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LARGE EARTHQUAKE RECURRENCE IN THE SPRIGG OROGEN,
SOUTH AUSTRALIA AND IMPLICATIONS FOR EARTHQUAKE
HAZARD ASSESSMENT
Dan Clark
Earthquake Hazard Project, Geoscience Australia
ABSTRACT
The Australian continent is actively deforming at a range of scales in response to far-field stresses associated
with plate margins, and buoyancy forces associated with mantle dynamics. On the smallest scale (several 10’s
of km), fault-related deformation associated with far-field stress partitioning has modified surface topography at
rates of up to approximately 100 m/Myr. This deformation is evidenced in the record of historical earthquakes,
and in the pre-historic record in the landscape. Paleoseismological studies indicate that few places in Australia
have experienced a maximum magnitude earthquake since European settlement, and that faults in most areas are
capable of hosting potentially catastrophic earthquakes with magnitudes in excess of 7.0. South Australia is well
represented in terms of its pre-historic earthquake record. Seismogenic faulting in the last 5-10 million years is
thought to be responsible for generating more than 30-50% of the contemporary topographic relief separating the
highlands of the Flinders and Mt Lofty Ranges from adjacent plains, and perhaps as much as a third of the strain
budget of the entire continent is accommodated there. Adelaide itself straddles several faults which are arguably
some of Australia's most active. Decisions relating to the siting and construction of the built environment should
therefore be informed with knowledge of the local neotectonics.
Figure 1: Modelled SHmax vectors (after Sandiford & Quigley, 2009) for the Indo-Australian Plate adapted from
solutions in Sandiford et al. (2005).
1
THE AUSTRALIAN CRUSTAL STRESS FIELD
Over the last 45 million years the Australian continent has drifted approximately 3000 km to the north at a rate
of approximately 6.5 cm/year - further and faster than any other continent (Quigley et al. 2010). In contrast to
other fast moving plates, such as the North and South American Plates, the maximum horizontal stress (SHmax)
orientation within the Australian Plate is not parallel to the plate motion vectors (Fig. 1) (Coblentz et al. 1995;
Hillis & Reynolds 2000; Hillis & Reynolds 2003; Sandiford et al. 2004). This complex pattern has been
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LARGE EARTHQUAKE RECURRENCE IN THE SPRIGG
IMPLICATIONS FOR EARTHQUAKE HAZARD ASSESSMENT
OROGEN,
SOUTH
AUSTRALIA AND
DAN CLARK
satisfactorily modelled in terms of a balance between driving and resistive plate boundary forces (Coblentz et al.
1995; Coblentz et al. 1998; Burbidge 2004; Sandiford et al. 2004; Dyksterhuis & Müller 2008; Hillis et al.
2008). Resistance to plate motion is seen in the mountain ranges of Papua New Guinea, the Himalaya and New
Zealand, whereas slab pull along the Indonesian margin, and ridge push to the south along the AustraliaAntarctic Discordance are drivers of plate motion (Coblentz et al. 1995; Coblentz et al. 1998; Hillis et al. 2008).
Structural and sedimentary evidence from southeast Australian basins suggests that the current crustal stress
regime was established in the interval 10-5 Ma (e.g. Dickinson et al., 2001; Dickinson et al., 2002; Sandiford
2003b, a; Sandiford et al., 2004; Hillis et al., 2008). A major unconformity related to substantial regional-scale
tilting, uplift, folding and reverse faulting of late Miocene and older strata occurs in all southeast Australian
basins (e.g. Gippsland and Otway Basins). Pliocene and Quaternary strata overlying the unconformity contain
neotectonic structures yielding palaeo-stress indicators consistent with the current in situ stress field, as
determined from seismicity (Denham et al. 1979; Denham & Windsor 1991; Leonard et al. 2002; Clark &
Leonard 2003) and down-hole stress determination techniques (e.g. Hillis & Reynolds 2000; Hillis & Reynolds
2003; Hillis et al., 2008). The time over which the current stress field has pertained is defined as the
Neotectonic Era, and a “neotectonic fault” is defined as a fault which has hosted displacement under conditions
imposed by the current Australian crustal stress regime, and hence may move again in the future.
2
MANIFESTATION OF STRESS AS DEFORMATION
Australia preserves a unique geomorphic record of intraplate tectonic activity. Three distinct modes of surface
deformation are recognised (Sandiford et al. 2009; Sandiford & Quigley 2009). At long wavelengths (several
1000’s of km) systematic variations in the extent of Neogene marine inundation imply the continent has tilted
north-down, southwest-up. This is most convincingly seen along the southern Australian margin, where
Miocene limestone of the Nullarbor Plain has been uplifted and subjected to marine erosion to form iconic cliffs.
At intermediate wavelengths (several 100’s of km) several undulations of approximately 100-200 m amplitude
have developed on the 1-10 Myr timescale, consistent with the buckling amplitude of the lithosphere. In two of
the more notable cases, the Lake Mackay palaeo-drainage is now internally draining in its northern reaches, and
palaeo-lake Billa Kalina now sits high on a watershed (Sandiford et al. 2009). At still shorter wavelengths
(several 10’s of km), fault related motion has produced local relief at rates of up to approximately 100 m/Myr
over a period several million years (Sandiford & Quigley 2009), as in the Flinders Ranges (Quigley et al. 2006)
and the Otway Ranges (Sandiford 2003a). While it has been proposed that the buckling mode of deformation is
associated with contemporary seismicity in India (Vita-Finzi 2004), this link has not been proven in Australia.
Hence it is the third mode that is of most interest in terms of assessing seismic hazard.
Figure 2: Earthquake epicentres (Geoscience Australia database), seismogenic neotectonic features (Clark et al.
2010), maximum horizontal stress vectors (Hillis & Reynolds 2003) and zones of elevated seismicity (Hillis et
al. 2008).
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LARGE EARTHQUAKE RECURRENCE IN THE SPRIGG
IMPLICATIONS FOR EARTHQUAKE HAZARD ASSESSMENT
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OROGEN,
SOUTH
AUSTRALIA AND
DAN CLARK
AUSTRALIA’S SEISMOGENIC NEOTECTONIC RECORD
Australia’s rich neotectonic record provides an opportunity to understand the characteristics of seismogenic
intraplate deformation, both at the scale of a single neotectonic fault and at the scale of the entire continent (Fig.
2). Over the last decade our knowledge of Australian intraplate faults has advanced significantly, with the
recognition that faults in different parts of the Australian continent respond in different ways to the imposed
stress. Six source zones (domains) spanning onshore continental Australia have been proposed (Clark 2006;
Clark et al. 2010) based upon geological and neotectonic data (Fig. 3a). A seventh offshore source zone is
defined based upon analogy with the eastern United States (Johnston et al. 1994; Wheeler 1996; Wheeler &
Frankel 2000). In principle, each source zone contains neotectonic faults that share common recurrence and
behavioural characteristics, in a similar way that source zones are defined using the historic record of seismicity.
The power of this domain approach lies in the ability to extrapolate characteristic behaviours from wellcharacterised faults (few) to faults about which little is known (many). These data, and conceptual and
numerical models describing the nature of the seismicity in each source zone, has the potential to significantly
enhance our understanding of seismic hazard in Australia at a time scale more representative than the snapshot
provided by the historic record of seismicity. This includes providing a means by which to estimate key
parameters underpinning the next generation seismic hazard maps for Australia, such as maximum magnitude
earthquake (Mmax) (Fig. 3b) and seismic source zone b values.
(a)
(b)
Figure 3: (a) Preliminary neotectonic domains with fault scarps from the Australian neotectonics database
overlain (Clark et al. 2010), (b) statistics for fault length, which is a proxy for Mmax. The median value of 40
km for the Flinders domain (D2) might be associated with an Mmax of ~7.1 (Leonard 2010).
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IMPLICATIONS FOR EARTHQUAKE HAZARD ASSESSMENT
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OROGEN,
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DAN CLARK
THE SEISMOGENIC NEOTECTONIC RECORD OF THE SPRIGG OROGEN
(DOMAIN 2)
Domain 2 is dominated by the Sprigg Orogen (Sandiford 2003b) (Fig. 3a). The Flinders and Mount Lofty
Ranges form the main topographic axis of the orogen, and are bound by N-S to NE-SW trending linear scarps.
These scarps provide dramatic testimony to the role of neotectonic faulting in shaping the landscape (Fig. 4).
Rare exposures of the range-bounding faults show evidence for moderate to steep reverse motion which has
juxtaposed ancient (> 500 million year old) basement rock above alluvial and colluvial sediments shed from the
developing upland systems in the last one-two million years (Fig. 4b,c) (e.g. Williams 1973; Bourman &
Lindsay 1989; Sandiford 2003b; Quigley et al. 2006). Fault-slip kinematics are consistent with the structures
having formed in response to the current stress regime, with SHmax trending between N080°E and N125°E
(Quigley et al. 2006). This motion is consistent with a subset of earthquake mechanisms, which show both
reverse and strike slip failure for this region (Clark and Leonard, 2003).
Figure 4: (a) Highly simplified geological map of the Flinders and Mount Lofty ranges region. Basement rocks
are shaded whereas recent sediments are white. Insets show sketches of exposures of range bounding thrust
faults from (b) Paralana, (c) Wilkatana, and (d) Burra. Note all faults are reverse and dip beneath the ranges
(after Celerier et al., 2005).
The cumulative vertical displacement on the fault network that forms the western front of the Mount Lofty
Ranges is estimated to be approximately 240 m, with approximately 80 m offset of early Quaternary ca. 1.6
million year old strata (Sandiford 2003b; Sandiford & Quigley 2009). Stratigraphic relations suggest that the
total displacement has accumulated in the last 5-6 million years (Sandiford et al., 2004), giving time averaged
displacement on bounding faults of approximately 40-50 m/Myr, in line with that inferred from historical
seismicity rates (Sandiford & Quigley 2009). Palaeoseismic studies show maximum magnitude earthquake
events of Mw 7.3 – 7.5 with recurrence intervals in the order of 104 years, averaged over several seismic cycles
(Quigley et al. 2006; Somerville et al. 2008).
The Flinders Ranges form part of a zone of anomalous surface heat flow (Neumann et al. 2000) reflecting
unusually elevated heat production in the Proterozoic basement rocks. Celerier et al. (2005) showed that
variations in both the absolute abundance and depth of heat producing elements provides a plausible thermal
control on lithospheric strength that helps to localise deformation in the Flinders Ranges. This characteristic
may explain repeated tectonic activity in this area, from the Late Proterozoic Adelaidian rift system (<850 Ma)
into which the dominant sedimentary rocks exposed in the ranges were deposited, to the Delamerian Orogeny
and associated granites and fold belt (520-480 Ma), to the currently building Sprigg Orogen (<10 Ma).
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IMPLICATIONS FOR EARTHQUAKE HAZARD ASSESSMENT
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OROGEN,
SOUTH
AUSTRALIA AND
DAN CLARK
QUATERNARY FAULTING PROXIMAL TO ADELAIDE
Several major neotectonic faults are located within 50 km of the Adelaide urbanised area (e.g. Para, EdenBurnside, Ochre Cove/Clarendon, Willunga, Millendella; Fig. 5)(Sandiford 2003b). Several more occur to the
north, proximal to the industrial corridor between Adelaide and Port Augusta (e.g. Alma, Redbanks,
Williamstown-Meadows, Wilkatana, Crystal Brook, Nectar Brook)(e.g. Somerville et al. 2008). Exposures of
the range bounding faults are rare near to Adelaide (Fig. 5, points 1-5). However, these exposures invariably
indicate that the range bounding faults are reverse faults and dip beneath the ranges (Fig. 5, cross section; Fig.
6). A recent palaeoseismological investigation of the Williamstown-Meadows Fault near Tarlee (Fig. 5, point 4;
Fig. 6) revealed evidence for a 1.8 m single event displacement which vertically offsets a young alluvial fan by
1.5 m (Geoscience Australia, unpublished data). This data is consistent with an MW 6.9 earthquake, a recurrence
of which would be catastrophic given its location approximately 15 km from the northern end of the Adelaide
urbanised area. Trenches across the Alma Fault revealed a similar single event slip, and hence earthquake
magnitude (Fig. 5, point 5).
Figure 5: Major neotectonic faults proximal to the Adelaide urbanised area (shown stippled) and schematic
cross section derived from 90 m SRTM DEM data. Fault exposures marked (1-3) show that the range bounding
faults are reverse in nature and dip beneath the range. 1 – Concordia/Gawler fault (Quigley et al. 2009), 2 –
Willunga Fault (Sandiford 2003b), 3 – Millendella fault (Bourman & Lindsay 1989). Points 4 and 5 mark the
Williamstown-Meadows and Alma Faults, which were recently trenched at locations to the north of this image
(Geoscience Australia, unpublished data).
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LARGE EARTHQUAKE RECURRENCE IN THE SPRIGG OROGEN, SOUTH AUSTRALIA AND IMPLICATIONS FOR EARTHQUAKE HAZARD ASSESSMENT
DAN CLARK
Figure 6: Selected neotectonic fault exposures near Adelaide (see Fig. 5 for fault locations). (1) the Concordia Fault at Gawler displacing Rowland Flat Sand (links into the
LARGE EARTHQUAKE RECURRENCE IN THE SPRIGG
IMPLICATIONS FOR EARTHQUAKE HAZARD ASSESSMENT
OROGEN,
SOUTH
AUSTRALIA AND
DAN CLARK
An interesting implication of this reverse geometry is that the 1954 M5.4 Adelaide earthquake may have
occurred at depth on the Para Fault, rather than on the Eden-Burnside Fault, as is the common wisdom (Fig. 7).
The Para Fault is of particular significance for seismic hazard assessment as it is situated almost entirely beneath
the Adelaide urbanised area and is associated with a prominent scarp developed in Quaternary sediments (Fig.
7). Borehole data (Sheard & Bowman 1996) suggests 27 m of offset of the Pleistocene Pooraka Formation
across the Para Fault, equating to a slip rate in the order of 0.2-0.8 mm/yr over the last 35-125 ka. The top of
underlying Hindmarsh Clay (>500 ka) is displaced by a similar amount, suggesting that the displacement relates
to a recent pulse of activity, similar to that seen in the last ca. 67 ka on the Wilkatana Fault (Quigley et al. 2006).
The borehole records further indicate that the underlying Pliocene Hallett Cove Sandstone has been displaced
across the fault by less than 200 m, implying longer term slip rates at least 2-5 times smaller than since the mid
Quaternary. If the borehole data is validated, and the Para Fault remains in an active period commensurate with
the last 35-125 ka, recurrences for earthquakes much larger than the 1954 event could be <10 ka, and perhaps as
little as a few thousand years.
Figure 7. Trace and schematic section through the Para fault, which underlies Adelaide. Easterly fault trace is the
Eden-Burnside Fault. Note that the Para fault displaces the Quaternary (Mid-Pleistocene) Hindmarsh Clay
(yellow unit), by tens of metres (inset from Belperio 1995). Red star marks the estimated epicentre of the 1954
M5.4 Adelaide earthquake (McCue 1996).
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OROGEN,
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DAN CLARK
SEISMIC HAZARD IMPLICATIONS
As implied in the previous section, there are several sources of seismic hazard to Adelaide that should be
considered alongside a relatively infrequent maximum magnitude rupture on the Para Fault. That this is so is
intuitively clear when it is considered that the damage radius of a relatively modest magnitude 6.5 earthquake is
approximately 50 km. Somerville et al. (2008) demonstrate that the contribution from several nearby
neotectonic fault sources in the central Flinders Ranges are greater than distributed seismicity (ie. the historic
earthquake record) in hazard calculations for return periods of 475 years and longer. For critical infrastructure
(return periods of 10,000 years and greater) the hazard from neotectonic fault sources dominates. A similar
scenario might be expected for the Adelaide area given the great density of neotectonic faults (Figs. 5 & 7). The
Para, Eden-Burnside, Ochre Cove/Clarendon, Willunga, Alma, Williamstown-Meadows and Milendella faults
all have Mmax exceeding MW 7.0. A large rupture on any of these faults has the potential to seriously impact
Adelaide. More frequent, smaller than Mmax events on the faults of the western range front of the Mt Lofty
Ranges, and beneath the coastal plane (e.g. the Redbank Fault extends south underneath Port Adelaide, Figure
5), such as the 1954 M 5.4 Adelaide earthquake, also have the potential to cause significant damage.
It has been demonstrated that neotectonic faults can contribute significantly to seismic hazard, even for return
periods appropriate to residential construction design (e.g. Somerville et al. 2008). Obtaining robust neotectonic
data for faults of interest, or nearby analogous faults, is critical to assessing the contribution that neotectonic
faults might make. For the Adelaide region, recurrence data for neotectonic faults is sparse. While the
preliminary neotectonics domains model discussed above is a valuable tool to identify appropriate analogues
where local fault data is not available, a significant amount of additional data is required to reduce uncertainties
and validate the model, in all the identified domains.
7
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IMPLICATIONS FOR EARTHQUAKE HAZARD ASSESSMENT
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