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A revised seismic site conditions map for
Australia
GEOSCIENCE AUSTRALIA
RECORD 2017/12
A. A. McPherson
Department of Industry, Innovation and Science
Minister for Resources and Northern Australia: Senator the Hon Matthew Canavan
Assistant Minister for Industry, Innovation and Science: The Hon Craig Laundy MP
Secretary: Ms Glenys Beauchamp PSM
Geoscience Australia
Chief Executive Officer: Dr James Johnson
This paper is published with the permission of the CEO, Geoscience Australia
© Commonwealth of Australia (Geoscience Australia) 2017
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ISSN 2201-702X (PDF)
ISBN 978-1-925297-49-2 (PDF)
eCat 103281
Bibliographic reference: McPherson, A. A. 2017. A revised seismic site conditions map for Australia.
Record 2017/12. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2017.012
Version: 1701
A revised seismic site conditions map for Australia
Contents
Abstract.....................................................................................................................................................1
1 Introduction ............................................................................................................................................2
1.1 History of the existing product .........................................................................................................2
1.2 Context for revision ..........................................................................................................................5
2 Input Geological Datasets .....................................................................................................................6
3 Workflow ................................................................................................................................................7
3.1 Data preparation and spatial referencing .........................................................................................7
3.2 Seismic site conditions classification ...............................................................................................9
3.3 Application of the weathering intensity index .................................................................................11
3.4 Data integration and quality control ...............................................................................................13
3.4.1 Data merge and checks ...........................................................................................................13
3.4.2 Spatial reference check ............................................................................................................14
3.4.3 Coastline clip ............................................................................................................................15
4 Sensitivity Testing of Higher Resolution Data .....................................................................................17
4.1 Bendigo ..........................................................................................................................................17
4.2 Melbourne ......................................................................................................................................20
4.3 Influence on amplification factors ..................................................................................................23
5 The Australian Seismic Site Conditions Map ......................................................................................24
5.1 Comparison between the ASSCM and NRSCM ............................................................................24
5.2 Comparison against measured VS30 data ......................................................................................26
6 Summary and Future Work .................................................................................................................29
6.1 Summary ........................................................................................................................................29
6.2 Future Work ...................................................................................................................................29
7 Acknowlegdements .............................................................................................................................30
8 References ..........................................................................................................................................31
Appendix A - Sources of Input Geological Data .....................................................................................35
Appendix B - Workflow for Site Class Generation ..................................................................................37
B.1 Generating Geological Unit Reclassification Table .......................................................................37
B.2 Generating Zonal Statistics for the Weathering Intensity Index ....................................................38
B.2.1 Converting the WII from Continuous to Discrete Raster Data .................................................38
B.2.2 Generating Zonal Statistics ......................................................................................................39
B.3 FME® Desktop Workflow ...............................................................................................................39
A revised seismic site conditions map for Australia
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Abstract
The Australian Seismic Site Conditions Map (ASSCM) uses information about surficial geology
(regolith) as a proxy for the potential behaviour of geological materials under the influence of seismic
ground shaking, predominantly in the context of amplification of earthquake energy.
The ASSCM represents a revision and upgrade of the existing National Regolith Site Classification
Map of Australia (NRSCM) published in 2007 (McPherson & Hall, 2007). Key improvements to the
new product include the integration of new and updated geological data in a more robust spatial
framework. In addition, a fully documented workflow has been developed, enabling the product to be
efficiently updated as required.
Comparison of geological data sets of different scales demonstrates the power of larger spatial scale
data to provide better discrimination of site conditions – an important factor in determining the
reliability of hazard and risk modelling outputs. The implementation of a weathering intensity index
captures the reduced strength (and possible minor increase in ground shaking potential) of substrate
in bedrock-dominated environments. Accuracy and consistency checking of the ASSCM (and the
NRSCM) against geotechnical data from the Newcastle (NSW) and Perth (WA) regions shows good
agreement between the ASSCM site condition (site class) mapping and field measurement. In both
areas the ASSCM out-performs the existing NRSCM.
The ASSCM data set provides a robust and publicly available representation of Australian seismic site
conditions. The generally improved resolution and spatial reliability relative to the existing NRSCM
establish the ASSCM as a key input for incorporating seismic site conditions into seismic risk and
impact analysis and modelling in Australia. It is, however, critical to recognise the limitations of this
national-scale product when applying at local scales.
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1 Introduction
1.1 History of the existing product
The Australian Seismic Site Conditions Map (ASSCM) uses information about the surficial geology
(regolith) in any given location as a proxy for the potential behaviour of geological materials under the
influence of seismic (earthquake) ground shaking. The product represents a revision of the existing
National Regolith Site Classification Map of Australia (NRSCM) published in 2007 (Figure 1.1;
McPherson & Hall, 2007; 2013).
Figure 1.1 Existing Australian National Regolith Site Classification Map (McPherson & Hall, 2007) showing
modified National Earthquake Hazard Reduction Program (NEHRP) site classes.
The original rationale for the development of a national scale site conditions map for Australia is
documented in McPherson & Hall (2007). By way of summary, the modification of seismic energy most
often takes the form of amplification resulting from impedance contrasts between bedrock and any
overlying materials (e.g. sediments, soil) (Figure 1.2). The significance of this is that any amplification
may result in increased damage to structures exposed to the ground shaking. This phenomenon is
referred to in simple terms as ‘site response’, and variation in this property is commonly (but not
exclusively) expressed as time-averaged shear-wave velocity in the upper 30 m of the Earth (VS30).
Geological materials exhibiting similar ranges of VS30 values are often grouped into ‘site classes’
(Borcherdt, 1994) which reflect variation in site conditions. Lower VS30 values generally imply higher
amplification potential, and are usually associated with fine-grained, unconsolidated materials such as
sediments and soil (often referred to as ‘soft’ materials or ‘soft soil’), while higher VS30 values are
usually associated with more competent (‘stiff’) materials such as bedrock (Kramer, 1996).
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A revised seismic site conditions map for Australia
Figure 1.2 Diagram illustrating the effect of impedance on earthquake shear-wave energy. In this example, an
increase in wave amplitude coincides with the transition from higher velocity rock (high impedance) to lower
velocity (low impedance) sediment (McPherson & Hall, 2007).
The site classes and defining parameters used to characterise their potential site response for
earthquake hazard purposes were originally defined by the United States National Earthquake Hazard
Reduction Program (NEHRP) (Building Seismic Safety Council, 2001; 2004; 2009; 2015) (Table 1.1).
These classes were later modified by Wills et al. (2000) (Table 1.2) to reflect a greater overlap in the
range of Vs30 values across a range of material types and ages, as shown by direct measurement of
several hundred shear-wave velocity profiles in California.
In Australia, our understanding of the relationship between geological materials and their response to
earthquake ground shaking is generally poor. This is due to both a paucity of ground motion data and
a lack of available geotechnical and geophysical data (particularly shear-wave velocity data) that could
be used to define typical VS30 ranges for different near-surface materials (McPherson & Hall, 2013).
Accordingly, the site classes applied in both the original NRSCM, and in the revised ASSCM
presented here, are essentially the modified NEHRP classes defined by Wills et al. (2000). However,
Australia’s stable continental setting means that a variety of regolith types are present which may not
be accurately classified by the Wills et al. (2000) Californian-based scheme, and consequently it is
necessary to adjust the site class definitions to account for some of these differences. Accordingly,
Table 1.3 has been modified slightly from Table 8 of McPherson & Hall (2007) to better reflect
geological age information and provide more explicit examples of material types.
In addition to accounting for the various geological material types, consideration was given to the
appreciable thicknesses of in-situ weathered material that can occur in different parts of the continent.
For example, weathering profiles in the Yilgarn Craton of Western Australia can exceed 70 m (Anand
& Paine, 2002), while profiles developed in Tertiary basalts in Queensland can be up to 40 m thick
(Willey, 2003). Where appropriate, site classes for the bedrock-dominated materials (i.e. Class B)
were adjusted by either a half site class unit (e.g. B to BC) or a full site class unit (e.g. B to C) to take
into account any possible in-situ weathering (McPherson & Hall, 2007) (refer to Appendix B for further
details). This was considered to be potentially important as the presence of such material can affect
the calculated earthquake site response (Davis, 1995; Steidl et al., 1996; Anbazhagan et al., 2013;
Manandhar et al., 2016), with period-dependent average acceleration response spectra on weathered
rock sites shown to be up to 20% higher than those at competent rock sites (Idriss & Silva in
Rodriguez-Marek et al., 2001; Somerville & Abrahamson in Rodriguez-Marek et al., 2001).
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Table 1.1 NEHRP site class definitions (Building Seismic Safety Council, 2001; 2004; 2009; 2015).
NEHRP Site Class
VS30 (m/s)
Material
A
> 1500
Hard rock
B
760 - 1500
Firm to hard rock
C
360 - 760
Very dense soil / soft rock
D
180 - 360
Stiff soil
E
< 180
Soft soil
F
-
Soils requiring site-specific testing and evaluation
Table 1.2 Modified NEHRP site classes, associated VS30 values and general groupings of geologic materials
associated with each class, based on 556 measured profiles from California (Wills et al., 2000).
Modified NEHRP Site Class
VS30 (m/s)
Geological Materials
B
> 760
Plutonic & metamorphic rocks; most volcanic rocks;
coarse-grained sedimentary rocks Cretaceous &
older
BC
555 - 1000
Franciscan Complex rocks except ‘melange’ and
serpentine; crystalline rocks of the Transverse
Ranges which tend to be more sheared;
Cretaceous siltstones or mudstone
C
360 - 760
Franciscan melange and serpentine; sedimentary
rocks of Oligocene to Cretaceous age, or younger
coarse-grained sedimentary rocks
CD
270 - 555
Sedimentary rocks of Miocene and younger age,
unless formation is notably coarse-grained; PlioPleistocene alluvial units; older (Pleistocene)
alluvium; some areas of coarse younger alluvium
D
180 - 360
Younger (Holocene) alluvium
DE
90 - 270
Fill over bay mud in the San Francisco Bay area;
fine-grained alluvial and estuarine deposits
elsewhere along the coast
E
< 180
Bay mud and similar intertidal mud
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A revised seismic site conditions map for Australia
Table 1.3 Modified version of the Wills et al. (2000) site classification scheme as applicable to Australian
geological conditions (after McPherson & Hall, 2007).
Modified NEHRP Site Class
VS30 (m/s)
Geological materials
B
>760
Fresh to moderately weathered rock units, including
plutonic and metamorphic rocks, most intrusive and
extrusive volcanic rocks, and coarse-grained
sedimentary rocks Cretaceous and older
BC
555-1000
Highly weathered rock units; some Cenozoic
volcanic rocks
C
360-760
Extremely weathered rock units; sedimentary rocks
of Paleogene age; younger coarse-grained
sedimentary rocks (defined as units dominated
texturally by coarse-sand, gravel and/or
conglomerate through to boulders, as well as
moderately to heavily indurated materials, e.g.
silcrete)
CD
270-555
Sedimentary rocks of Neogene and younger age,
unless notably coarse-grained (refer to C); PlioPleistocene sedimentary units including older
(Pleistocene) alluvium and colluvium; some coarse
younger materials
D
180-360
Late Pleistocene to Holocene sediments (e.g.
alluvium, colluvium, dunes, lunettes deposits)
DE
90-270
Fine-grained alluvial, lacustrine, deltaic and
estuarine sediments
E
<180
Inter-tidal and back-barrier swamp (lagoonal)
deposits
1.2 Context for revision
The validity of characterising site conditions using VS30 (or proxies for VS30) has been questioned in
recent times (e.g. Benjumea et al., 2008, Castellaro et al., 2008; Lee & Trifunac, 2010; Luzi et al.,
2011; McPherson & Hall, 2013; Thompson & Wald, 2016, Volti et al., 2016), and proxies other than
geology have been suggested (e.g. Wald & Allen, 2007; Allen & Wald, 2009; Hassani & Atkinson,
2016). In the Australian context, however, there is still a general absence of suitable geotechnical or
geophysical data to permit the application of any viable alternative method. Notwithstanding its known
limitations, the geological proxy-based product is still considered adequate for site condition
classification for building code purposes (Idriss, 2011). The new product is referred to herein as the
Australian Seismic Site Conditions Map (ASSCM), bringing the name of the product into line with the
nomenclature more commonly found in the literature (e.g. Wills et al., 2000, Wills & Clahan, 2006;
Wald & Allen, 2007; Allen & Wald, 2009).
Key updates incorporated into the Australian Seismic Site Conditions Map (ASSCM) include:

Integration of new and revised geological data published since 2007 (refer to Appendix A), along
with a quantitative, nationally-consistent product for characterising weathering of bedrock.

Improved spatial referencing of the input data, reducing errors resulting from merging data sets of
different vintage and varying spatial scale.

A streamlined workflow, with accompanying documentation, enabling more effective reproduction
and modification of the product in future (Appendix B).
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2 Input Geological Datasets
Figure 2.1 provides a visual representation of the distribution and scale of the input geological data
sets incorporated into the ASSCM. A full list of input geological data sets can be found in Appendix A.
Owing largely to the use of the Surface Geology of Australia 1:1,000,000 base data set, which is
compiled from 1:250,000 scale geological mapping (Raymond et al., 2012), the ASSCM provides
higher spatial definition baseline coverage across most of the country compared to the NRSCM
(McPherson & Hall, 2007; 2013).
The ASSCM takes advantage of several new or updated input data sets, including:

Expanded 1:50,000 scale coverage in the Perth region, which now extends from Cape Leschenault
(Guilderton) in the north to Cape Naturaliste (Busselton) in the south.

1:100,000 scale data for south-eastern South Australia and large areas of Queensland.

1:50,000 scale coverage across ~40% of Victoria.

1:25,000-1:250,000 scale coverage across eastern (UTM Zone 56) New South Wales,
incorporating coastal Quaternary geological mapping (Troedson & Hashimoto, 2008).
There are a small number of high resolution (i.e. ~1:10,000 scale) geological data sets incorporated in
the NRSCM which are absent from the ASSCM. Very localised areas in and around Launceston
(Geological Survey of Tasmania, 1996), Darwin (Vanden Broek, 1980) and Canberra (Henderson
1980, 1981, 1986) are not included, as the data are unavailable in digital form.
Figure 2.1 Distribution and scale of geological data sets incorporated in the Australian Seismic Site Conditions
Map.
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A revised seismic site conditions map for Australia
3 Workflow
The following sections provide a brief summary of the key steps in generating the ASSCM, covering:
1. Data preparation and spatial referencing.
2. Seismic site conditions classification.
3. Application of the weathering intensity index.
4. Data integration and quality control.
A detailed workflow describing the revised processes is presented in Appendix B.
3.1 Data preparation and spatial referencing
Eight input geological data sets were sourced from their respective custodians (refer to Appendix A for
details) in Esri® ArcGIS™ compatible formats (shapefile or geodatabase). Data sets which were not
already in ESRI file geodatabase format were converted to that form. All data sets were converted into
geographic coordinates with reference to the Geocentric Datum of Australia 1994 (GDA 94).
An inspection of the extents of each data set was undertaken with reference to the coastline as
represented in the Australian Standard Geographical Classification (ASGC) Digital Boundaries – State
and Territory (Australian Bureau of Statistics, 2011). This was completed primarily to test the integrity
of the data with respect to the coastline, where many areas of high population and infrastructure
exposure occur. This requirement came about as a result of the identification of isolated spatial
discrepancies in the NRSCM, such as the example in Figure 3.1 from the western Eyre Peninsula in
South Australia. These examples result in spatial mismatches of up to 3.5 km between the coastline
and the boundaries of the input geological data set. Similar checks for the ASSCM (refer to Section
3.4) found that the spatial properties of the input data sets resulted in some small over- and under-lap
between the polygons and the coastline, although the scale of these errors was significantly less than
that in the existing NRSCM (compare Figure 3.1a & b). No attempt was made to modify any of the
spatial extents of the input geological data.
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Figure 3.1 Comparison of (a) NRSCM and (b) ASSCM outputs for the western Eyre Peninsula (South Australia).
Spatial discrepancies of up to 3.5 km between the coastline and edge of geological data of have propagated
through from the original NRSCM input data (see (a)). Note the greater level of detail and more accurate match to
the coastline in the equivalent ASSCM data.
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3.2 Seismic site conditions classification
Site condition classification of each geological data set was undertaken manually by a geologist. The
attribute table for the data set was exported from the spatial data set in Esri® ArcMap™ to Microsoft®
Excel, and the geological mapping units filtered using a pivot table. Each geological unit was assigned
a site class on the basis of the criteria in Table 1.3. The Surface Geology of Australia 1:1,000,000 data
set (Raymond et al., 2012) was classified first to provide a ‘base map’ at national scale. Larger spatial
scale (higher definition) data sets were then classified and quality controlled using the national scale
data set as a reference frame.
A class labelled ‘OTHER’ was introduced to capture poorly attributed geological units and structures or
landforms resulting from human processes (e.g. mining stockpiles, fill, quarried deposits, dam walls,
airport runways, etc.), which are difficult to classify without specific knowledge. It also permits a level
of screening of water bodies (e.g. lakes, reservoirs) and geological units beyond the shoreline (e.g.
tidal delta deposits) which may have been captured in the original geological data. In order to facilitate
spatial and numerical analysis a numerical code equivalent (Table 3.1) was also assigned to each
unit. The updated attribute data were then re-joined to the spatial data in ArcMap.
Table 3.1 Numeric and alpha codes assigned to the reclassified geological data.
Modified NEHRP Site Class
Numeric ID
B
0
BC
1
C
2
CD
3
D
4
DE
5
E
6
OTHER
999
Figure 3.2 and Figure 3.3 respectively show the existing Australian National Regolith Site
Classification Map (McPherson & Hall, 2007) and the new Australian Seismic Site Conditions Map
(ASSCM). It is important to recognise that the NRSCM (Figure 3.2) has had a weathering modifier
applied, whereas in this example the ASSCM has not. A key observation is that thee ASSCM (Figure
3.3) provides a notable increase in detail across large areas of the country; mainly the result of the
aggregation of 1:250,000 scale geological information which underpins the Surface Geology of
Australia 1:1 million scale data set.
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Figure 3.2 National Regolith Site Classification Map of Australia (NRSCM - McPherson & Hall, 2007). This
product has a weathering modifier applied.
Figure 3.3 Australian Seismic Site Conditions Map (ASSCM) with no weathering intensity index applied.
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A revised seismic site conditions map for Australia
3.3 Application of the weathering intensity index
The weathering intensity index (WII) data set of Wilford (2012) was developed at a 100 m resolution
using regression models based on airborne gamma-ray spectrometry imagery and the Shuttle Radar
Topography Mission (SRTM) elevation data. The reader is referred to the original paper for further
detail. It is important to recognise that the WII was developed primarily for characterising erosional
landscapes (Wilford, 2012), and thus its application is restricted to bedrock-dominated ‘erosional’
environments (i.e. site class B).
The WII data set required conversion from continuous to discrete raster data (i.e. floating point to
integer values) and categorisation of pixel values such that areas of limited, moderate and intense
(strong) weathering are represented (Figure 3.4). Weathering class values from Wilford (2012), which
draw heavily on the weathering classes of Eggleton (2001), were aggregated as shown in Table 3.2.
Table 3.2 Calculated discrete weathering intensity index (WII) integer values and corresponding aggregate class
values.
Weathering Intensity
Weathering Intensity Index Value(s)
Aggregate Class Value
Limited
0-4
0
Moderate
5-6
1
Strong
7
2
NoData
NoData
The aggregated WII raster data set was then analysed in conjunction with each input geological data
set. For every feature (i.e. polygon) in the geological input data set the statistical majority raster value
was calculated, and that value assigned to the feature. The final step in the process was to convert the
numeric site class values (with or without a WII modifier applied) back into modified NEHRP site class
text (alpha) values (refer to Table 1.3).
The Australian Seismic Site Conditions Map (ASSCM), with the weathering intensity index applied, is
presented in Figure 3.5. Comparison with the unmodified ASSCM (Figure 3.3) shows that many
bedrock-dominated areas (i.e. site class B in Figure 3.3) are modified to some extent by application of
the weathering index, most notably in the western and northern parts of the continent. In most
circumstances the application of the weathering index results in a half site class unit increase (e.g. B
to BC; Figure 3.4) indicating a shift towards less stiff (‘softer’) site conditions.
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Figure 3.4 Weathering intensity index data (Wilford, 2012) aggregated into classes representing the degree of
weathering and corresponding modification factor for site class B polygons; 0 = limited weathering with no
change, 1 = moderate weathering with half site class unit change (i.e. B to BC), 2 = strongly weathered with full
site class unit change (i.e. B to C).
Figure 3.5 Australian Seismic Site Conditions Map with weathering intensity index applied. When compared with
the unmodified ASSCM (Figure 3.3) note the relative increase in areas of site classes BC and C.
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3.4 Data integration and quality control
The final data set was compiled by integrating the site condition maps generated from each input data
set. This combined data set was then subjected to a series of checks to confirm the validity of it spatial
and attribute characteristics.
3.4.1 Data merge and checks
In terms of prioritising the data sets for the integration process the following rules were implemented:

Any data set at a scale larger than 1:1,000,000 scale (e.g. 1:250,000, etc.) takes priority over the
1:1,000,000 scale data.

The New South Wales UTM Zone 56 seamless geology (1:25,000-1:250,000) data set takes
priority over the 1:100,000 scale Queensland data set where they overlap in the border region.
This is to preserve the more detailed 1:25,000 scale mapping in the [higher exposure] coastal
areas of northern NSW.

1:50,000 scale data for Victoria takes priority over the 1:250,000 scale data for the same area.
1
A Python script was developed to combine each of the input reclassified site condition data sets
(detailed in Appendix A). The order in which the data sets were updated is as below. This order is
important as data sets further down the list will overwrite data sets above in the list, if they overlap.
1. Surface Geology of Australia, 1:1 000 000
2. Northern Territory, 1:100 000
3. Victoria - Seamless Geology ,1:250 000
4. Queensland - Detailed surface geology , 1:100 000
5. South Australia - Surface Geology , 1:100 000
6. New South Wales Zone 56 Seamless Geology, (1:25 000 – 1:250 000)
7. Victoria - Seamless Geology, 1: 50 000
8. Perth Metropolitan Region, 1:50 000 environmental geology series
9. Tasmania - digital geology, 1:25 000
Quality control was managed within the Python script by way of completing a spatial join following
each data set being updated into the emerging national data set. The number of features selected
with the spatial join was compared to the number of features in the source data set. As a second test,
the new rows in the emerging national data set were calculated and backwards matched to the source
data set. This method therefore imposed both a spatial and an attribute check.
The results of the quality control process indicated that, as expected, the only data sets that were
overwritten in part were:

Surface Geology of Australia, 1:1 000 000

Victoria - Seamless Geology 1:250 000

Queensland - Detailed surface geology 1:100 000
There were some minor variations in the results of the spatial join versus attribute query for the New
South Wales data (20 features) and the Queensland data (4 features). These features were visually
1
https://github.com/GeoscienceAustralia/SpatialApps/commit/63c589e1ca661b6c685d864dfd692d75dd9bf4d2
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inspected and due to the attribute query returning the same number of rows as the source data sets
these spatial join count variations were considered to be minor feature geometry variations violating
the rules of the spatial join. The spatial join selected features where they were identical to the source
data set.
A further visual inspection of the national data set confirms the following priority has been observed
with regard to data overwrites:

New South Wales Zone 56 Seamless Geology (1:25 000 – 1:250 000) has taken priority over the
Queensland - Detailed surface geology 1:100 000;

Victoria - Seamless Geology 1: 50 000 has taken priority over the Victoria - Seamless Geology
1:250 000.
This priority is as per the process design.
A visual inspection of the data set identified a small number of polygons (< 6) with WII_NEHRP
attribute field values equal to ‘Null’. Such features were subsequently identified as being in the source
data and were addressed as below:

SA – delete the ‘Null’ polygons on the eastern boundary of the data set (these polygons actually
exist across the border in Victoria);

WA – recalculate ‘Null’ to ‘OTHER’ following visual inspection and confirmation of the ground
conditions.
The Python script was then re-run to regenerate the combined national data set.
Final checks confirmed the following.

No records were attributed with null site class values.

Higher definition data sets are present in the national data set (visual check).

Feature outlines and attribution are consistent between source and resulting national data set
(visual check).
3.4.2 Spatial reference check
The variability in spatial scales of the input geological data made it necessary to cross-check the
combined ASSCM output against a standard spatial reference frame. The Australian Standard
Geographical Classification (ASGC) Digital Boundaries – State and Territory (Australian Bureau of
Statistics, 2011) was utilised as it represents an accepted geographical and spatial standard.
The combined ASSCM was clipped using the ASGC data set, and the resulting slivers assessed. To
test the validity of the final data set from an end-user perspective, the sliver data was also intersected
with the National Exposure Information System (NEXIS) (http://www.ga.gov.au/scientifictopics/hazards/risk-impact/nexis) building exposure (point) data set.
The results of the analyses demonstrate that while the inevitable sliver data set was spatially
extensive (mainly around the coastline) the actual discrepancies introduced by the ASGC clip are, at
most, in the order of a few hundred metres horizontally. Visual inspection shows that the largest
slivers (by area) usually relate to geological mapping units that cross the coastline, and in some
instances where units are mapped offshore (e.g. between the mainland and Fraser Island in central
Queensland), or areas where the ASGC data set records the presence of offshore islands not
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A revised seismic site conditions map for Australia
captured in the input geological data (e.g. coral atolls of the northern Great Barrier Reef, offshore
Queensland).
Comparison of the ASGC-clipped data against the building exposure point data (Figure 3.6)
demonstrates that out of a NEXIS data set of more than 8.6 million buildings, only 0.013% (1134
buildings) fall outside the boundaries defined by the input geological data, but within the boundaries
defined by the ASGC data. The number of NEXIS structures identified as falling outside the
boundaries of the ASGC, but within the boundaries defined of the geological data was two orders of
magnitude smaller; these 6 buildings accounting for just 0.000069% of the NEXIS data set.
Two important conclusions can be drawn from these results.
1. The spatially integrity of the ASSCM data clipped to the ASGC data is high.
2. It is critical to recognise the composition and limitations of this national-scale product when
applying at local scales.
Figure 3.6 Map showing the relative occurrence of NEXIS buildings captured by sliver polygons resulting from the
intersection of the ASSCM and ASGC data sets. Points indicate the presence of features, not the number of
features. Blue triangles indicate buildings that fall outside the bounds of the ASGC data, but within the bounds of
the ASSCM; red circles indicate buildings that fall inside (within) the bounds of the ASGC data, but outside the
bounds of the ASSCM. Total numbers of for each type are given in the legend.
3.4.3 Coastline clip
The final step involved clipping the national data set using a modified version of the Australian
Standard Geographical Classification (ASGC) Digital Boundaries – State and Territory (Australian
Bureau of Statistics, 2011). This ensured that the spatial extent captured only conterminous Australia
(i.e. mainland Australia, Tasmania and near-shore islands), consistent with the previous product.
Accordingly, the modification of the ASGC data set involved the removal of polygons representing all
A revised seismic site conditions map for Australia
15
offshore External Territories (plus Lord Howe Island, which is part of New South Wales). This includes
Christmas Island, Macquarie Island, Heard Island, Norfolk Island and numerous coral cays off the east
coast of northern Queensland.
16
A revised seismic site conditions map for Australia
4 Sensitivity Testing of Higher Resolution Data
While the ability to produce a consistent continental-scale product has value for national- to regionalscale seismic hazard and risk assessments, perhaps a more important goal is to generate site
condition (site class) data at the largest spatial scale possible (subject to the limitation of the available
geological input data). In this section we utilise three different spatial scales of geological data for
Victoria (or parts thereof) and explore what influence input data scale has on the distribution and
proportional representation of different site classes for two population centres. This is significant as the
occurrence (or non-occurrence) of different classes, particularly those representing softer materials
with a higher probability of amplifying ground shaking, may impact hazard and damage estimates
more significantly.
Applying the method outlined in Section 3 the following datasets were included in the analysis:

Surface Geology of Australia 1:1,000,000 (Raymond et al., 2012).

Seamless Geology of Victoria (including 1:250,000 and 1:50,000 data) (Victorian Department of
State Development, Business and Innovation, 2014).
For the purpose of this exercise data sets have been attributed with the modified NEHRP classification
only (i.e. no weathering intensity index is applied).
4.1 Bendigo
The City of Greater Bendigo in central Victoria (Figure 4.1) is one of the largest non-metropolitan
centres in the state, with a population of over 80,000 people. The region has historical and current
associations with agriculture and mining, and as a result of rapid, gold-rush fuelled growth from the
mid-1800’s, an appreciable proportion of the older building stock comprises seismically vulnerable
unreinforced masonry structures. The city is not adjacent to any major river, but is drained by several
smaller tributary streams, and as a result of the early mining has notable areas of disturbed ground,
including mullock heaps (mining spoil) and alluvium redistributed by gold working. Many of the gold
prospects worked alluvial gold in buried paleochannels – former river channels now buried several
metres to tens of metres below the ground surface (for examples refer to Canavan, 1988).
The three scales of geological data available for the area—1:1,000,000, 1:250,000 and 1:50,000—
have been re-mapped to modified NEHRP site classes. Figure 4.2 presents the three data sets and
demonstrates visually the difference that the larger spatial scale data produces in terms of the area
attributable to each site class, particularly in terms of an increase in the area assigned to softer site
classes.
Statistics derived from the spatial data (Table 4.1) support the visual assessment, with much of the
reduction in area of site class B being accounted for by an increase in site class CD—a consequence
of higher resolution input data which better discriminates geological materials. The significance of this
observation is that scenario earthquake hazard modelling may be influenced by the scale of the
geological data upon which the site class map is based, with a relative increase in area of softer site
classes (in this case, site class CD) ultimately favouring a higher (more conservative) hazard estimate.
A revised seismic site conditions map for Australia
17
Figure 4.1 Location and extent of the Greater Bendigo region of central Victoria (red polygons).
Table 4.1 Calculated areas (km2) for each modified NEHRP site class at different spatial scales (1:1,000,000,
1:250,000 and 1:50,000) for the Greater Bendigo region. The percentage of the total area (45.2 km2) covered by
each site class at each scale is also presented. Note the relative decrease in areas of bedrock-dominated site
classes (e.g. B, C) and the increased occurrence of softer site classes (e.g. CD, D) with the inclusion of more
detailed geological mapping data.
1:1,000,000
% total area
1:250,000
% total area
1:50,000
% total area
B
41.7
92.3
37.9
83.8
31.5
69.7
BC
0
0
0
0
0
0
C
0.6
1.3
0.1
0.2
0.1
0.3
CD
2.9
6.3
6.3
14.0
7.0
15.5
D
0
0
0.9
1.9
6.6
14.5
DE
0
0
0
0
0
0
E
0
0
0
0
0
0
Site Class
18
A revised seismic site conditions map for Australia
Figure 4.2 Seismic site conditions maps for the Greater Bendigo region of central Victoria at (a) 1:50,000, (b)
1:250,000 and (c) 1:1,000,000 scales.
A revised seismic site conditions map for Australia
19
4.2 Melbourne
The City of Greater Melbourne is the metropolitan capital and the largest city in Victoria (Figure 4.3).
With a population of more than 4 million people, it is also the second largest city in Australia. The
region has historical and current associations with manufacturing, trade and finance. Similar to
Bendigo, much of Melbourne’s historical development was fuelled by the Victorian gold rushes of the
1800’s, and as a result older structures in Melbourne are also commonly built of unreinforced
masonry. The city centre is located at the confluence of the Yarra River and Port Phillip Bay. Various
parts of the city, particularly the CBD, are known to be built upon Neogene and younger sediments,
particularly in proximity to Port Phillip Bay (Geological Survey of Victoria, 1967).
Figure 4.3 Location and extent of the Greater Melbourne area of Victoria (red polygons). Grey outlined boxes
indicate the extent of 1:50,000 scale geological mapping data within the vicinity of Greater Melbourne.
Two scales of geological data are available for the area—1:1,000,000 and 1:250,000, while the
1:50,000 scale data provide only partial coverage in the north of the area (Figure 4.3). Accordingly, the
modified NEHRP site class maps for the different scales of data (Figure 4.4) have been limited to the
extent of the 1:50,000 data (refer to Figure 4.3) in order to permit comparison across scales.
Statistics derived from the spatial data (Table 4.2) again demonstrate the difference that the higher
resolution mapping produces in terms of the area attributable to each site class. Areas assigned to site
class B show little variation across scales, however the more detailed geological mapping results in
the identification of (albeit small areas) of much softer site classes (i.e. DE, E). Critically, these softer
site class areas occur close to either the CBD or adjacent populated areas. The significance of this
outcome is that any relative increase in area of softer site classes will influence the ground-shaking
potential and ultimately favour higher (more conservative) hazard estimates and earthquake-related
losses.
20
A revised seismic site conditions map for Australia
Table 4.2 Calculated areas (km2) for each modified NEHRP site class at different spatial scales (1:1,000,000,
1:250,000 and 1:50,000) for the Greater Melbourne region (within the bounds of 1:50,000 scale mapping). The
percentage of the total area (1233.4 km2) covered by each site class at each scale are also presented. Note the
relative consistency in the proportion of site class B across spatial scales, and the increasing occurrence of softer
site classes (e.g. DE, E) with the availability of more detailed geological mapping data.
1:1,000,000
% total area
1:250,000
% total area
1:50,000
% total area
B
816.3
66.2
793.3
64.3
799.8
64.9
BC
0
0
0
0
0
0
C
235.9
19.1
6.9
0.6
7.5
0.6
CD
0
0
238.4
19.3
217.1
17.6
D
181.2
14.7
157.6
12.8
174.2
14.1
DE
0
0
0
0
0.6
<0.1
E
0
0
37.1
3.0
33.8
2.7
OTHER
0
0
0
0
0
0
Site Class
A revised seismic site conditions map for Australia
21
Figure 4.4 Seismic site conditions maps for the northern part of the Greater Melbourne region at (a) 1:50,000, (b)
1:250,000 and (c) 1:1,000,000 scales. Refer to Figure 4.3 for spatial context (i.e. limit of 1:50,000 mapping).
22
A revised seismic site conditions map for Australia
4.3 Influence on amplification factors
One of the potential applications of the ASSCM is in the development of amplification factors for use in
seismic hazard modelling. To demonstrate the influence on variation in ground-shaking amplification
we apply the non-linear site amplification model of Seyhan & Stewart (2014) for site classes BC and
softer, while the ground-motion prediction equations of Atkinson & Boore (2006) are used for site class
B. The calculations apply representative VS30 values for each site class (Table 4.3), with B and E using
the values of Seyhan & Stewart (2014), normalised to a reference shear-wave velocity of 760 m/s for
site class BC, for a range of periods and peak ground accelerations (PGAs).
Table 4.3 Site class and representative VS30 values used to generate amplification factors.
Site Class
Representative VS30 (m/s)
B
1100
BC
760
C
560
CD
412
D
270
DE
180
E
115
Figure 4.5 shows plots of calculated amplification factors relative to reference BC site condition for
PGAs of 0.1 g and 0.4 g at a range of periods. For the lower PGA, at periods significant to low-rise
structures (e.g. 0.2 s), amplification exceeding the reference site condition by up to 70% is predicted,
while at periods significant to high-rise structures (e.g. at 1.0 s) amplification is up to 4 times.
Conversely, at higher ground-shaking, significant de-amplification in the softer soil classes is predicted
for shorter periods. These results further demonstrate the importance of capturing the maximum
possible level of spatial detail in the input geological data, as the relative presence/absence of
different site classes may result in spurious estimates of ground-shaking hazard and biased
assessments of relative exposure.
Figure 4.5 Amplification factors calculated relative to a reference BC site condition for PGA values of 0.1 g and
0.4 g using the models of Sayhen & Stewart (2014) and Atkinson & Boore (2006).
A revised seismic site conditions map for Australia
23
5 The Australian Seismic Site Conditions Map
5.1 Comparison between the ASSCM and NRSCM
Figure 5.1 presents the new Australian Seismic Site Conditions Map (ASSCM). Comparing this
against the existing Australian National Regolith Site Classification Map (NRSCM; McPherson & Hall,
2007) (Figure 5.2) the main observation is that the ASSCM represents a more conservative product,
particularly from a building code and hazard modelling perspective. There is a significant reduction in
the areas covered by intermediate the site classes (BC, C, CD) balanced by an increase in the ‘rock’
site class B (Table 5.1). Perhaps more importantly, however, there is a significant increase in the
areas mapped to softer site conditions, particularly site class D (orange colour). The prevalence of site
class D is consistent with published data from numerous studies (e.g. Wills et al., 2000; Chapman et
al., 2006; Wills & Clahan, 2006; Holzer, 2010; Volti et al., 2016) reporting that (Late) Pleistocene and
younger sediments from a variety of geomorphic environments relatively consistently record VS30
values within a range of approximately 150-300 m/s.
Table 5.1 Comparison of total area (in square kilometres) for each site class in both the existing National Regolith
Site Classification Map (NRSCM) and the new Australian Seismic Site Conditions Map (ASSCM). Note the
relative increase in area of site class B and the softer site classes (D, DE, E) at the expense of the intermediate
classes (BC, C, CD). Total areas are not equivalent owing to the spatial extents of the respective data sets.
Site Class
NRSCM (area – km2) ASSCM (area – km2)
B
767,053
1,418,360

BC
1,161,114
74,1334

C
1,203,874
49,3044

CD
2,737,842
1,716,865

D
1,776,573
3,148,340

DE
19,344
116,711

E
292
51,126

OTHER (e.g. water)
5678
213

TOTAL
7,671,770
7,685,992
Increase/Decrease
The ASSCM as a whole generally provides a better coverage of larger spatial scale data across most
of the country, owing largely to the Surface Geology of Australia 1:1,000,000 base data set, which is
compiled from 1:250,000 scale geological mapping (Raymond et al., 2012). There are a small number
of very large spatial scale (e.g. 1:10,000) geological data sets incorporated in the NRSCM which are
absent from the ASSCM. Localised areas within Launceston (Geological Survey of Tasmania, 1996),
Darwin (Vanden Broek, 1980) and Canberra (Henderson 1980, 1981, 1986), which have isolated
engineering geology mapping at scales of ~1:10,000, are not included owing to the data being
unavailable in digital form.
24
A revised seismic site conditions map for Australia
Figure 5.1 The Australian Seismic Site Conditions Map (with weathering index applied).
Figure 5.2 Existing National Regolith Site Classification Map of Australia (McPherson & Hall, 2007).
A revised seismic site conditions map for Australia
25
5.2 Comparison against measured VS30 data
In order to test for accuracy and consistency, both the ASSCM and the existing NRSCM were
compared against measured geotechnical data for the Newcastle (NSW) and Perth (WA) regions - two
of only a few locations in Australia where such data are available. Specifically, the geotechnical data
used are averaged VS30 measurements acquired from seismic cone penetration tests (SCPT). For
further details the reader is referred to Crumb (2001) and McPherson & Jones (2005) respectively.
In both areas, comparison of the ASSCM against these point data suggest that the new product
provides a good match to the field measurements and also appears to out-perform the existing
NRSCM (Figure 5.3 and Figure 5.4). This distinction is particularly notable in Newcastle, where the
ASSCM more accurately captures the sediments (modified NEHRP site class D) proximal to
outcropping bedrock (compare Figure 5.3a & b). This change may also reflect updated geological
mapping in that area, providing better discrimination of these materials.
In the Perth region the increased spatial coverage of modified NEHRP site class D (compare Figure
5.4a & b) provides better agreement with the distribution of measured VS30 values.
The most notable issue highlighted by this study is the difficulty in applying the modified NEHRP site
classes. From the test cases in Newcastle and Perth, VS30 values suggest that a majority of sites could
fall into either class D or DE on the basis of the published cut-off values (refer to Table 1.3Table 1.3
Modified version of the Wills et al. (2000) site classification scheme as applicable to Australian
geological conditions (after McPherson & Hall, 2007).). The impact of such decisions on hazard and
risk modelling estimates has yet to be quantified.
26
A revised seismic site conditions map for Australia
Figure 5.3 Seismic site conditions maps for the Newcastle region comparing (a) the new ASSCM and (b) the
existing NRSCM. Coloured points represent seismic cone penetration test (SCPT) locations labelled with the VS30
value (in units of m/s) for each site. Points coloured blue indicate VS30 values between 180-270 m/s (i.e. sites
which could be considered site class DE on the basis of the SCPT measurements).
A revised seismic site conditions map for Australia
27
Figure 5.4 Seismic site condition maps for the Perth region comparing (a) the new ASSCM and (b) the existing NRSCM. Coloured points represent seismic cone penetration
test (SCPT) locations labelled with the VS30 value (in units of m/s) for the site. Points coloured blue indicate VS30 values between 180-270 m/s (i.e. sites which could be
considered site class DE on the basis of the SCPT measurements).
28
A revised seismic site conditions map for Australia
6 Summary and Future Work
6.1 Summary
Generating the revised seismic site conditions product for Australia—the Australian Seismic Site
Conditions Map (ASSCM)—provided an opportunity to:

Integrate both new and updated geological data at a range of scales.

Apply a nationally-consistent and quantitative measure of regolith weathering distribution and
intensity.

Create a documented workflow for the development of the product.
The application of a weathering intensity index captures the reduced strength (and probable minor
influence on ground shaking potential) of substrate in bedrock-dominated environments. Comparisons
of variable resolution geological input data sets demonstrate the importance of incorporating as much
high resolution data as possible in order to best represent the variation in site conditions. Validation of
the ASSCM site condition classification using available field data suggest that the map more
accurately represents the distribution of measured values than the existing NRSCM. As a result of the
relative increase in areas mapped to softer site class units, the ASSCM also represents a more
conservative product than the NRSCM, particularly from a building code and hazard modelling
perspective. ,.
Despite the limitations of the geological proxy method for estimating ground shaking potential, and the
absence of a small number of higher spatial resolution geological data sets (of very limited extent), the
new ASSCM represents a repeatable and publicly available data set representing Australian seismic
site conditions. The generally improved resolution and spatial reliability relative to the existing NRSCM
establish the ASSCM as a key input for incorporating seismic site conditions into seismic risk and
impact analysis and modelling in Australia.
The Australian Seismic Site Conditions Map digital data is available in ESRI (file geodatabase and
shapefile) and open-source OGC Geopackage formats through the Geoscience Australia website.
6.2 Future Work

Sensitivity testing of hazard and risk modelling estimates to the variable resolution geological data
might usefully quantify the value of investing in the digital capture of existing high resolution
geological data, such as engineering geology mapping in major population centres.

Acquisition of new geotechnical or geophysical data in key locations/environments could be
considered for further validation or modification of the product.
A revised seismic site conditions map for Australia
29
7 Acknowledgements
The author gratefully acknowledges the following significant contributions:

Dan Connolly for compiling many of the input geological data sets.

Dan McIlroy for support in developing the FME scripts.

Lisa Hall for geological classification cross-checks, technical review of the data product and
constructive review of this document.

Trevor Allen for generating the amplification factors and for useful feedback on the manuscript.

Duncan Moore and Paul Zanni for substantial effort in the integration and QA/QC of the final
product.
30
A revised seismic site conditions map for Australia
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Resources Tasmania, Hobart. http://maps.thelist.tas.gov.au/listmap/app/list/map?layoutoptions=LAYER_LIST_OPEN&cpoint=147.43,42.85,10000&srs=EPSG:4283&bmlayer=3&layers=263. Last accessed 20 December 2016.
Pietsch, B. A. 1983. Darwin 1:100,000 geological sheet 5073. 1:100,000 Geological Series. Northern
Territory Geological Survey, Darwin.
Pietsch, B. A. 1985. Koolpinyah 1:100,000 geological sheet 5173. 1:100,000 Geological Series.
Northern Territory Geological Survey, Darwin.
Pietsch, B. A. 1986. Bynoe 1:100,000 geological sheet 5072. 1:100,000 Geological Series. Northern
Territory Geological Survey, Darwin.
Raymond, O. L., Liu, S., Gallagher, R., Zhang, W. and Highet, L. M. 2012. Surface Geology of
Australia 1:1 000 000 scale dataset (2012 edition). http://www.ga.gov.au/metadatagateway/metadata/record/74619/. Last accessed 19 December 2016.
Rodriguez-Marek, A., Bray, J. D. and Abrahamson, N. A. 2001. An empirical geotechnical seismic site
response procedure. Earthquake Spectra 17(1), 65-87. http://dx.doi.org/10.1193/1.1586167.
Seyhan, E. and Stewart, J. P. 2014. Semi-empirical nonlinear site amplification from NGA-West2 data
and simulations. Earthquake Spectra 30(3),1241-1256. http://dx.doi.org/10.1193/063013EQS181M.
State of Queensland. 2016. Detailed Surface Geology – Queensland [digital data set]. Department of
Natural Resources and Mines, Brisbane. http://qldspatial.information.qld.gov.au/catalogue//. Last
accessed 19 December 2016.
Steidl, J. H., Tumarkin, A. G. and Archuleta, R. J. 1996. What is a reference site? Bulletin of the
Seismological Society of America 86(6), 1733-1748.
Thompson, E. M. and Wald, D. J. 2016. Uncertainty in Vs30-based site response. Bulletin of the
Seismological Society of America 106 (2), 453-463. http://dx.doi.org/10.1785/0120150214.
Troedson, A. L. and Hashimoto T. R. 2008. Coastal Quaternary Geology — north and south coast of
NSW. Geological Survey of New South Wales Bulletin 34. Geological Survey of New South Wales,
Maitland. 94 p.
Vanden Broek, P. H. 1980. Engineering geology of the proposed Darwin East urban development
area, Northern Territory. Bureau of Mineral Resources, Geology and Geophysics Bulletin 203.
Australian Government Publishing Service, Canberra. 33 p.
Victorian Department of State Development, Business and Innovation. 2014. Victoria - Seamless
Geology 2014. Bioregional Assessment Source Dataset.
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Last accessed 19 December 2016.
Volti, T., Burbidge, D., Collins, C. D. N., Asten, M., Odum, J., Stephenson, W., Harris-Pascal, C. and
Holzschuh, J. 2016. Comparisons between Vs30 and spectral response for 30 sites in Newcastle,
Australia, from collocated seismic cone penetrometer, active- and passive-source Vs Data. Bulletin
of the Seismological Society of America 106(4), 1690-1709. http://dx.doi.org/10.1785/0120150073.
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Wald, D. J. and Allen, T. I. 2007. Topographic slope as a proxy for seismic site conditions and
amplification. Bulletin of the Seismological Society of America 97(5), 1379-1395.
http://dx.doi.org/10.1785/0120060267.
Wilford, J. 2012. A weathering intensity index for the Australian continent using airborne gamma-ray
spectrometry and digital terrain analysis. Geoderma 183-184, 124-142.
http://dx.doi.org/10.1016/j.geoderma.2010.12.022.
Willey, E.C. 2003. Urban geology of the Toowoomba conurbation, SE Queensland, Australia.
Quaternary International 103, 57-74. http://dx.doi.org/10.1016/S1040-6182(02)00141-6.
Wills, C.J., Petersen, M., Bryant, W.A., Reichle, M., Saucedo, G.J., Tan, S., Taylor, G. and Treiman, J.
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Appendix A - Sources of Input Geological Data
AUSTRALIA
Data Set
Scale
Reference
Surface Geology of Australia 1:1,000,000
(2012 edition)
1:1,000,000
Raymond et al. (2012)
Data Set
Scale
Reference
New South Wales Zone 56 Seamless
Geology**
1:25,0001:250,000
Colquhoun et al. (2015)
NEW SOUTH WALES
** As a result of structure of the data set, attribute tables were stripped of map symbol, geological unit,
unit description and age information and merged into a combined product for use in the reclassification
process.
VICTORIA
Data Set
Scale
Reference
Victoria - Seamless Geology 2014
1:250,000 &
1:50,000
Victorian Department of State Development,
Business and Innovation (2014)
Data Set
Scale
Reference
Perth Metropolitan Region 1:50 000
environmental geology series
1:50,000
Geological Survey of Western Australia (various
years)
Data Set
Scale
Reference
1:25,000 digital geology
1:25,000
Mineral Resources Tasmania (2010)
Data Set
Scale
Reference
Detailed surface geology – Queensland**
1:100,000
State of Queensland (2016)
WESTERN AUSTRALIA
TASMANIA
QUEENSLAND
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** Owing to a lack of detail in the NEHRP reclassified data relative to that produced from the Surface
Geology of Australia 1:1,000,000 data, the area covering the isolated Cunnamulla 1:250,000
geological sheet in southern-most central Queensland was removed from this data set.
NORTHERN TERRITORY
Data Set
Scale
Reference
Bynoe
1:100,000
Pietsch (1986)
Darwin
1:100,000
Pietsch (1983)
Koolpinyah
1:100,000
Pietsch (1985)
Noonamah
1:100,000
Doyle & Lally (2003)
Data Set
Scale
Reference
100K Surface Geology**
1:100 ,000
Geological Survey of South Australia (2012)
SOUTH AUSTRALIA
** Owing to inconsistencies between mapping units across numerous map sheet boundaries, this data
set was clipped to the extents covered by the eight 1:250,000 map sheets listed in the table below
(see also graphical representation). The intention was to maintain coverage across the region of
highest population and infrastructure density within the State, while eliminating the majority of data
with cross-boundary inconsistencies.
1:250,000 scale geological sheets (South Australia)
Adelaide
Kingscote-Barker
Naracoorte
Pinnaroo
Barker
Maitland-Kingscote
Penola
Renmark
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Appendix B - Workflow for Site Class Generation
This Appendix provides a documented workflow for generating modified National Earthquake Hazard
Reduction Program (NEHRP) (Building Seismic Safety Council, 2001; 2004) site class units (Wills et
al., 2000; this document) from digital geological map inputs using Esri® ArcGIS™, Microsoft® Excel
and FME® Desktop. The workflow permits the generation of both ‘raw’ sites classes (i.e. where no
weathering intensity index has been applied) and ‘weathering-modified’ NEHRP site classes, wherein
a weathering intensity index has been applied to the high shear wave velocity (i.e. bedrock) geological
units.
NOTES:
The steps outlined in B.1 and B.2 below must be completed prior to application of the FME® Desktop
workflow in B.3.

B.1 below must be completed for any input geological data set, while B.2 requires completion only
once (unless the weathering intensity index requires a different discrete categorisation).

Instructions below are written using the Surface Geology of Australia 1:1,000,000 data set as the
input geological data – attribute field names, such as ‘MAPSYMBOL’, will vary between input data
sets.
B.1 Generating Geological Unit Reclassification Table
Geological units within the input data set require interpretation and attribution with a modified NEHRP
site classification (Wills et al., 2000; McPherson & Hall, 2007). This is completed in both Esri®
ArcGIS™and Microsoft® Excel environments.

A copy of the full attribute table from the input geological data set exported from the source feature
class (geodatabase) in ArcMap and imported into Excel.

In Excel a Pivot Table is generated to summarise the ‘MAPSYMBOL’ (or equivalent) field. To
facilitate reclassification it is helpful to also include fields holding descriptive and age information.

A new worksheet is generated with a list of the ‘MAPSYMBOL’ units, along with two new fields
labelled ‘NEHRP’ and ‘CLASSNO’. The ‘NEHRP’ (text; 10 chars) field is populated with ‘B’ as a
default, while ‘CLASSNO’ is populated with ‘0’ (zero).

An appropriately skilled person (e.g. geologist) manually assigns each geological unit into a
modified NEHRP site class, and labels the respective ‘NEHRP’ and ‘CLASSNO’ fields according to
Table B.1 (refer to Section 3.2 previously). In most instances only Cenozoic geological units (e.g.
Qa, Czs, etc.) will require a rigorous assessment for site class, as the underlying assumption is
that the majority of geological units represent ‘hard’ bedrock and will retain the default ‘B’
(CLASSNO = ‘0’) classification.

Once completed, the worksheet containing the ‘MAPSYMBOL’, ‘NEHRP’ and ‘CLASSNO’ fields
should be imported as a table into the working file geodatabase for that geological data set.
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Table B.1 Modified NEHRP site classes (text values) and their numeric equivalents.
Modified NEHRP Site Class [‘NEHRP’]
Site Class Number [‘CLASSNO’]
B
0
BC
1
C
2
CD
3
D
4
DE
5
E
6
Other
999
B.2 Generating Zonal Statistics for the Weathering Intensity Index
The weathering intensity index (WII) (Wilford, 2012) requires conversion from continuous to discrete
raster data (i.e. floating point to integer values) and needs to be categorised to represent areas of
limited, moderate and strong weathering. Zonal statistics then then need to be generated to permit
assessment of where the WII is to be applied. This is completed within the ArcGIS environment.
B.2.1 Converting the WII from Continuous to Discrete Raster Data

Open ArcTools and navigate to Spatial Analyst Tools > Math > Plus. Load the raster (e.g.
‘wii_oz2’_test) and apply a value of 0.5. This will add 0.5 to the floating point values already
assigned to the raster cells (and essentially ensures that the next step rounds the values). Save
output raster (e.g. ‘wii_oz_pl05’).

In ArcTools navigate to Spatial Analyst > Math > Int. Load the raster ‘wii_pl0_5’ and run the tool.
This will truncate the existing floating point values leaving integer values only. Save output raster
(e.g. ‘wii_oz_int’).

In ArcTools navigate to Spatial Analyst > Reclass > Reclassify. Load the raster ‘wii_oz_int’ and set
Reclass Field as ‘VALUE’. Specify the value ranges based on Table B.2 then run the tool. Save
output raster (e.g. ‘reclass_wii’).
Table B.2 Discrete weathering intensity index (WII) integer values and corresponding aggregate class values.
Discrete WII Integer Values
Aggregate Class Value
0-4
0
5-6
1
7
2
NoData
NoData
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A revised seismic site conditions map for Australia
B.2.2 Generating Zonal Statistics
NOTE: Depending on the size of the input geological data set, this process may fail in ArcGIS 10.2,
and will require access to Arc Professional or equivalent to enable the necessary processing.

Open ArcTools and navigate to Spatial Analyst Tools > Zonal > Zonal Statistics as Table.

Set the input geological data set (vector) as the Input Feature Zone Data.

Set the Zone Field as OBJECTID. This ensures that every unique polygon will have statistics
calculated for it.

Set the reclassified raster ‘reclass_wii’ as the Input value raster.

Name the Output Table and have it generated in the same geodatabase as the input geological
data.

Set the Statistics Type to ALL. This will give a full range of statistics for analysis. In this case the
main statistic of interest is MAJORITY, which determines the majority raster cell value inside each
polygon. If desired, MAJORITY alone could be selected.
B.3 FME® Desktop Workflow
The FME workflow can be summarised in the following steps (refer to Figure B.1):
1. Define a vector Esri file geodatabase as the geological data input source.
2. Generate a new attribute field (‘CLASSNO’) within the vector data set and populate this field using
the pre-generated reclassification table. This input table assigns a numeric code (0, 1, 2, etc.) that
corresponds with the modified NEHRP site class (e.g. B, BC, C, etc.) for each geological unit
within the input data set.
3. Generate another two new attribute fields (in this case ‘MAJORITY’ and ‘MEDIAN’) within the
vector data set and populate these using the pre-generated zonal statistics table.
4. Filter the data to identify which records need to be processed with respect to the Weathering
Intensity Index. In this step a new field (‘WII_val’) is generated. Records with ‘CLASSNO’ = 0 are
processed - for these records the field ‘WII_val’ is populated with the sum of the fields ‘CLASSNO’
and ‘MAJORITY’. For the remaining records the field ‘WII_val’ is populated with their unmodified
‘CLASSNO’ value.
5. Create and populate attribute data fields with NEHRP classification text values. The first step uses
a set of domain rules based on Table 1 to convert ‘CLASSNO’ numeric values to their text
equivalent. The second step applies the same logic to the ‘WII_val’ field. This generates
respectively the ‘RECLASS_NEHRP’ and ‘WII_NEHRP’ fields; the former representing an
unmodified NEHRP classification value and the latter a weathering intensity index-modified
NEHRP classification for each polygon.
6. Following a test to ensure that ‘WII_val’ field output values are valid (999 is assigned to any value
that fails this test) the output is written to a new feature class within a pre-defined geodatabase.
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Figure B.1 FME Desktop workflow for assessing and applying the weathering intensity index (WII) to NEHRP reclassified geological data (this example - Surface Geology of
Australia 1:1,000,000.
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A revised seismic site conditions map for Australia