Earthquake hazard rating relates to the general vicinity of the property address. It is calculated using Risk Frontiers QuakeAUS loss model. Poor building construction and condition, especially older unreinforced masonry structures such as double brick buildings, can experience damage from even modest levels of ground shaking. Newer engineered structures or smaller wood-framed buildings are much less susceptible to damage. This report does not take the condition and construction details of buildings into account.
Based on the property address, the following individual risk measures are considered when calculating the overall risk rating for an earthquake impacting the property:
- 500-year ARI Peak ground acceleration
- Soil zonation
Earthquakes generate seismic waves whose ground motions cause sideways forces on buildings, potentially leading to damage. One measure of ground motion especially relevant to low level structures of one or two stories is Peak Ground Acceleration (PGA), because it is directly related to the force on the building caused by the earthquake. PGA is measured in terms of metres per second squared (m/s2) or as a fraction of the acceleration of gravity (%g). The choice of a 500-year ARI PGA is based on the seismic criteria governing building codes in Australia (AS1170.4), which address life-safety considerations. Significant damage is expected to occur at an ARI of 500 years.
In this report, the term soil zonation describes the stiffness of the ground. Hard rock and very dense soils are stiff and give rise to lower ground motion levels than do deep soils and soft soils, which are more flexible and give rise to higher ground shaking levels. The soil zonation used in the report is supplementary information; it is already explicitly included in the PGA estimation.
This risk report has adopted a five-point peril scale based on the Global Seismic Hazard Assessment Program. PGA figures are arrived at using Risk Frontiers QuakeAUS model that takes into account the soil stiffness in the general vicinity of the address.
- Very High (5) – PGA larger than 4.0 m/s² or approximately 40% of the acceleration of gravity
- High (4) – PGA between 2.4–4.0 m/s²
- Medium (3) – PGA between 0.8–2.4 m/s²
- Low (2) – PGA between 0.2–0.8 m/s²
- Negligible (1) – PGA less than 0.2 m/s²
A simplified version of soil zonations are used to calculate risk is outlined in the table below.
- Zone 5 – Soft soil (very high potential to amplify bedrock ground shaking)
- Zone 4 – Stiff soil (high potential to amplify bedrock ground shaking)
- Zone 3 – Very dense soil/soft rock (moderate potential to amplify bedrock ground shaking)
- Zone 2 – Firm to hard rock (low potential to amplify bedrock ground shaking)
- Zone 1 – Hard rock (very low potential to amplify bedrock ground shaking)