Scars left by Australia’s undersea landslides reveal future tsunami potential

The Byron Scar, left behind by an undersea landslide. Colours indicate depths.
Samantha Clarke, Author provided

Samantha Clarke, University of Sydney; Hannah Power, University of Newcastle; Kaya Wilson, University of Newcastle, and Tom Hubble, University of Sydney

It is often said that we know more about the surface of other planets than we do about our own deep ocean. To overcome this problem, we embarked on a voyage on CSIRO’s research vessel, the Southern Surveyor, to help map Australia’s continental slope – the region of seafloor connecting the shallow continental shelf to the deep oceanic abyssal plain.

The majority of our seafloor maps depict most of the ocean as blank and featureless (and the majority still do!). These maps are derived from wide-scale satellite data, which produce images showing only very large features such as sub-oceanic mountain ranges (like those seen on Google Earth). Compare that with the resolution of land-based imagery, which allows you to zoom in on individual trees in your own neighbourhood if you want to.

But using a state-of-the art sonar system attached to the Southern Surveyor, we have now studied sections of the seafloor in more detail. In the process, we found evidence of huge underwater landslides close to shore over the past 25,000 years.

Generally triggered by earthquakes, landslides like these can cause tsumanis.

Into the void

For 90% of the ocean, we still struggle to identify any feature the size of, say, Canberra. For this reason, we know more about the surface of Venus than we do about our own ocean’s depths.

As we sailed the Southern Surveyor in 2013, a multibeam sonar system attached to the vessel revealed images of the ocean floor in unprecedented detail. Only 40-60km offshore from major cities including Sydney, Wollongong, Byron Bay and Brisbane, we found huge scars where sediment had collapsed, forming submarine landslides up to several tens of kilometres across.

A portion of the continental slope looking onshore towards Brisbane, showing the ‘eaten away’ appearance of the slope in the northern two-thirds of the image, the result of previous submarine landslides.
Samantha Clarke

What are submarine landslides?

Submarine landslides, as the name suggests, are underwater landslides where seafloor sediments or rocks move down a slope towards the deep seafloor. They are caused by a variety of different triggers, including earthquakes and volcanic activity.

The typical evolution of a submarine landslide after failure.
Geological Digressions

As we processed the incoming data to our vessel, images of the seafloor started to become clear. What we discovered was that an extensive region of the seafloor offshore New South Wales and Southern Queensland had experienced intense submarine landsliding over the past 15 million years.

From these new, high-resolution images, we were able to identify over 250 individual historic submarine landslide scars, a number of which had the potential to generate a tsunami. The Byron Slide in the image below is a good example of one of the “smaller” submarine landslides we found – at 5.6km long, 3.5km wide, 220m thick and 1.5 cubic km in volume. This is equivalent to almost 1,000 Melbourne Cricket Grounds.

This image shows the Byron Slide scar, located offshore Byron Bay.
Samantha Clarke

The historic slides we found range in size from less than 0.5 cubic km to more than 20 cubic km – the same as roughly 300 to 12,000 Melbourne Cricket Grounds. The slides travelled down slopes that were less than 6° on average (a 10% gradient), which is low in comparison to slides on land, which usually fail on slopes steeper than 11°.

We found several sites with cracks in the seafloor slope, suggesting that these regions may be unstable and ready to slide in the future. However, it is likely that these submarine landslides occur sporadically over geological timescales, which are much longer than a human lifetime. At a given site, landslides might happen once every 10,000 years, or even less frequently than this.

A collection of submarine landslide scars off Moreton Island.
Samantha Clarke

Since returning home, our investigations have focused on how, when, and why these submarine landslides occur. We found that east Australia’s submarine landslides are unexpectedly recent, at less than 25,000 years old, and relatively frequent in geological terms.

We also found that for a submarine landslide to generate along east Australia today, it is highly likely that an external trigger is needed, such as an earthquake of magnitude 7 or greater. The generation of submarine landslides is associated with earthquakes from other places in the world.

Submarine landslides can lead to tsunamis ranging from small to catastrophic. For example, the 2011 Tohoku tsunami resulted in more than 16,000 individuals dead or missing, and is suggested to be caused by the combination of an earthquake and a submarine landslide that was triggered by an earthquake. Luckily, Australia experiences few large earthquakes, compared with places such as New Zealand and Peru.

Why should we care about submarine landslides?

We are concerned about the hazard we would face if a submarine landslide were to occur in the future, so we model what would happen in likely locations. Modelling is our best prediction method and requires combining seafloor maps and sediment data in computer models to work out how likely and dangerous a landslide threat is.

Our current models of tsunamis generated by submarine landslides suggest that some sites could represent a future tsunami risk for Australia’s east coast. We are currently investigating exactly what this threat might be, but we suspect that such tsunamis pose little to no immediate threat to the coastal communities of eastern Australia.

This video shows an animation of a tsunami caused by submarine landslide.

That said, submarine landslides are an ongoing, widespread process on the east Australian continental slope, so the risk cannot be ignored (by scientists, at least).

Of course it is hard to predict exactly when, where and how these submarine landslides will happen in future. Understanding past and potential slides, as well as improving the hazard and risk evaluation posed by any resulting tsunamis, is an important and ongoing task.

In Australia, more than 85% of us live within 50km of the coast. Knowing what is happening far beneath the waves is a logical next step in the journey of scientific discovery.

Samantha Clarke, Associate Lecturer in Education Innovation, University of Sydney; Hannah Power, Lecturer in Coastal Science, University of Newcastle; Kaya Wilson, , University of Newcastle, and Tom Hubble, Associate professor, University of Sydney

This article was originally published on The Conversation. Read the original article.

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A marine heatwave has wiped out a swathe of WA’s undersea kelp forest

Scott Bennett, Curtin University; Julia Santana-Garcon, and Thomas Wernberg, University of Western Australia

Kelp forests along some 100km of Western Australia’s coast have been wiped out, and many more areas damaged, by a marine heatwave that struck the area in 2011.

The heatwave, which featured ocean temperatures more than 2℃ above normal and persisted for more than 10 weeks, ushered in an abrupt change in marine plant life along a section of Australia’s Great Southern Reef, with kelp disappearing to be replaced by tropical species.

As we and our international colleagues report in the journal Science, five years on from the heatwave, these kelp forests show no signs of recovery.

Instead, fish, seaweed and invertebrate communities from these formerly temperate kelp forests are being replaced by subtropical and tropical reef communities. Tropical fish species are now intensely grazing the reef, preventing the kelp forests from recovering.

Kelp forest before (left) and after (right) the marine heatwave.
Author provided

Assessing the damage

We and our team surveyed reefs along 2,000km of coastline from Cape Leeuwin, south of Perth, to Ningaloo Reef between 2001 and 2015.

Up until 2011, temperate reefs were clearly defined by the distribution of kelp forests which formed dense, highly productive forests as far north as Kalbarri in WA’s Mid West.

Since 2011, the boundary between these temperate reefs of southern WA and the more tropical reefs (including Ningaloo) to the north has become less clear-cut. Instead, the sharp divide has been replaced by an intermediate region of turf-dominated reefs.

Infographic illustrating the impacts of the heatwave, kelp loss and tropicalisation of temperate reefs.
[Produced by Awaroo]((www.awaroo.com))

This has implications for the Great Southern Reef (GSR), which extends more than 8,000km around the southern half of Australia from the southern half of WA all the way to southern Queensland – a coastline that is home to around 70% of Australians.

Kelp forests are the GSR’s “biological engine”, feeding a globally unique collection of temperate marine species, not to mention supporting some of the most valuable fisheries in Australia and underpinning reef tourism worth more than A$10 billion a year.

But our research shows that on the GSR’s western side, kelp forests are being pushed towards Australia’s southern edge, where continued warming puts them at risk of losses across thousands of kilometres of coastline because there is no more southerly habitat to which they can retreat.

While the 2011 marine heatwave affected some 1,000km of Western Australia’s temperate coastline, it was a stretch of roughly 100km extending south of Kalbarri on the state’s Mid West coast that was most severely affected.

In this area alone an estimated 385 square km of kelp forest have been completely wiped out.

Further south, from Geraldton to Cape Leeuwin, the extent of kelp loss was less severe, despite an estimated total area of 960 square km having been lost in the region.

Northern regions towards Kalbarri were more severely affected because these kelp forests were closer to their limit, and also because this area is closer to the tropical regions like Ningaloo Reef, meaning that tropical species could more easily move in.

A school of tropical rabbitfish moves through the affected area.
Thomas Wernberg, Author provided

The problem was exacerbated by the southward-flowing Leeuwin Current, which helps tropical species move south while making it harder for temperate species to move north and recolonise the affected areas of the GSR.

The combination of these physical and ecological processes set within a background warming rate roughly twice the global average, compounds the challenges faced by kelp forests in the region.

The plight of WA’s kelp forests provides a strong warning of what the future might hold for Australia’s temperate marine environment, and the many services it provides to Australians.

Scott Bennett, Marie Curie Fellow at the Spanish National Research Council (CSIC), Curtin University; Julia Santana-Garcon, Postdoctoral research associate in Marine Ecology, and Thomas Wernberg, ARC Future Fellow in Marine Ecology, University of Western Australia

This article was originally published on The Conversation. Read the original article.

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