A fault avoidance zone is an area created by establishing a buffer zone either side of the known fault trace (or the identified likely fault rupture zone that appears on the land surface). These Guidelines recommend a minimum buffer zone of 20 metres either side of the known fault trace or likely fault rupture zone.
Twenty metres has been chosen because intense deformation and secondary ruptures are commonly experienced as a result of fault movement within this distance from the primary plane of the fault rupture. These effects can occur because near-surface weak materials deform instead of breaking cleanly, and structures built near an area of fault rupture can cause surface rupture to divert around them unpredictably. Twenty metres also represents a precautionary approach to ensure a level of safety in regard to the protection of life. However detailed fault studies which may be undertaken may show that fault deformation is less extensive than 20 metres from the end or edges of the fault trace allowing a reduction in the size of the fault avoidance zone.
Figure 6.1: A fault avoidance zone on a district planning map
Defining a fault avoidance zone on district planning maps, and supporting it with policies and methods (including rules) will allow a council to:
- restrict development within the fault avoidance zone
- take a risk-based approach to development in built-up areas.
The determination of the extent of a fault avoidance zone is closely related to fault complexity (refer section 8). A wide and complex likely fault rupture zone is likely to have a significant fault avoidance zone.
Displacement across a fault usually decreases with its distance from the fault trace. The fault avoidance zone can be reduced if a detailed fault study shows that the zone of intense deformation and secondary rupture is less than 20 metres from the likely fault rupture zone.