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Earthquake could trigger Qld tsunami, warns study

QUEENSLAND scientists say a large earthquake causing the collapse of a sediment block deep under the sea could trigger a tsunami with waves of up to 11m high.

But James Cook University scientist Dr Robin Beaman has told the Cairns Post it would take a 'catastrophic' event for the large block to collapse.

In that event, it would take the waves just over an hour to travel the 70km to shore, hitting near Mission Beach first.

The discovery was made during 3D mapping of deep parts of the Great Barrier Reef.

Scientists found the site, dubbed the Noggin block, was already showing signs of collapse due to erosion caused by sediment falling from the reef above.

The block, which is thousands of years old, is about 100km southeast of Cairns, between 340m to 470m deep and measures one cubic km.

Age-date testing on deep water corals growing on the block show it is thousands of years old.

The research, which was carried out with Spain's University of Granada and the University of Sydney, has been published in the Natural Hazards journal.

Map of offshore Cairns showing earthquakes events from 1866 to 2011 (Earth Systems Science Computational Centre; Geoscience Australia). Yellow star marks the location of the Noggin block. Observe that some of these recent earthquakes have occurred relatively close to the Noggin block at less than 50 km
Map of offshore Cairns showing earthquakes events from 1866 to 2011 (Earth Systems Science Computational Centre; Geoscience Australia). Yellow star marks the location of the Noggin block. Observe that some of these recent earthquakes have occurred relatively close to the Noggin block at less than 50 km

WHAT THE REEF COLLAPSE STUDY SAYS

  • Analysis of high-resolution multibeam bathymetry and seismic profiles in the Noggin Passage region, north-eastern Australia, has identified a small area (Noggin block) in the upper-slope offshore Cairns that may potentially collapse and generate a tsunami wave.
  • The Noggin block extends from 340 to 470 m depth covering a roughly circular (2.4 km long and 3.7 km wide) area of about 5.3 km2.
  • The well-defined margins of the block correspond to different bounding seabed features.
  • These features include steep headscarps, small landslides and a group of aligned circular pockforms up to 500 m wide and 20 m deep.
  • Slope stability simulations indicate that the Noggin block is stable under normal present-day gravitational conditions on the upper slope.
  • However, block failure may result under external loads, such as those produced by earthquakes. Failure modelling shows that critical peak horizontal accelerations of 0.2-0.4 g could lead to the collapse of the Noggin block.
  • In north-eastern Australia, these acceleration values would involve earthquakes generated at short hypocentral distances and short periods.
  • The collapse of the potential sediment slide mass of about 0.86 km3 (162 m average thickness) may lead to the formation of a landslide-generated tsunami wave.
  • Semi-empirical equations indicate the collapse of this mass would yield a 7-11-m high three-dimensional tsunami wave.
  • These waves could reach an estimated run-up height at the coast of 5-7 m.
  • Our first-order approach highlights the potential consequences for nearby coastal communities, the need for better sediment characterisation in the study area, and the systematic identification of other areas prone to slope failures along the Great Barrier Reef margin

Topics:  cairns earthquake editors picks great barrier reef tsunami



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