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These maps summarise the degree of hazard at a particular location as a
single number. This is the ‘hazard factor’ or ‘Z factor’, which is equivalent to
an acceleration co-efficient with an annual probability of exceedance of 1/500,
(i.e. a 10% probability of exceedance in 50 years). In lay terms, it is the
amount of earthquake induced movement that, on average, would be
exceeded only once in 500 years. Several aspects of this assessment should
be noted.
First, there is considerable inter-regional variation in the degree of hazard. On
the basis of existing knowledge, the most active seismic zone within mainland
Australia is the Meckering region, located about 150 kilometres east of Perth.
It is an area of roughly 300 by 500 kilometres of inter-plate seismic activity
that cuts across south-west WA in a north-west to south-east direction (see
figure 1.1). This was the site of a 1968 earthquake that measured 6.9 on the
Richter scale and destroyed the small town of Meckering, population 240. It
caused ground rupturing over a distance of nearly 40 km. The maximum
heave was 2.4 metres and the maximum vertical displacement was 2.0
metres. The Meckering region returns the highest Z factor that GA records for
Australian territory (0.22). There are also high Z factors for certain island
territories but, otherwise, the Z factor varies from 0.03 for Hobart to 0.13 for
Tennant Creek 4 . The Z factors for the major capitals are:
• Hobart – 0.03
• Brisbane – 0.05
• Canberra – 0.08
• Melbourne – 0.08
• Sydney– 0.08
• Perth – 0.09
• Darwin – 0.09
• Adelaide – 0.10
Newcastle experienced Australia’s most damaging earthquake in 1989,
measuring 5.6 on the Richter scale, and has a Z factor of 0.11.
Second, the Z factor does not indicate that a given location will experience
earthquakes of a particular magnitude. All locations are exposed to
earthquakes that range from minor tremors to major disasters, with the Z
factor indicating the magnitude of the earthquake that, on average, would be
exceeded only once in 500 years.
Third, GA acknowledges that key aspects of Australia’s earthquake hazard
are poorly understood (Leonard 2003: page 1). Leonard’s broad assessment
of the state of knowledge is that:
4
These values are reported in table 3.2 of Australian Standard AS1170.4–1993 (Minimum
design loads for structures: earthquake loads).
ABCB Regulation Impact Statement (RIS 2007-03)
2