The mine is currently operated underground, but is due to be expanded into the world’s largest open cut uranium mine by owners BHP-Billiton with the active support of the South Australian state Labor government.
A 35 kilometre long Mashers Fault line runs through the centre of the ore body and the proposed open pit.
Natural seismic activity, and changes in groundwater pressures caused by the massive pumping of artesian water supply by the mine, could equally result in a radioactive event.
Seismic activity is common throughout parts of South Australia.
The suburbs of the capital city of Adelaide sit astride several major fault lines. The most notable earthquakes have been in 1954 (magnitude 5.5) and 2010 (magnitude 3.8). Earthquakes in SA regional centres have included Beachport (1897 mag. 6.5) and Warooka (1902, mag. 6.0).
The most active area for earthquakes in SA is the Flinders Ranges (see here) where the Chinese-backed Marathon Resources hopes to tunnel into a wilderness sanctuary to mine uranium.
But it is the potential for disturbance at Olympic Dam that has excited recent comment. Edward Cranswick, a retired geophysicist, has been warning of the dangers of expanding the Olympic Dam into one of the world’s largest open pit mines.
These details are taken from the Coober Pedy Regional Times:
OLYMPIC DAM: Mashers Fault
Mashers Fault and the Seismicity Anticipated to be Stimulated by the Proposed Open Pit Mine at Olympic DamBy: Edward Cranswick (30DEC2009) REVISED* Public Seismic Network (Geophysicist, US Geological Survey, resigned)
(Figure 1 Mashers Fault schematic SSE-NNW cross-section through the Olympic Dam ore body, no vertical exaggeration )
Abstract: The proposed excavation at Olympic Dam of one of the largest open pit mines on Earth, 4.1 km long, 3.5 km wide, 1 km deep, at a bend in the steeply dipping, 35-km-long Mashers Fault, and the associated perturbation of the local groundwater pore pressures in a region of horizontal compressive stress would most likely stimulate local seismicity.
(Figure 2 Map of Olympic Dam showing existing underground mine and outline of the proposed open pit in bold black, the potentiometric contours (labeled by elevation, m, above sea-level) in blue, and the trace of the NE-SW Mashers Fault in yellow (the eastern-most 5 km have been truncated). Black arrows indicate the principal horizontal stress direction (Bunger et al. 2008)
Faulting, perhaps related to variations of pore pressure, has occurred intermittently on structures in Australia that are difficult to recognise, and open pit mines have caused earthquakes in other countries.
Removing 1 km of rock at Olympic Dam would reduce the vertical stress by ~25 MPa, increase the deviatoric stress and facilitate thrust-type faulting in the vicinity of the open pit, and possibly stimulate strike-slip failure on the Mashers Fault, triggered by the extensive pumping and disposal of ground water.
["potentiometric contours" in Figure 2 refers to contours of water table with respect to sea level]
Relatively small (magnitude <6), href="http://www.bhpbilliton.com/bb/odxEis.jsp">http://www.bhpbilliton.com/bb/odxEis.jsp )
Introduction: Broken Hill Proprietary (BHP Billiton) currently owns and operates the existing underground copper-uranium-gold-silver Olympic Dam mine (coordinates: –30.44°, +136.87°) 10 km north of Roxby Downs in South Australia.
The company has proposed in their Olympic Dam Expansion Draft Environmental Impact Statement 2009 (ODXdEIS; BHP Billiton 2009) to expand the mining operation over a period of 40 years and create an open pit mine 4.1 km long, 3.5 km wide, 1 km deep – making it one of the largest open pit mines on Earth.
The Olympic Dam ore body is a hematite breccia within the basement of Palaeoproterozoic and Mesoproterozoic crystalline rocks that are unconformably overlain by ~300 m of Neoproterozoic to Cambrian age, flat-lying sedimentary rocks of the Stuart Shelf and a veneer of surficial deposits on the eastern margin of the Gawler Craton (Reynolds 2001). The mined ore is ground into particles and treated with acid to dissolve and remove copper, uranium, gold and silver, and the resulting tailings are a slurry that is pumped to the Tailings Storage Facility (TSF). There the tailings solids are allowed to settle and the liquid component is removed by evaporation and seepage into the surficial layer of sands and clays (~3 m thick) and the underlying shallow aquifer of the Andamooka Limestone, and some excess liquid drains into evaporation ponds (ODXdEIS).
Next to the ore body, the most prominent geological structure at Olympic Dam is the 35-km-long Mashers Fault that passes through the middle of the ore body and the proposed pit. The excavation of the open pit mine and the associated perturbation of the local groundwater pore pressures would most likely stimulate some level of seismicity in the vicinity of the mine, and possibly slip on the Masher Fault itself. I use the term “stimulate” as defined by McGarr et al. (2002):
‘“induced” describes seismicity resulting from an activity that is comparable in magnitude to the ambient shear stress acting on a fault to cause slip, whereas “triggered” is used if the stress change is only a small fraction of the ambient level … By “stimulated” we refer generally to seismicity either triggered or induced by human activities.’
To continue: Included at the following link is a recent peer-reviewed scientific paper, “Mashers Fault and the Seismicity Anticipated to be Stimulated by the Proposed Open Pit Mine at Olympic Dam” by Edward Cranswick, presented at the annual meeting of the Australian Earthquake Engineering Society held in Newcastle, NSW, in December 2009 to commemorate the 20th anniversary of the 1989 Newcastle Earthquake (in January 1990.
Mr. Cranswick participated in the tail-end of the then on-going aftershock investigation of the Newcastle earthquake). His presentation slides from the annual meeting of the Australian Earthquake Engineering Society held in Newcastle, NSW can be found at this website: http://cranswick.net/MashersSeismicityAnticipatedOlympicDam/
Further Seismic information by this author is included here as a submission on the application licenses for a National Radioactive Waste Repository (at Woomera)
13 Nov 2003
The Project Management Officer
PO Box 655
MIRANDA NSW 1490
RE: Public Submission on the application licenses for a National Radioactive Waste Repository Edward Cranswick
I am opposed to the Australian Radiation Protection and Nuclear Safety Agency’s (ARPANSA) granting of a license for the construction and operation of the National Radioactive Waste Repository (NRWR) at Site 40a in South Australia (SA). There remain serious unresolved issues regarding the impact to the health and safety of people and the environment by the proposed shallow burial of radioactive waste near Woomera. In particular, I am concerned about the inadequate analysis of seismic hazard in the NRWR Draft Environmental Impact Statement (EIS).
I recently retired from the US Geological Survey where I worked as a geophysicist for 22 years and investigated the strong ground motions produced by earthquakes.
The whole discussion of seismic hazard is limited to less than one page in the Main Report of the EIS (Environment Assessment, Chapter 8, Physical Environment, 8.1.3 Seismicity, p. 157), the text of which is reproduced in its entirety below:
The level of seismic activity in Australia is generally considered to be low when compared to the seismically active areas of the world.
The most seismically active areas of South Australia are associated with the Adelaide Geosyncline in an area extending from the Flinders Ranges in the north to Kangaroo Island in the south; the eastern portion of Eyre Peninsula; and the southeastern region of the state around Mt Gambier.
The area between Quorn and Leigh Creek has the highest number of seismic events (considered to be related to zones of crustal weakness), with several earthquakes ranging in magnitude from Richter Local Magnitude (ML) 4.5 to 5.7 between 1939 and 1983. Activity west of the Torrens Hinge Zone, in the areas of the proposed repository, range from ML 1 to 2 (with ML 2 being the lowest magnitude able to be felt). Discussion with the South Australian Office of Minerals and Energy Resources has indicated that the cluster of predominantly ML 1 recordings are likely to be related to blasting activities associated with mining at Olympic Dam and Mt Gunson.
In Eyre Peninsula the earthquakes appear to be associated with the Lincoln Fault Zone, the highest recording being the 1959 Mambin earthquake of magnitude ML 4.9. In the South East, seismicity is related to the western margin of the Otway Basin and an onshore volcanic belt. The highest recorded earthquakes are the 1897 Beachport–Kingston earthquake of magnitude ML 6.5 and the 1948 Robe earthquake of magnitude ML 5.6.
The Standards Association of Australia AS 1170.4-1993, Minimum Design Loads on Structures, Part 4 Earthquake Loads indicates that a ground acceleration coefficient of 0.08 would be appropriate for Site 52a, and between 0.085 and 0.09 for the eastern sites. There is a 10% probability that the aboveground acceleration levels would be exceeded in a 50-year period.
The repository and buildings would be designed in accordance with AS 1170.4-1993.
The repository and buildings would be designed in accordance with AS 1170.4-1993.
In the following submission, I will review this text.
(Fig. 4. Earthquake hazard map of Australia. The numbers, e.g., "greater than 0.10”, refer to the ground acceleration (measured as a fraction of the Earth’s gravitational acceleration, g, i.e., 1.0 g = 9.8 m/s2) with a 10% probability of being exceeded in 50 years (Geoscience Australia).
Site 40a is approximately 20 km east of Woomera. Instead of being located within one of the least seismically hazardous zones of Australia, the site is adjacent to the one of the most hazardous zones, that which encompasses the Flinders Ranges (Figs. 4 and 5).
Indeed, the Federal Member for the South Australian electorate of Mayo and Minister for Foreign Affairs, Alexander Downer, recently said, “There have been more moderate-sized earthquakes near Adelaide over the last 50 years than anywhere else in Australia” (Geoscience Australia, 2003a).
(Figure 5 – Earthquake hazard map of Australia. The numbers, e.g., “greater than 0.10”, refer to the ground acceleration (measured as a fraction of the Earth’s gravitational acceleration, g, i.e., 1.0 g = 9.8 m/s2) with a 10% probability of being exceeded in 50 years (Geoscience Australia, 2003b).
SA earthquake hazard map of region surrounding location marked by the black square. The yellow, brown, and red dots represent earthquakes (Geoscience Australia).
Figure 5 – Earthquake hazard map of region surrounding Site 40a whose location is marked by the black square. The yellow, brown, and red dots represent earthquakes (Geoscience Australia, 2003b).
The PIRSA earthquake maps (PIRSA, 2003) for years 2000 and 2001 show events to the west of Lake Torrens, and the EIS states “Activity west of the Torrens Hinge Zone, in the areas of the proposed repository … are likely to be related to blasting activities associated with mining at Olympic Dam and Mt Gunson” (my italics). However, I note the following: 1) not all these events are located at either Olympic Dam or Mt Gunson; 2) whether all these events are explosions should be explicitly determined by consulting the records of the respective licensed blasters; 3) mining activities can trigger seismic events; 4) Olympic Dam has a sophisticated seismic array to monitor earth movements at the mine (S. Eldridge, 2003, personal communication), and data from this array can be used to constrain the nature of local seismic sources, i.e., those within a hundred km.
Because of the historical paucity of seismograph stations, the occurrence and exact locations of earthquakes and their corresponding wave propagation characteristics in northern SA are not well known, and an intensive field campaign of earthquake monitoring has recently commenced “to improve estimates of seismic hazard in the region” (Geoscience Australia, 2003c).
In addition to the hazard posed by the known sources of seismic activity in the Flinders, there is the hazard posed by large intraplate earthquakes that could occur anywhere throughout the Australian craton, such as the 1987-1988 Tennant Creek earthquake sequence that included three magnitude 6+ events: “The Tennant Creek area had no history of earthquake activity before 1987.” (Seismology Research Centre, 1998). Cummins et al. (2003) argue that there are “large uncertainties … regarding an earthquake hazard map for Australia in which we can have complete confidence”, and they conclude, “Recent neotectonic investigations in areas of low topographic relief, for example, indicate that seismicity must be transitory in both space and time over large parts of Australia.”
Not only strong ground shaking, but also faulting and ground rupture can pose a significant hazard to the NRWR. Ground rupture such as that which damaged the gas pipeline during the earthquakes at Tennant Creek can damage waste containment structures. The 1992 ML 5.6-5.8 Little Skull Mountain Earthquake, whose epicenter is within 20 km of Yucca Mountain, Nevada, USA, a potential high-level NRWR, is the largest tectonic event that has occurred within 50 km of the site in historic time, and it has raised concerns about “coupled processes that may lead to the future release of radioactive material to the accessible environment.
A few examples of tectonically coupled processes are: co-seismic changes in the water table; changes in infiltration rates due to changes in fracture characteristics … ” (Whitney and Keefer, 2000). Similar abrupt changes in the water table were observed after the 1998 magnitude 5.3 Pymatuning earthquake in Pennsylvania, USA, and Fleeger et al. (1999) argued that these changes could be explained by faulting that fractured the aquitard and drained the aquifer. Faulting plays an important role in the natural occurrence of mound springs in the Great Artesian Basin (GAB) 200 km to the north (James Cook University, 2000).
The last sentence of the “Seismicity” section of the EIS is, “The repository and buildings would be designed in accordance with AS 1170.4-1993.” However, the abstract of the AS 1170.4-1993 states (Standards Australia, 2003):
Sets out data and procedures for determining minimum earthquake loads on structures and their components, and also minimum detailing requirements for structures. It does not consider related phenomena such as settlement, slides, subsidence, liquefaction or faulting in the immediate vicinity of a structure. It does not include nuclear reactors, dams, transmission towers, bridges, piers and wharves, which may require special consideration. The Standard is in limit states format. New earthquake maps are defined in terms of an acceleration coefficient instead of the zoning system used in the previous Standard AS 2121. Domestic structures are now included.
Therefore, AS 1170.4-1993 is essentially irrelevant to the design of the repository because the repository would clearly be one of those structures which “require special consideration.” The title, “Seismicity”, of this section about seismic hazard is itself a misnomer because the term seismicity refers to the temporal, spatial, and magnitude distribution of earthquakes but does not directly refer to their damaging effects.
Site 40a is not a site of minimal earthquake hazard. There is much uncertainty at present about the seismic hazard in the region, and much of the information that is available has not been included in the EIS. Furthermore, it is premature to attempt to evaluate the earthquake hazard until results are obtained from the recently commenced tectonic and seismic studies of the Flinders Ranges.
Most critically, the misunderstanding concerning AS 1170.4-1993 indicates that the EIS Study Team is not competent to evaluate seismic risk. The EIS Study Team did not include any geophysicists – it is essential that the EIS be approved by seismologists, and my Australian colleagues who are familiar with the Australian land are the ones to do that. In conclusion, as characterized by the Draft EIS, ARPANSA should not grant a license for the construction and operation of the NRWR at Site 40a.
(for bibliography, see Coober Pedy Regional Times link above).