24 October 2022
The Barry Arm landslide in Alaska: increased rates of movement
Posted by Dave Petley
The Barry Arm landslide in Alaska: increased rates of movement
On Friday the Alaska Division of Geological & Geophysical Surveys (DGGS) posted an update on the movement of the Barry Arm landslide. It provides strong guidance for people located on or close to the fjord in the vicinity of the landslide:-
- Due to the ongoing hazard, we ask people to exercise their best judgement and, where possible, limit travel in the Barry Arm area, including Harriman Fjord, Barry Arm, College Fjord, and upper Port Wells.
The Barry Arm landslide is a 500 million cubic metre landslide situated above Prince William Sound in Alaska. The Alaska DDGS has a quite detailed description of the landslide, which has been moving since at least 1957. However, one section is causing particular concern, as per this image:-
The rapidly deforming section of the Barry Arm landslide in Alaska. Image by Katreen Wikstrom Jones of the DGGS, State of Alaska.
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This section of the slope is known as the Kite, although adjacent areas to the right on the image are also actively moving. A very large rear scarp has formed at the crown of the Kite, as indicated by the arrow in the image.
On 16 September 2022, the DGGS reported that the landslide was moving at 40 to 70 mm per day. As the image above shows, the section that is active is located directly above the fjord. A major failure would likely trigger a displacement wave, although the size will depend on the volume of failure and the dynamics of the movement.
The landslide is located at 61.139, -148.169, and is clearly visible on Google Earth. There is much that is unknown, most notably the thickness of the moving mass. Unfortunately, access is currently restricted due to the risk associated with being on the water in the vicinity of the landslide.
Forecasting the likely future behaviour of large, unstable rock masses is very difficult. Loyal readers will remember the trials and tribulations faced by the monitoring team, and the local residents, of the Velsemannen rockslide in Norway, which repeatedly went through phases of accelerated movement that did not lead to failure. Indeed, in this case a large-scale collapse is not inevitable, but the risk is high.
The DGGS will post a further update on Friday “unless significant changes warrant earlier updates”.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Good post – this is an extraordinary situation to watch unfold, and makes very clear the huge gaps that exist in understanding how a slowly deforming slope transitions to a catastrophic rock avalanche.
Minor point, but I think we should stop using the term “displacement wave.” When a subaerial landslide hits the water, the process that transfers landslide energy to the water to form a tsunami is usually only to a modest extent displacement. The exception would be slow landslides entering deep water, which doesn’t produce much of a wave, but it might be reasonable to call the minor disturbance in such a situation a “displacement wave.” To get larger waves, much of the energy is transferred during the time when the landslide is moving faster than the shallow-water wave speed in the water body. This is very much a dynamic process, better described as a “splash”. And the result – both the splash itself and the wave that propagates further away – is a tsunami (whether it’s in a lake or the ocean.)
[Many thanks,. Your point about a displacement wave is interesting as on other forums people have objected to the use of the term tsunami in this context. For example, the National Ocean Service definition (https://oceanservice.noaa.gov/facts/tsunami.html) is “…a series of waves caused by earthquakes or undersea volcanic eruptions”, so by its definition the potential event at Barry Arm is not a tsunami. D.]
There are geotechnical satellite services now that can survey movement daily.
There are satellite services that can measure geotechnical data daily.
Does the National Ocean Service (and many others who limit the causes of a tsunami to earthquakes or undersea volcanic eruptions) know that land can slide (if into water thus causing longwave waves = tsunami) and tectonic reverse faults or normal faults of the sea floor also can cause a tsunami?
The semantics around tsunami aren’t great. There are definitions that sound pretty good like “An impulse-generated wave.” However, typically the extreme runup in Lituya Bay is considered tsunami runup, though it was not a wave in the sense of a propagating dynamic disturbance – the runup was due to a mass of water that was ejected from the bay and collided with a mountain. That’s why I’d call it a splash. Also, we don’t call calving waves at glaciers tsunamis, though they are impulse generated, and we increasingly use the term “meteotsunami” to describe waves that are generated by atmosphere-ocean coupling and commonly have a large harmonic component – ie they aren’t impulse generated. So I’d absolutely agree that “tsunami” is a very messy term.
Going back to its roots can help – the translation in Japanese is “harbor wave,” which essentially means a wave that is long enough period to propagate into bodies of water normally thought of as protected, and thus used as harbors. So you could simply say a tsunami is a wave with a period greater than something like 20 seconds, the upper limit of wind waves, and this works pretty well (but would include seiches…)
It’s still better than “displacement wave” which, like “tidal wave”, is a phrase composed of words that describe something other than the phenomenon at hand. There are many ways of defining “tsunami” but there is general agreement that long-period waves that pose a coastal hazard are tsunamis, and that’s what we’re talking about here.
Interesting that this NOAA page omits landslides and meteor impacts – usually both fall on the list of generic for-the-public definitions of tsunamis. Of note is that the National Tsunami Warning Center, part of NOAA, is actively involved in monitoring the Barry Arm situation and considers the potential event a tsunami…
One more note on “tsunami” terminology – I reached out to Dave Snider, who heads the public communications for the National Tsunami Warning Center, to ask about the definition you cited. He said this wasn’t the official NOAA definition – they do consider landslides and meteor strikes as potential tsunami sources. You can see some more nuanced discussion on the Tsunami.gov FAQ (https://tsunami.gov/?page=tsunamiFAQ) – about how tsunamis are generated it says:
“A tsunami is caused by a large and sudden displacement of the ocean. Large earthquakes below or near the ocean floor are the most common cause, but landslides, volcanic activity, certain types of weather, and near earth objects (e.g., asteroids, comets) can also cause tsunamis.”
Interesting discussion on wave type and description however, all that aside, one cannot escape the fact that if this large mass fails catastrophically the resulting wave could be potentially gargantuan. This mega tsunami, much like the one in Lityua Bay in 1953, would surely result in untold damage down to the bedrock throughout the immediate fjords. I certainly wouldn’t want to be anywhere near it bar a very high elevation on the opposite peaks – what an incredible sight that would be to witness! I’m surprised no keen geologist is camping out there somewhere to witness this possible once in a lifetime event – if I lived nearby I’d certainly give it a chance.