issue 89 Mar 2008 Revealing the continental shelf off New South Wales ks ac cr t n oa io Cross-agency work increases understanding ns m Bedrock outcrop e al T n io carp s of tsunami hazard and risk S ro n E Bulli Slide row C Depositiona Kriton Glenn lee ridge erosionPost groovefailure s l

Shovel slide The results of a 15-day marine survey off the New South Wales coast a very slow sedimentation rate,200m sca rp have provided a regional understanding of the morphology and has been prone to extensive Depositional toe Tensiocracksn mass wasting history of the continental slope between Jervis Bay and sediment mass wasting over time. 07-2464-1 Forster (figure 1). 500 0 10 km

NT ) QLD While providing a view of previously unknown submarine rivers, New features 1000 bedrock outcrops and multiple deepwater canyons, the survey discoveredSA 1500

gathered baseline data that helped Geoscience Australia assess the NSW Water depth (m probability and implications of localised submarine mass failures or Many remarkableACT features were2000 underwater landslides. also seen forVIC the first time, such Harnessing expertise from several areas across Geoscience as mid slope channels off the Australia as well as university specialists, the survey team assessed Hunter region. Some of these the continental slope, particularly in areas where it is thought that channels have levees and a V submarine mass failures could generate tsunamis. The data were shaped morphology, suggesting presented and discussed at the recent international Submarine Mass that both active erosion and Movements and their Consequence Symposium in Greece. deposition are taking place. The physiography of the east Australian margin is largely controlled Additionally, a series of large by the original shape of the basement. The survey data reveal that the pockmarks (~600 metres in New South Wales continental slope, despite being characterised by diameter and ~70 metres deep) were found in water depths

ks exceeding 1300 metres. The ages ac cr t n oa of the pockmarks are hard to io ns m Bedrock outcrop e al T n determine, as they are formed io carp s S ro as a result of an ongoing gas or n E Bulli Slide row C Depositiona liquid escape from much deeper lee ridge erosionPost groovefailure s in the geological profile. Seismic l data indicate that the fluid Shovel slide is migrating along faults and 200m sca rp Depositional toe escaping via these features into the Tensiocracksn 07-2464-1 water column. The profiles show 500 0 10 km little infilling of the steep walls

NT ) QLD of the pockmarks, indicating an 1000 active erosional process. SA 1500

NSW Water depth (m ACT 2000 Submarine landslides VIC revealed The swath bathymetry also Figure 1. The slope failure architecture and slip-plane geometry of the Shovel Slide. Inset: the survey area off the New South Wales coast. revealed the slope failure

Revealing the continental shelf off New South Wales 1 issue 89 Mar 2008

architecture and slip plane geometry of several submarine mass failure investigation program that sites. The sites that have failed include the Bulli (~20 km3), Shovel seeks to determine if and when (~7.97 km3; figure 1), Birubi (~2.3 km3) and Yacaaba (~0.24 km3) tsunamis have occurred along the slides. New South Wales coast. The sub-bottom profiles illustrate the nature of the failures and The results of this type of highlight two distinct types: those related to the sediment bedding scientific enquiry provide a planes in the Cainozoic sediment wedge and those related to the model for further assessment of critical dynamics of the seaward face of volcanic highs, slope angle tsunami risk along the Australian and sediment load. continent. The survey also helped to identify potential failure sites across much of the continental slope. Sites identified as potential failure For more information zones displayed retro-gradational failure and surface cracking, and phone Kriton Glenn on some are subject to localised slope undercutting. Taking these factors +61 2 6249 9379 into consideration and assessing the location of vulnerable sediment email [email protected] accumulations, future programs will seek to identify and assess additional areas susceptible to failure along Australia’s continental slope. Data acquired: Predicting tsunami risk • ~9200 square kilometres swath bathymetry This survey provided essential information for developing models to • 3414 kilometres sub-bottom assess the risk of tsunamis to the Australian coastline. The data were profiles coupled with tsunami model outputs to develop a palaeo-tsunami

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Revealing the continental shelf off New South Wales 2