Fault mapping for Taupō District
The Taupō
District is particularly vulnerable to seismic, geothermal and volcanic
activity. As a result, it has many fault lines running through it, especially
to the north-east and southern ends of Lake Taupō. These are areas where the
ground has the potential to rupture due to earthquakes. They represent a
potential hazard to building and development, in addition to general earthquake
shaking which is a risk everywhere.
As your local authority, Taupō District Council has responsibilities under the Resource Management Act 1991 to identify and provide information on natural hazards – including fault lines. In August 2020, GNS Science – Te Pu Ao, as technical experts in this area, provided active fault mapping to Taupō District Council. The fault mapping identifies Fault Avoidance Zones and Fault Awareness Areas for Taupō District.
The fault mapping shows areas where there are definite, likely or possible active faults. If a fault were to rupture due to an earthquake this may present a potential hazard to building and development, in addition to ground shaking from an earthquake, which could be experienced anywhere.
In locations of active faults, site-specific geotechnical and engineering information is likely to be required in order to obtain resource or building consent, so that the Council can be assured that activities, such as subdivision and building, can be and are undertaken in a way that mitigates the risks associated with active faults.
The fault mapping identifies:
- Fault Avoidance Zone (possible tectonic origin)
A Fault Avoidance Zone is an area created by establishing a 20 m buffer (setback) zone either side of the identified possible fault rupture zone of an identified or suspected active fault. This fault is classed as having “possible” tectonic origin, which means it is suspected that the mapped feature is an active fault, but it may have another, non-tectonic, origin, such as river or lake erosion, or land sliding. Confirming whether it is an active fault requires further investigation. - Fault Avoidance Zone (likely tectonic origin)
A Fault Avoidance Zone is an area created by establishing a 20 m buffer (setback) zone either side of the identified possible fault rupture zone of an identified or suspected active fault. This fault is classed as having “likely” tectonic origin, which means that the mapped feature is likely an active fault, but other explanations for its origin, such as river or lake erosion, or land sliding, cannot be ruled out. - Fault Avoidance Zone (definite tectonic origin)
A Fault Avoidance Zone is an area created by establishing a 20 m buffer (setback) zone either side of the identified possible fault rupture zone of an identified or suspected active fault. This fault is classed as having “definite” tectonic origin, which means that there is strong evidence (e.g., exposure in a trench or streambank, or geomorphology that cross-cuts other features such as streams) that the mapped feature is an active fault. - Fault Awareness Area
A Fault Awareness Area highlights that an active fault is known, or suspected, to be present, but existing mapping is not accurate enough to be sure of its exact location.
In the map
you can find the classifications of any specific fault by clicking on it. The
report contains information on the maps and fault classifications, as well as
their underlying assumptions, uncertainties and implications
To explain
and support the mapping, GNS Science has provided the following report:
Active Fault Hazards in the Taupō District. GNS Science Consultancy Report 2020/31
Uncertainty
The active fault mapping has been done as a desktop study. Where available, GNS Scientists used LIDAR data (light detection and ranging data – which provides high resolution maps of land contours) to identify geographical features that are likely to be active faults. Very limited information is available for ground truthing (e.g. site inspections, digging and trenching to verify the existence of earthquake faults and frequency of rupture events). There is a degree of uncertainty in both the accuracy of the fault mapping, the likelihood or frequency of possible fault rupture, and the potential scale of ground movement in an earthquake event. Fault Avoidance Zones have been mapped in different colours according to the certainty that they are active faults (by the tectonic origin classifications of definite, likely, or possible).
Not all of the mapped faults are confirmed as definite active faults. For many, potential alternative explanations for the identified geographical feature have not been ruled out. Where there is a higher degree of uncertainty in the location of the feature, the feature is identified as a Fault Awareness Area. The classification for Fault Awareness Areas can be seen by clicking on the fault in the mapping tool on the Council’s website.
Timeframes / likelihood for events
Paleoseismic data, which are required to determine the frequency of ground-surface rupturing earthquakes (known as the recurrence interval), are currently only available for a small proportion of the faults in the Taupō District. Alongside this information, GNS Science has used the New Zealand National Seismic Hazard Model to estimate the recurrence interval for some of the major faults in Taupō District.
Most of the mapped faults in the Taupō District are classified as having a recurrence interval of less than or equal to 2000 years. GNS Science acknowledges that the classification of less than or equal to 2000 years is a conservative minimum in many instances. Some of the mapped faults will have longer recurrence intervals. This is because many of the faults in Taupō District have multiple strands of fault traces at the surface, however, it is possible that not every surface fault trace will rupture in every earthquake on the fault. More detail can be found on pages 41 to 45 of the GNS Report.
Impact in event of an earthquake
Ground-surface ruptures resulting from moderate to large earthquakes on active faults in the Taupō District are likely to range from centimetre to metre scale.
This deformation could, in places, be either relatively concentrated (i.e. discrete) or spread-out (i.e. distributed). Both the amount of displacement, and the manner in which that displacement is either concentrated or distributed influence the impact that surface rupture may have on structures sited on or across a fault. Whether an individual fault is classified as ‘well-defined’ or ‘distributed’ can be seen by clicking on the fault in the mapping tool on the Council’s website. More details of these classifications can be found on pages 19 to 21 of the GNS Report
Future building and land use
Expert advice is that building a new house in a Fault Avoidance Zone, especially if the fault is ‘well-defined’, would require a geotechnical assessment to assess the tectonic origin of the mapped fault (if it is a ‘possible’ fault), the recurrence interval, and/or locate the likely rupture zone more precisely.
If you apply for a building consent in a Fault Avoidance Zone, you will be required to get geotechnical and engineering advice to assure Council that new building work can be done and engineered in a way that mitigates the risks associated with the active fault.
