An extensive undersea mapping program of the Australian coast has revealed some surprises about the deep Great Barrier Reef, including a dense network of submarine canyons, the remains of numerous undersea landslide scarps where large parts of the continental slope have given way, and some areas which may be prone to future underwater landslides. The research, published in the latest edition of the journal Natural Hazards, was undertaken by Angel Puga-Bernabeu, from the University of Granada in Spain, Dr Jody Webster from the University of Sydney and Dr Robin Beaman from James Cook University.
One place in particular caught the attention of the team of marine scientists, as they investigated areas lying further offshore than the more familiar coral reefs. About 100 km south-east of Cairns, a large sediment block found on the upper continental slope, appears to be in the early stages of collapse marked by a chain of deep pockmarks lining the block. Nearby is evidence of smaller landslides undercutting the steep lower face of the block and the resulting debris downslope.
“Over the past several years, we have mapped hundreds of kilometres along the Great Barrier Reef margin, marvelling at the dense network of canyons and landslide scarps, but this block really stood out as a site for a potential collapse,” said Dr Jody Webster, from the School of Geosciences at the University of Sydney.
“We used multibeam 3D mapping and sub-bottom profiling data to study the detailed geomorphology – or shape – of the roughly one cubic kilometre sized block and its surrounds,” said Dr Webster.
“The block appears to be stable under normal present-day conditions and we concluded from our modelling that it would take a large earthquake in the close vicinity, which is quite unlikely, to cause the block’s collapse.”
Dr Robin Beaman, from James Cook University, said, “Hypothetically, if there was a catastrophic collapse of the block it would cause a tsunami that would locally affect the adjacent Queensland coast, taking just over an hour to travel the 70 kilometres directly across the width of the Great Barrier Reef.”
“We stress that this is only a modelled scenario and not a cause for alarm for the communities along the North Queensland coast. All the evidence we have, including age dates from deep-water corals growing on the remains of other debris blocks found nearby, show that these submarine landslides are many thousands of years old,” said Dr Beaman.
The team have named the landslide area as the Noggin block due to its location in the Noggin Passage region off north-eastern Australia, and found that it extends from 340 metres to 470 metres in depth, covering a roughly circular area of about 5.3 kilometres squared.
“The block is stable currently, but the block could fail under external loads, like that from an earthquake. Underwater slope failure is a common process and plays an important role in shaping seafloor environments and the exchange of sediment between the shelf and the deep-sea basin,” explained Dr Webster.
“The Australian coast has experienced modern tsunamis, like the 2007 earthquake-generated Solomon tsunami, which reached the eastern coast of Australia, but just as a small wave of less than one metre in height,” said Dr Webster.
“Studying underwater landslides has important implications for assessing the potential tsunami hazard facing populated coastal areas, so we’re engaged in an ongoing effort to understand the level of onshore hazard caused by landslide-generated tsunamis to the Australian coast.”
The team now has plans to investigate the rest of the Great Barrier Reef margin to identify other areas that might be prone to collapse. Other sites, both north and south of the Noggin block, show numerous landslide features with adjacent debris blocks lying in the deep Coral Sea.
The results of the 3D mapping project, which started in 2007, have also been used to help guide the 2010 Integrated Ocean Drilling Program expedition to the Great Barrier Reef, and to provide important geological baseline data for the Coral Sea Commonwealth Marine Reserve.