2023 is an earthquake year under review

When Alaska has fewer than 50,000 earthquakes in a given year, we consider it quiet. 2023 was a quiet year for Alaska, with the Alaska Earthquake Center reporting 45,546 seismic events in Alaska and surrounding areas (Figure 1). This is about 1,500 fewer than in 2022, and about 8,900 fewer than the 2018 record (Figure 2).

Most notable earthquakes: Largest earthquake: July 16, with a magnitude of 7.2 in the Alaska Peninsula area. Although we have informally referred to it as the “Sand Point Earthquake,” we consider it to be a late aftershock of the 2020 M7.8 Semyonov earthquake. The next two larger events were the October 16 M6.4 and December 21 M6.1 earthquakes in the Andrianov Islands area of Aleutian Islands. The largest earthquake in mainland Alaska was a magnitude 5.4 in the lower Cook Inlet area on March 19. A total of 225 earthquakes were reported, most in south-central Alaska communities (Figure 3). The largest number of DYFI (did you feel it) responses — 2,191 — came from the M4.5 earthquake that occurred on July 3 at 14:47:29 UTC (5:47 a.m. Alaska time on July 4), about 2 miles south of West of the Eagle River in the Cook Inlet area. Notable events and sequences

Aftershocks continue, albeit at a slower rate, within the 2018 M7.1 Anchorage, 2018 M6.4 Kaktovik, 2018 M7.9 Offshore Kodiak, 2020 M7.8 Simeonof, and 2021 M8.2 Chignik aftershock sequences. The Purcell Mountains earthquake swarm and the Wright Glacier group northeast of Juneau also remain active.

2020 M7.8 Simeonof aftershock sequence

The Simonov earthquake produced the largest number of aftershocks in 2023, with 1,503 aftershocks reported with a magnitude between 1.1 and 7.2 (Figures 4 and 5). About 20 of them had a magnitude higher than 4.0. The largest magnitude 7.2 aftershock occurred on July 16 south of Sand Point and within the western part of the M7.8 aftershock zone. While the aftershock rate of the Simonov earthquake continued to decline in the first half of 2023, it increased after the M7.2 event, which generated its own aftershock sequence. This aftershock was felt throughout the Alaska Peninsula and the Eastern Aleutian Islands. This prompted tsunami evacuations, and generated a notable 0.5 foot tsunami at both King Cove and Sand Point. We expect the Simonov aftershock sequence to continue into 2024.

2018 M7.1 Anchorage aftershock sequence

November 30, 2023 was the fifth anniversary of the M7.1 Anchorage earthquake. Aftershocks have continued at an average rate of about 11 aftershocks per week, which is about 20% fewer than in 2022. The largest aftershock was a magnitude 3.9 on April 3. Nearly 585 aftershocks were reported in 2023, bringing the total number of aftershocks to more than 14,000. About 17 of these aftershocks have been reported in 2023. While original estimates of the duration of the aftershock sequence were on the order of 2 to 2.5 years, the seismicity rate remains high compared to the baseline rate before the M7.1 mainshock. (Figure 6). We expect this sequence to continue at a decreasing rate in 2024.

Volcanic events

The year 2023 saw an increase in seismic activity associated with volcanic processes (Figure 7). We described 2,626 earthquakes associated with volcanic activity (M = 0.6 to 4.3), which is four times as many as occurred in 2022. The first and most active sequence began in February and continued until March beneath the Tanaga-Takawanga volcanic complex in the center of the country. Aleutians. About 900 earthquakes in total have been reported, six of which have a magnitude between M4.0 and M4.4. About 1,200 events have been associated with the Katmai volcanic field, which has a magnitude of 4.2 and activity continues throughout the year. An unusual, but short-lived, swarm associated with Bogoslof volcano occurred in late October. The most recent eruption of Bogoslof volcano was in 2017. Most events were fairly small, with a few reaching M2.8. About ten earthquakes (M1.3–2.4) were recorded beneath Mount Edgecomb in southeastern Alaska in May and June. The Alaska Volcano Observatory installed a local monitoring network at this volcano in 2022, after observations of ground deformation consistent with swelling of a magma body beneath the volcano. Seismic events have been recorded at many other volcanoes as well.

Glacial earthquakes and the Wright Glacier Group

Alaska, due to its large extent of glacial areas, has great potential for recording glacial earthquakes. In 2023, we reported 1,028 icequakes, ranging in magnitude from 3.1 M. Most glacial activity occurs where glaciers end at water; These are tidewater glaciers and are common in the area of ​​Prince William Sound, Icy Bay and Yakutat Bay (Figure 8). The “ice quake” activity follows seasonal fluctuations and peaks at different times in different regions (Figure 9). This year, glacial seismic activity in Prince William Sound peaked in October and November, later than usual in the season.

We continued recording events in a group beneath Wright Glacier, located about 40 miles northeast of Juneau. 2023 activity began in late May and saw two different episodes of high rates in July and October. Activity in 2023 continued much later in the season than observed in 2020 or 2021, but similar to 2022. Overall, seismic activity in this group was only half the rate of that observed in 2022 or 2021. Periodic earthquakes have been observed in This area has been around since then. 1970s, when event rates typically peak in summer and early fall. These earthquakes tend to cluster near the Spell River, where it drains the glacial areas of Mount Ogden. However, activity levels are not the same every year. For example, the earthquake rates observed in 2020-2022 have not been observed since 2011-2012.

Landslides

Ten reported seismic events were classified as landslides (M1.1–3.0), most based on visual analysis of seismic waveforms, some with ground confirmations. Landslides have a seismic signal that is different from earthquakes, avalanches and explosions.

On September 13 at 18:06 AKDT, a large landslide occurred near Peters Dome, on the north side of Denali Peak in Denali National Park. This event was verified the next day by National Park Service officials during the bridge. Preliminary estimates indicate a volume of approximately 4 to 6 million cubic meters of displaced rock and soil. The propagation of this event – i.e. the distance traveled by the landslide – during the bridge was estimated to be approximately 3 km. The estimated equivalent earthquake magnitude for this event is 2.6. Smaller landslides were recorded over the next few days in the area of ​​the initial large landslide.

Another major landslide occurred on September 13, at 11:19 a.m. AKDT, near Yakutat Bay/Lucia Glacier. The initial volume of this landslide is estimated at 10 million cubic metres. The estimated equivalent earthquake magnitude is 3.0. This event has been verified using Planet.com images.

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