Gigantic submarine landslide offshore western Tasmania: risk mitigation for shelf derived tsunami in Australia.

The CSIRO research vessel RV Investigator conducted the 38-day IN2023_V02 voyage to SW Tasmania in March-April 2023. The primary objectives of this voyage were to survey and sample a gigantic submarine landslide offshore the SW Tasmanian shelf. Secondary objectives were multiple and included survey of the continental shelf to map the continuation of onland rock formations, assess upwelling and nutrient input in the ocean waters on the Tasmanian shelf, identify biological habitat zones, establish a baseline survey of marine mammal and sea bird occurrence, assess the impact of trawling on seabed habitats, and identify and survey the 1973 shipwreck of the MV Blythe Star. The voyage operated in extremely favourable waters, allowing for maximum recovery of scientific data. Minimum loss of science days resulted from a combination of efficient interchangeability between day-time and night-time operations, minimum technical difficulties, minimum transit time being less than 120 nautical miles from port (Hobart), and excellent weather.
The main operations were coring and seismic reflection surveys, which were operated by day only. Dredging, deep-towed camera surveys, CTDs, SUNA, box core, epibenthic net and surface Neuston net were carried out during night-time, or after completion of the main day-time operations. Deep camera surveys and CTDs included SUNA where for <2000 mbsl surveys. Multibeam bathymetry (EMs 122 and 710), sub-bottom profiler, gravimeter, and atmospheric surveys were continuously acquired.

The scientific objectives of this multidisciplinary voyage were:
1. Slope failure and submarine landslide
Offshore SW Tasmania, the continental shelf consists of a smooth, 20–40 km wide area indented by numerous but relatively small canyons. However, >50 km of length of continental shelf shows an abrupt headscarp failure associated with a gigantic multi-100 km3 submarine landslide deposit. The submarine landslide and associated turbidites were deposited 30–120 km downslope and at 2000–4500 m under water. The headscarp is >400 m high at places, attesting of a single (or a few), catastrophic event(s) with minimal post-event sediment remobilisation. The morphology of the submarine landslide deposit suggests to be relatively recent (Pleistocene-Pliocene). A smaller submarine landslide occurs 40 km laterally of this slide and was also be targeted, and unfailed and ‘lesser failed’ slope were also surveyed. The main submarine landslide is arguably the biggest recent slide offshore Australia. It is more than twice the size of the 1929 Grand Banks slide offshore Newfoundland that created a 3–8 m tsunami 400 km away and ruptured seafloor infrastructures. The SW Tasmania submarine landslide has never been examined in detail before, despite being a remarkably sharp morphology on bathymetric maps.

This voyage sampled and surveyed the landslide in its entirety, including its scarp, the main deposit at the base of the continental slope, and the turbidites sheets on the abyssal plain. Sediment cores enabled sampling of the turbidites and were collected to (1) date the collapse event(s) by dating the overlying pelagic mud, (2) assess transport and sedimentation processes, (3) calculate the volume of the landslide, and (4) measure the physical properties of the failed and unfailed mud. Seismic reflection and sub-bottom profiler provide exceptional insights on the internal morphology of the continental slope, its failure, and the landslide deposits. The seismic surveys can be correlated to a nearby Ocean Drilling Program (ODP) Hole 1168 and multiple seismic lines, which have very well constrained ages and stratigraphy. This knowledge will allow for characterisation of the causes, age, volume and extent of the landslides. We dredged cliffs to recover rocks and sediments from the various landslide scarps, giving more insights on what material failed and/or facilitated the slide. In addition to on-deck operations, bathymetry and sub-bottom profiler surveys provided important context for the sedimentation processes associated with the landslide, as well as for the reconstruction of sub-seafloor morphology in this area.
2. Tsunami mitigation
The presence of such a large submarine landslide offshore Tasmania is a superb opportunity to assess risks relative to shelf collapse and has a direct applicability to Australia and globally. Tsunami can destroy coastal and underwater infrastructures hundreds of km from source. Tsunamigenic shelf failures occur at proximity to the coast and may not be associated with large earthquakes. This is a double issue, because (i) submarine landslides may not be identified as a threat by the global seismometer network, and (ii) the source of the tsunami is very close to shore, reducing the ability to plan evacuations in a timely manner. This project provides novel data on catastrophic tsunamigenic submarine mass-wasting events to be used by modelers for tsunami mitigation. Based on surface morphology, the submarine landslide seems to have formed over very large-scale failures and possibly a single failure event, and thus capable to produce catastrophic tsunami.
3. Geology of Tasmania
The targeted area is key to reconstructing an important part of the geological puzzle of Tasmania. The area is split by major faults that juxtapose the Mesoproterozoic Tyennan block (east) with the Cambrian Mount Read Volcanics intruded by granites (west). The Mount Read Volcanics have provided enormous resources to the State from its various mineralisations in noble and base metals. Bathymetric surveys and dredging on the continental shelf and slope will allow to continue the geology offshore and improve interpretations on large-scale tectonic boundaries in SW Tasmania. This novel data will shed light on important structural connections that will have profound implications for the structural fabric of Tasmania and help in the palinspastic reconstruction of the Tabberabberan orogeny during the Devonian, and the Gondwana breakup. To improve and extend the geological map of Tasmania has profound implications to strengthen exploration for resources on the continent.
4. Biodiversity, seafloor sustainability and ocean productivity
This voyage has improved our understanding of ecological processes ranging from habitat/assemblage vulnerability to climate change through to providing further information for future monitoring. The data collected will make major improvements in our understanding of the links between geomorphic features and seafloor biodiversity - including the spatial scales at which these processes operate on this unique coastline. The new surveys of the shelf and upper slope area are of national significance.
Little is known about the habitats on the shelf in this region. Such information is of direct importance to the sustainability of commercial fisheries operating on the west coast of Tasmania. Bottom trawling has implications for bryozoan turf/thicket habitat on the shelf. Currently, no protection of bryozoan turf habitats occurs through formal fishery spatial management regulations, although minor protection occurs through the Australian Marine Park network.
Upwelling of nutrient-rich deep ocean that enhance ocean productivity have been suspected to occur at SW Cape canyon. This location matches the small landslide we surveyed. Data from the voyage will improve our understanding whether this upwelling could be detected, and its scale. The nutrient concentrations was surveyed as multiple locations along the shelf and down to 2,000 mbsl. It also included control locations where upwelling was not expected.
Marine mammals and marine bird observations will enable us to establish benchmark statistics for biodiversity in SW Tasmania. These observations will enable us to calculate the role and effects of climate change in this poorly surveyed region.
5. Blythe Star
The RV investigator formally identified and surveyed the shipwreck of the MV Blythe Star, which sank off the SW Cape in 1973.