OceanMOOC | 5.3 | Submarine Landslides and Tsunamis

Hi, I’m Colin Devey, I’m a marine geologist.
And with me is Morelia Urlaub, who’s an expert on sea floor landslides. Now we know
that tsunamis are generated in the ocean. I guess most of us would think they’re to
do with earthquakes, but some of them to do with landslides. Why is that the case and
why are they so dangerous? So it’s similar to earthquakes. Submarine
landslides displace a huge amount of mass and thereby bring the water into action so
to say. And they are particularly dangerous because they, these landslides can occur anywhere
where the sea floor is inclined. So even on very gentle slopes. And they are, they are
pretty much unpredictable. We don’t know when they occur. We don’t know where they
occur. And this makes us pretty much completely unprepared for these disasters.
Okay, will every landslide produce a tsunami or is there some way if differentiating?
No, because the tsunamigenic potential of a landslide depends on its initial acceleration,
it depends on the volume of the landslide and on the water depth…depth in which the
landslide occurs. Okay, does that mean then that deeper ones
are more dangerous or is it the shallow ones that…..
Oh no, it’s the other way around. So the, the shallower they occur generally the more
dangerous they are. Okay, and can we predict where landslides
are going to happen? Do we have any idea to sort of say, okay this is a place we want
to be careful of? Well we, we can go to our geological invent…inventory
of the past, so we can look where landslides have occurred in geological times. But the
factors that cause these landslides are very little understood, so we are just currently
trying to understand the mechanisms that lead to sea floor failure. And this is particularly
interesting because these landslides are not, are not only much larger than any landslides
on land, but they also occur on very gentle slopes as I said before. So they occur on…on
slope angles of one or two degrees, which is about that of a FIFA certified football
pitch. And you wouldn’t even notice that, that there is a slope.
So and these, these slopes fail catastrophically. Okay. And is there something geological, are
there particular geological settings where you would say, okay that’s something you
need to be careful of? We have particular….. We don’t have particular
sites, because as I said, they, they can occur anywhere where the slope is inclined. We have
made some progress in in getting to know where very large landslides tend to occur and where
rather small landslides tend to occur. So interestingly, on active margins, so near
subduction zones we observe rather small landslides, but many of them. And on conti…on passive
continental margins, so where you don’t have many earthquakes, these landslides tend
to be extremely big. So one example is this Storegga slide of Norway. So that occurred
around 8150 years ago. And that removed or the
landslide moved a volume that, of debris that would cover the whole New York metropolitan
area in about 200 meters of debris. So that’s the scale we’re talking about.
Okay. And is there any relationship to climate change? I mean if we see sea level rise, if
we see warming of the sea water, is that going to affect how landslides work in any way?
That’s quite a difficult question because we are not very certain about the…the courses
of these landslides. So what we can say is sea level rise as such does not affect the
stability of slopes. But of course anything that goes with a sea level rise, so all the
changes that…the differences in, in sedimentation rates for example or, or similar things. So
gas hydrates for example, they make the sediment more stable. So they provide stability. But
once they dissociate, this stability is likely to get lost. So we are not entirely sure what
happens when these gas hydrates dissociate and the stability gets…gets lost.
And also earthquakes can trigger submarine landslides. And a rise in sea level is likely
to increase the seismicity. So there could be a relationship with that.
Yeah when we’re, when we’re talking about earthquakes we have something for measuring
earthquakes in front of us I believe, right? Yeah, so this is a, this is an ocean bottom
seismometer. We call it OBS. They sink to the sea floor. And stand at the sea floor
for long times and for maybe a year or so, up to a year. And they measure the seismic
activity. They are basically a seismometer as we use on land, but for use at the sea
floor. Okay, and with bit orange parts on it, this
is the floatation, so they come back up. You don’t just throw them over the side, you
do actually want to get them back again….. We do want to re-collect them because we want,
of course to get the data. And yes, so this is for floatation and there
is an acoustic release that is basically a little hook which attaches this thing to an
anchor. And this can be released with an acoustic signal, so that all of this comes up and the
anchor stays on this. Thank you Morelia. Lots of things I didn’t
know about landslides and how they could probably generate tsunamis. Very interesting, thank
you. Thank you.

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