Researchers continue to investigate a massive landslide and tsunami in August that hit a fjord popular among sightseeing cruises in Southeast Alaska.
A veritable mountainside of rock crashed into the water at the end of Tracy Arm, near the terminus of South Sawyer Glacier, generating a tsunami wave that scoured the shoreline of vegetation in the surrounding area and even disrupted tides in Juneau, about 75 miles away.
It was early in the morning, and Alaska state seismologist Michael West says, luckily, no cruise ships or tour boats were in the fjord at the time.
This interview has been lightly edited for length and clarity.
Michael West: I feel pretty comfortable saying that anything that was actually at the terminus of this glacier, right at the base of this landslide, would have been absolutely obliterated. I cannot see any way around that. Fortunately, we do not have reports of folks who were really close to the slide, say, you know, within a mile or something, but I can only imagine that would have been an extremely violent affair.
Casey Grove: Yeah. And how big was it? How would you describe the size of the landslide itself?
MW: I don't have an intuitive conceptual understanding of those, so I tend to go to, you know, pretty simple analogies. The one that I think of is a giant cube of rock that is something on the order of three to four football fields on all three sides. Well, that's huge. What is that? Something like a quarter mile in each dimension?
CG: There's been some time, I guess, now, to go directly look at the aftermath. And there was this huge wave that was created. I mean, do we have any sense of really how big that that wave was, and what did it actually do there?
MW: So this slide occurred on one side of the fjord, and when it struck the water, that water, think of it as sloshing up onto the other side of the fjord. You know, it sloshed well over a thousand feet, something probably on the order of 1400 feet up the mountainside, which, again, is a tremendous, just a tremendous height.
When you've got water moving at speeds like this, it strips out absolutely everything in its way. It doesn't matter how big the trees were, they're gone when water like that hits. So the mountainside where there was forest is just stripped down to bare rock.
CG: Wow. So, I mean, talking about the risk to people and infrastructure, does climate play a role in these landslides and in the subsequent tsunamis?
MW: So the climate piece of this story is a very active research area right now. We know that we have retreating glaciers in, well, most glaciers in Alaska, not all, but most are retreating. And we have a surprising number of data points where these landslides have occurred, not kind of near the end of the glacier, but right there, right above the terminus of the glacier. This one (in Tracy Arm), last fall, Surprise Glacier in Prince William Sound. We have a host of other examples.
And so I think that that it is not a coincidence that there's a significant number of these slides that are happening right at the end of glaciers. Now, the exact mechanisms of that, the exact explanations, that's a, let's just say, a very energetic area of research right now.
CG: Speaking of which, there are areas like the Barry Arm, close to Whittier, that that scientists are keeping a pretty close eye on. How could you possibly spot something like this happening before it falls? And I guess this glacier, the Sawyer Glacier, it's visited often by cruise excursions, so that seems like a pertinent question, you know, whether or not you could be able to predict something like that happening in a place where people are active.
MW: Yeah, this is the million dollar question. How do we live, coexist, with this kind of hazard? There's a handful of different tools that can be used to identify problematic slopes, I'm going to call them in advance.
A lot of the best tools come from satellite remote sensing. So there are places where we see mountainsides that have begun to inch down, you know, each year or in little bursts, and that we can get from satellites. And what that allows us to do is to identify kind of hot spots or hazard areas. You know, here are the 50 locations across coastal Alaska where we see something that suggests signs of movement.
That works as long as the landslide actually shows motion before it lets go.
To my knowledge, so far, we have not found, for Tracy Arm, evidence that this had been creeping or sliding over the past several years. This happened, geologically speaking, quite suddenly.
One of the things that has us really intrigued by the Tracy Arm slide is that it is now in just a handful of large landslides worldwide that showed seismic signals. Think of this as pops and creaks, if you will, in the hours and probably days before failure. These seismic precursors are a big deal. The seismic records from that morning are burned in my memory, and next time, if I stumble across a record like that from somewhere in the state, you can bet your butt it's got my attention.
