Numerous M5-6 earthquakes offshore the Aleutian Islands
We look at an interesting, if a bit remote, earthquake sequence along the subduction zone
Citation: Hubbard, J. and Bradley, K., 2024. Numerous M5-6 earthquakes offshore the Aleutian Islands. Earthquake Insights, https://doi.org/10.62481/bc91e648
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A swarm of earthquakes rattled a remote area south of the Aleutian Islands on December 8-9, 2024 (UTC time). The Aleutian Islands are the archipelago that extends for about 1,900 km west from Alaska, acting as the border between the Bering Sea to the north and the Pacific Ocean to the south.
The earthquakes occurred south of the Andreanof Islands, and about 80 km south-southwest of Adak Island, population 171 (as of the 2020 census). The USGS received four Did-You-Feel-It responses from Adak for the first earthquake, an extremely high per capita response rate!
Adak Island is geologically famous, lending its name to adakites — volcanic rocks with a particular chemical composition. These types of volcanic rocks are quite rare, and while their origin is still controversial, they sometimes seem to form when subducted basaltic crust actually gets hot enough to melt. This is inconvenient for the many science communicators who have worked to dispel the persistent myth that volcanoes arise from melting of the subducting slab. Usually (like almost all the time), it is actually the mantle above the slab that melts, due to release of water from the slab below. Adakites are therefore myth realized.
Kanaga Island, to the west of Adak, is about the same distance away from the earthquakes, but is unpopulated. That is, unless you count sea otters, sea lions, seals, and the large, active volcano.
A regular earthquake sequence, or a seismic swarm?
Earthquakes are not surprising here: this island chain is a volcanic arc. In other words, the islands (and their volcanoes) sit on top of the Aleutian subduction zone, where the Pacific Plate moves northward beneath the North America Plate, carrying into the mantle the fluids responsible for the melting of the mantle and the formation of arc volcanoes. What is surprising is the number and magnitudes of the earthquakes in this sequence.
Most earthquake sequences follow a similar pattern: a large mainshock, which may or may not be preceded by smaller foreshocks, and which is then followed by a regular decay of aftershocks caused by the local stress changes.
Sometimes, however, there is no clear ‘mainshock.’ Instead, multiple earthquakes of similar magnitude can happen over time, without a defined mainshock-aftershock sequence. These kinds of events are called seismic swarms, and they tend to happen when some continuous process, like movement of fluids at depth, is causing the earthquakes.
The December 8 sequence started with a Mw6.3 at 18 km depth (Dec. 8 19:57 UTC), followed by some mostly shallower aftershocks, including a Mw5.0 (20:01 UTC), Mw5.4 (20:39 UTC), and Mw5.8 (21:02 UTC). Then, about four hours after the sequence began, another Mw6.3 struck (Dec. 9 00:15 UTC). More aftershocks rattled the area, again at shallower depth, — including three above Mw5 — and then, after 23 minutes, a Mw6.1. Since then, we have seen six more events above Mw5, including a Mw5.7/Mw5.6 pair, about five minutes apart, and trailing Mw5.7 and Mw5.3 earthquakes several hours after the sequence had apparently stopped.
We usually observe that the largest aftershock of an earthquake is about one magnitude value lower than the mainshock — so, a Mw6.3 might have a Mw5.3 as its largest aftershock. In about 5% of cases, an earthquake might trigger an even larger event; then, following that larger event, we’ll still see the usual decay. This observation is tied to how nearby faults respond to the stresses imposed by the earthquakes.
Here, with three earthquakes between Mw6.1 and 6.3, and thirteen between Mw5.0 and 5.9, it is clear that the earthquakes are not following that typical pattern. However, the sequence did start with one large-ish earthquake, which could have triggered some of the later behavior. This sequence therefore seems to fall somewhere between a typical mainshock-aftershock sequence and a seismic swarm.
When we see earthquakes occurring in a swarm, we can usually intepret that there is some other, underlying process at work triggering the earthquakes, rather than the stress changes from the earthquakes themselves. The usual suspect is the movement of some kind of fluid at depth, either magma or pressurized water.
While there are volcanoes not so far away (red triangles on the map above), these earthquakes are much too far south to be associated with the magmas of the Aleutian volcanic arc. Instead, their locations and focal mechanisms match nicely with the megathrust: the great subduction fault that forms the boundary between the two plates.
So, one possible interpretation is that this sequence is related to fluids moving along or across the subduction megathrust fault. Subduction faults are expected to be full of fluids — derived from water trapped within sediments being carried down with the sinking slab, or contained within hydrated minerals. When those fluids are released, they make their way into and out of pore spaces in the crust, pressurized by the mass of rock above, and moving slowly from areas of higher pressure to areas of lower pressure. This can strongly affect faults: when fluids under high pressure fill fractures in the crust, they essentially push the two sides of a fracture apart, making it easier for the fracture to slip sideways, even if the shear stress has not increased. Earthquakes can in turn cause crustal fluids to move suddenly, accessing new parts of the fault, or suddenly becoming more pressurized, triggering new events.
What can we expect next? Well, the “swarmy” behavior seemed to abruptly end with a Mw5.1 at 05:56 UTC. A few more earthquakes have been recorded in the more than twelve hours since then, including two above Mw5. Whether these more intermittent events are associated with swarm behavior, or are simply regular aftershocks of the earlier sequence, is not clear to us.
We might expect the earthquake sequence to return to “normal” behavior — i.e., something like what is shown in the current USGS aftershock forecast. In other words, there should be a good chance of some more M3-4 earthquakes, a reasonable chance of a M5, and a slim chance of a M6+. Note that this forecast was put out prior to the latest Mw5.7 and Mw5.3.
Alternatively, it is also possible that the swarmy behavior will resume, or that it will simply die out and no further significant earthquakes will occur. These kinds of earthquake sequences are particularly hard to predict, because we don’t really know what drives them.
Great earthquakes
While these earthquakes are somewhat unusual and interesting, they don’t really compare to the most exciting (read: terrifying) earthquakes that this particular subduction zone can produce. This sequence occurred within the 1200-km-long rupture zone of the great 1957 Mw8.6 Aleutian Islands earthquake (also sometimes known as the Andreanof Islands earthquake, since the epicenter was near this part of the Aleutians).
A paper about the earthquake published last July argued that the megathrust slipped between 12 and 26 meters over a 600-km-long section in the east (Yamazaki et al., 2024). The earthquake raised a tsunami with 6-12 meters high waves that impacted both the Aleutian Islands and Hawaii. In the figure below, you can see how the maximum wave amplitude near shore can be extremely complicated. This is because the details of the underwater terrain become important as the tsunami waves approach the shore.
While the earthquake and tsunami did cause some damage, only two lives were lost, both in Hawaii. The story, as related in the March 10, 1957 issue of the Honolulu Star-Advertiser, combines tragedy with heroism, and is worth a brief mention.
As the trans-oceanic tsunami approached, Star-Bulletin reporter and Korean War correspondent Sarah Park, Star-Bulletin photographer Jack Matsumoto, and pilot Paul Beam, alerted by the tsunami warning system, flew out to sea in a small plane. The plane stalled and fell into the ocean, which had withdrawn significantly ahead of the incoming tsunami. Rescuers (including a 16 year old young man) rushed out from land to the plane, successfully extracting Jack (conscious) and Paul (injured and unconscious). A Marine helicopter also rushed to the rescue, and the co-pilot J.V. Kaup actually dove headfirst from the helicopter into the sea, hero-mode engaged. While he did manage to grab hold of someone in the water, they were ripped from his grasp by the arriving tsunami wave, and he was washed to shore. Sarah Park died at the scene, and Paul Beam died in the hospital the next day.
This earthquake is among the first for which the US-operated tsunami warning system, which began operations in 1949, saved lives: officials in Hawaii were forewarned, and were able to evacuate coastal areas well in advance. Today, the successor institution, the Pacific Tsunami Warning System, issues tsunami warnings not just on the basis of projections from earthquakes, but also incorporating direct measurements of sea level from both coastal stations and deep-ocean buoys and pressure recorders.
While some warnings prove to be false alarms (for instance, the one following the December 5 Mw7.0 earthquake offshore California), it is always better to be safe than sorry!
References:
Pacific Coastal Marine Science Center, 2024. Revisiting the 1957 Aleutian Earthquake: New insights into tsunami hazards for Hawai’i. https://www.usgs.gov/centers/pcmsc/news/revisiting-1957-aleutian-earthquake-new-insights-tsunami-hazards-hawaii
Yamazaki, Y., Lay, T., Cheung, K.F., Witter, R.C., La Selle, S.M. and Jaffe, B.E., 2024. A Great Tsunami Earthquake Component of the 1957 Aleutian Islands Earthquake. Earth and Planetary Science Letters, 637, p.118691. https://doi.org/10.1016/j.epsl.2024.118691
Thank you for the info on these many Adak quakes. Living in California and having family in Japan, I watch the earthquakes apps but they only give minimal info. Your info really helped.