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Australia, the geologically safest place for nuclear waste?
Greenhouse related global warning has opened a ‘window of opportunity’ for the nuclear energy industry - placing the issue of nuclear waste storage firmly on the agenda once again. It is frequently asserted that Australia provides the geologically safest environments for nuclear waste storage [1], reflecting the widely held perception that Australia is the most tectonically stable of all continents. We ask just how well-informed is this view?
The key message is that Australia is relatively stable but not tectonically-inert, and appears to be less stable than a number of other continental regions. Some places in Australia are geologically surprisingly active indeed, with earthquakes in the recent geologic past demonstrably changing the face of our land.
We occasionally get surprisingly big earthquakes in Australia (up to about magnitude 7) and the big ones have tended to occur in somewhat unexpected places like Tennant Creek. The occurrence of such earthquakes imply that we still have much to learn about our earthquake activity. From the point of view of long-term waste disposal this is very important, since prior to the 1988 (M 6.8) quake, Tennant Creek might have been viewed as one of the most appropriate parts of the continent for a storage facility [7]. Paradoxically, Tennant Creek may be more safe now as a consequence of having generated a large earthquake in recent times, since recurrence times for rupturing of intraplate faults like the Tennant Creek fault are likely to be of the order of 10,000 years or so [2].
Image showing Australian earthquake occurrences
with magnitude greater than 3. Information from the Geoscience Australia
database.
On the basis of the historical earthquake occurrence we can make
some educated guesses about relative stability of different geographical
regions. The appropriate measure is the somewhat scary sounding "seismic
moment release rate" - basically the energy released by earthquake
activity in any given region of the earth over a given time. The table
below (compiled by Arch Johnston [3]) is
one such attempt to estimate this for those part of the continents that
are generally considered to be stable, such as Australia, Northern Europe,
etc. In this compilation Johnston only considers those parts of the continents
that do not lie at, or close to, plate tectonic boundaries, and hence
are relatively stable. The lower the seismic moment release rate,
the greater the stability.
Table 1: Estimates of seismic moment release rates for those parts of continents considered to be relatively stable. [3]
It is important to realize that these are highly uncertain calculations and we would very much like to improve on them. Indeed, with colleagues I am working on this for the Australian case currently. Also, as indicated by the patchy distribution of recorded earthquakes in Figure 1, the seismic moment release rate is not uniform across Australia. Some regions, such as the Flinders Ranges and the north-western seaboard, have significantly elevated seismic moment release rates, while some of the intervening regions have no significant historical earthquake activity (as was the case for Tennant Creek prior to 1988).
At face value, these calculations do suggest that Australia is less stable than much of northern Europe (i.e. to the north of the European Alps), which would surprise most people. However, the risk of a typical site in Australia being within 50 kms of a large earthquake (magnitude 6.5 or higher) over a period of 1000 years is still very small.
Magnitude greater than 5 earthquakes since
1960, CNSS database.
The reason for Australia's relatively high seismic moment release rate in comparison to some of the other notionally stable continental regions is the relatively highly stressed state of our continent, reflecting its place in a very dynamic tectonic plate with complex interactions with the neighboring plates.
In answer to our question Australia is not the most stable of continental regions, although the levels of earthquake risk are low by global standards. To the extent that past earthquake activity provides a guide to future tectonic activity, Australia would not appear to provide the most tectonically stable environments for long-term waste facilities. However, earthquake risk is just one of the 'geologic' factors relevant to evaluating long-term integrity of waste storage facilities, and other factors such as the groundwater conditions, need to be evaluated in any comprehensive assessment of risk.
Are there any geological signs of young
tectonic activity in continent, reflecting ongoing earthquake activity?
Indeed
there are, and some are quite dramatic. For example, earthquake activity
along the Cadell Fault near Echuca dammed and diverted the Murray River
about 50,000 years ago to give rise to the Barmah forest. the earthquake
(or more likely series of earthquakes) responsible for this faulting
must have been very large, possibly above magnitude 7. Intriguingly this
(these) quake(s) occurred at about the time the country was first being
populated with this unique natural wetland system, so important to the
Yorta Yorta people, only created at around the time the first people
arrived in the land.
Estimate of relative size of an earthquake
responsible for the Cadell Fault, relative to the Meckering 1968 M6.8
quake in Western Australia that ruptured a surface breaking fault some
35 km long with a slip of up to 2 m. It is not known whether the Cadell
Fault ruptured in a single large event or in several smaller events.
As a community, Australian geologists have tended to overlook this recent aspect of our geological heritage because it is subtle and perhaps not as exciting as the older geological stories of our continent. However, this is changing and several people are now seriously trying to assess how young tectonic activity has affected our continent. My own research, funded through an ARC Professorial fellowship, is directly aimed at tackling these issues [5].
I touched on these issues in a public lecture at Melbourne University on October 13th, 2005, which can be viewed at this link.
[1]. For example, Bob Hawke was recently quoted as saying “ Australia has the geologically safest places in the world for the storage of waste”, http://www.abc.net.au/news/newsitems/200509/s1468931.htm
[2] although there is also the possibility of clustering of large quakes that exposes a place like Tennant Creek to the possibility of a series of large shocks over a very short duration, followed by long periods of inactivity - emphasizing that we just do not know enough yet to be certain how our earthquakes work!
[3] see Johnston, A.C. (1994a) The stable
continental region earthquake database.The earthquakes of stable continental
regions, 1. Assessment of large earthquake potential, Report TR-102261-1,
Electric Power Research Institute, Palo Alto, CA, pp. 3-1 to 3-80. and
Johnston, A.C. (1994b) Seismotectonic interpretations and conclusions
from the stable continental region seismicity database. The earthquakes
of stable continental regions, Report TR-102261-1, Electric Power Research
Institute, PaloAlto, CA, pp. 4-1 to 4-102.
[4] You can find some images of relevance to understanding young Australian earthquake activity on my web site - as part of the inaugural lecture (the Mawson lecture) I gave at the last Geological Society of Australia Convention. http://jaeger.earthsci.unimelb.edu.au/msandifo/Talks/2004/mawson/mawson.html
[5] ARC Discovery Project grant DP0556133, "The Neotectonics of the Indo-Australian Plate".
[6] For discussion on Australian earthquakes see http://www.quakes.uq.edu.au/gen_info/lge_quakes/list_aust.html
[7] Emphasizing the problems of understanding our earthquake hazard, these images show a before (1985) and after (1988) Tennant Creek scenario for determining a seismologically optimum region of stability!
