geometry model, Science, 362(6410), 58-61. DOI: 10.1126/science.aat4723
Herman, M. W. and Furlong, K. P. (2021). Triggering an unexpected earthquake in an uncoupled subduction zone, Science Advances, 7(13), DOI: 10.1126/sciadv.abf7590
