5 Mapping Active Faults
5.1 The importance of mapping
Faults must be accurately located, and mapped at a scale appropriate for end-use purposes to enable planners to make decisions about land use on or close to active faults.
Geologists with particular experience of mapping faults are the most appropriate professionals to investigate, locate and assess active faults. Engineers with recognised qualifications and experience in geotechnical engineering are also able to investigate faults.
Active faults are complex and often have multiple breaks. A number of methods and evaluative tools need to be used in investigation.
Once a fault has been accurately located and assessed, the fault features should be clearly marked out (for example, pegged) so they can be surveyed onto cadastral maps.
5.2 Required scale of fault maps
For planning purposes, faults should be mapped and classified at a minimum scale of 1:10,000. At present, few local authorities have mapped active faults to this scale, instead relying on existing fault maps for indicative purposes. This can create severe limitations for land use planning.
Most of New Zealand's major active faults are mapped on small-scale geological maps (such as 1:250,000 or 1:50,000 scale). This does not provide adequate detail for planning purposes, where detail to at least property boundary level is often required. Lines representing fault lines on maps of 1:250,000 scale could, for example, be the width of two or three residential sections if laid out on the actual ground (at 1:1 scale) and could end up being wider than the fault they represent.
A map should only be interpreted at the scale it is compiled at. Geological maps in New Zealand are often published at the 1:250,000 scale. If a 1: 250,000 scale map is reproduced at a 1:10,000 scale (as it often is, especially on photocopiers) the fault will be inaccurately portrayed and its placement interpreted wrongly. The fault will appear much wider that it is in reality and the accuracy of the true alignment can be lost. A key mistake that can result is thinking that a fault intersects a particular property when it does not. Faults shown on planning maps at 1:10,000 scale must be compiled, and features located, at a scale consistent with the intended end use.
