New LiDaR release along the Willunga Fault, SA.

[Dan]

The Elvis Elevation portal (https://elevation.fsdf.org.au/) has recently made available a 2019 LiDaR capture along the Willunga Fault scarp, south of Adelaide. The below screen grab, which extends from Sellicks Beach in the south, to Maslin Beach/McLaren Vale in the North, clearly shows that the scarp is much more sharply defined to the south. This may be taken as evidence for segmented rupture on the Willunga fault, with the southern part being the most recently active part of the fault. South of the screen grab, the fault goes offshore, so at this stage it is not clear what the length of the most recently active section might be.

A zoom in to the northern end of the most recently active segment shows the scarp diverting east into the range. A more topographically subdued (older), range front parallel section of scarp can be seen continuing to the northeast. Just south of the 'bend' into the range is the site where the 2020 AEES field excursion was proposed. Here's hoping that we can visit it this year!

[Dan]

I should add that this fault has been the subject of investigations led by Associate Professor Mark Quigley from the University of Melbourne for the last several years. Initial paleoseismic work at the northern end of the fault involving shallow geophysics, trenching and 3D modelling contributed to a hazard assessment of the Mt Bold Dam for SA Water. The results of these investigations are summarised in the below papers:

TERZIC Z., QUIGLEY M. C. & LOPEZ F. 2017 Detailed Seismic Hazard assessment of Mt Bold area: comprehensive site-specific investigations on Willunga Fault. Australian National Committee on Large Dams Incorporated (ANCOLD) General Assembly Full Length Conference Paper. pp. 14 p. Hobart, Tasmania.

MACKLIN S. R., Quigley, M., Terzic, Z., Barter, J. F., & Buchanan, P. 2019 A Multi-Disciplinary Approach to Active Fault Rupture Risk Characterization: 3D Geological Modelling of the Willunga Fault, Mt Bold Dam, South Australia. ICOLD 2019 Annual Meeting/Symposium: Sustainable and Safe Dams Around the World. June 9-14, Ottawa, Canada.

As is often the case in earthquake science, the information you need is not always readily available at the site you need it. Subsequent investigations to the south of Mt Bold, involving additional trenches, sought to obtain additional magnitude, recurrence and segmentation data for the fault. These studies are documented in two excellent Masters Theses (Mark Q is going to get back to me with links to these shortly):

WILSON A. J. 2018 Plio-Quaternary Faulting and Neotectonic Behaviour of the Willunga Fault, Mt Lofty Ranges, South Australia. School of Earth Sciences, The University of Melbourne. Masters Thesis. pp. 210.

KUANG K. 2018 Paleoseismology and Landscape Evolution of the Willunga Fault, Mount Lofty Ranges. School of Earth Sciences, The University of Melbourne. Masters Thesis. pp. 150.

 Characterising these range front faults has proven challenging as the range front is very dynamic in terms of erosion and deposition relative to the fault slip rates. There is only a few degrees in slope difference between highly erosive and highly depositional environments (see below cartoon – vertical scale is ~ 50 m). In the deposition environments, fault tips are quickly buried beyond the depth of trenching, whereas in the erosional environments, sediments are stripped off in the interseismic interval.

 

We were fortunately able to capture the last event on the range front fault on the central, more subdued section, about 10 km south of Mt Bold:

 

Note small pocket of colluvial sediment overthrust by bedrock with unfaulted brick-red soil development over the top. This soil forms during sea level low stands when carbonate rich dust is blown off nearby exposed coastal plains. It is pervasive along the Mt Lofty Ranges.

  

On the southern, fresher-looking segment, we were not able to hit the most-recent-event displacement on the range front fault as the tip had been deeply buried by colluvium. However, we found a hanging wall flexural graben with a sedimentary infill. Below is a log of the uphill bounding fault of the graben, with graben-fill sediments to the left of the fault (refer to cartoon at top for location with respect to the range front fault). The important bit here is that the brick red soil is displaced across the fault, indicating an event younger than seen in the northern trench (consistent with the LiDAR expression. The graben contained evidence for  several displacement events, which can be related to movement on the underlying range front fault.

 

 We are hoping to revisit this site in the near future to obtain further age control…

 

 

Together, these investigations (which are ongoing) have enormously improved our understanding of the behaviour and characteristics of the Willunga Fault (e.g. location of fault traces, fault displacement hazard, magnitudes, recurrence behaviour, segmentation etc). SA Water’s engagement with this work represents a gold standard in engaging with the ANCOLD 2019 Guidelines for design of dams and appurtenant structures for earthquake. Further, this work will inform decisions built into the National Fault Source Model, which is a fundamental component of the National Seismic Hazard Assessment.

 

Cheerio,

Dan.