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Associate Professor Dr Hannah Power onboard the CSIRO Research Vessel Investigator. contributed.

Scientists study seafloor for tsunami threat

Rodney Stevens

 

As the Clarence Valley was bathed in warming winter sun last week, Associate Professor Hannah Power was relishing the conditions more than 20 kilometres offshore, sampling the seafloor for signs of tsunami potential.

Dr Power was among a team of 32 scientists and university students who were in Clarence Coast waters over the past week in CSIRO Research Vessel Investigator, mapping the ocean floor to investigate the East Coast tsunami risk.

Leading the 37-day mission along the coastline from Hobart to Noosa, was Chief Scientist Associate Professor Tom Hubble from the University of Sydney, with Dr Power as Alternate Chief Scientist and Dr Mike Kinsela as a Principal Investigator from the University of Newcastle.

The University of Newcastle’s Dr Power said one of the mission’s aims was to understand how our continent evolves, as the edge of the landmass is underwater and sediment gets deposited on our coastlines, with some ending up in the deep oceans.

The RV Investigator mapping the sea floor off Australia’s east coast. contributed

“We’re here to understand that process, and some of the things we are looking at are underwater canyons and large underwater landslides,” she said.

“One of the reasons we are interested in these underwater landslides is they have the potential to form a tsunami.”

Commissioned in 2014, the CSIRO’s RV Investigator is a 94 metre, purpose built floating scientific research vessel supporting biological, oceanographic, geological and atmospheric research, which is capable of 10,000km or 60 days without resupply with 60 onboard. 

The ship has a variety of research equipment including two multibeam underwater mapping systems, acoustic mapping systems, a sub bottom profiler, echosounder, cranes, a gondola, dredges, drop keels and drop cameras capable to 6000 metres.

A multi-faceted approach is used, Dr Power said, to gather data, samples and map the ocean floor.

Scientists measuring a core sample taken from the ocean floor during a study investigating tsunamis on RV Investigator. contributed.

“We are using multi-beam echo sounding, like a larger version of a fish finder, we’re using sub bottom profiling, which allows us to see a few hundred metres into the sediment, allowing us to see through the sediment layers and into the sea floor below,’ she said.

“We’re doing seafloor sampling, so we are taking samples of sediment from the sea floor using cores up to 15 metres long, and we’re also doing some dredging, collecting rocks in areas where there isn’t sediment cover.

“We’re also using the deep tow cameras to collect some images from the sea floor.”

While compiling all the research will take many weeks, Dr Power said the crew had identified some features off the Clarence Coast.

“There are some smaller landslide features off the coast of Yamba, where we are now, and we’ve collected a lot of really nice data while we’ve been here at sea,” she said.

“We’ve been remapping an area (off Yamba) that was mapped 20 years ago to look for differences.

“It was mapped over 10 years ago and then we remapped it to see if it has changed, to see if any of this material has moved and we are also taking some samples while we’re out here.

“When you’re working in very deep water it can take a couple of hours for a sampling device to get down to the bottom, collect a sample, then another couple of hours to come back up again, so an operation like that can take up to five or six hours just to get one sample.

“Because we’ve been working on the slope up to 50km off Yamba, the depth varies quite a bit, so we’ve worked into the hundred metre depths, and right out to 4600 metres.

“The continental shelf goes out to about 100 to 200 metres deep, then you go down the continental slope which takes you down to 3000 to 4000 metres deep, then you go out onto the abyssal plain, which is the deep ocean where you get beyond 4000 metres deep.” 

The underwater landslides off the Clarence Coast, Dr Power said, are a common feature.

“We see these similar features all the way from Ulladulla in the south to Fraser Island in the north,” she said.

“It’s hard to get a fixed date as to when some of these landslides occurred because we need to get right down into the sediment.

“We’ve got a range of dates, but we think these events happen around every 10,000 to 50,000 years, and they have the potential to be catastrophic, that’s why we’re investigating them.

“There is a very small but significant risk of a local tsunami, but it’s not something that people should be worried about or lose sleep over.”

Movement of the water following an earthquake was the most likely sign of a tsunami, Dr Power said.

“The first sign would be a large earthquake and then if you saw any unusual movement of the water, so the water receding a long way, or starting to come in more than you think was normal,” she said.

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