This section examines how two territorial authorities within the Wellington Region, Wellington City Council (WCC) and Kapiti Coast District Council (KCDC), have used these guidelines when reviewing active fault hazard provisions in their district plans. The case studies are preceded by an explanation of the unique tectonic setting in the Wellington region to help explain the fault rupture hazard.
12.1 The Wellington Region's Tectonic Environment
Both Wellington City Council and Kapiti Coast District Council sit within the Wellington region; the jurisdiction of Greater Wellington - The Regional Council. The tectonic environment within the Wellington region is very active given its location astride the constantly moving Pacific and Australian plates. As a result, a large number of active faults of varying complexity and recurrence interval classifications are present within the region.
The most active fault in the region (ie, the one with the shortest recurrence interval) is the Wellington Fault which extends northwards from the Cook Strait (its most southernmost known location) past the south Wellington shoreline, through Wellington and the Hutt Valley and through the Tararua Range to the Manawatu River. At this point, the name of the fault changes but continues north to the Bay of Plenty coastline.
The Wairarapa Fault, the source of the great 1855 Wairarapa earthquake, extends northeastward along the base of the eastern flank of the Tararua Ranges. With a recurrence interval of about 1500 years, it is a Class 1 active fault. Its average slip rate of just under 10mm/year means it is moving faster than the Wellington Fault. Past surface rupturing earthquakes on the Wairarapa Faults have resulted in up to 10 metres or more of lateral slip at the fault trace, with regional uplift and tilting east of the fault.
The Ohariu Fault extends approximately 70km north-northeastward from offshore of the Wellington south coast, through Porirua to Waikanae (Heron et al. 1998, Begg & Johnston 2000) and probably continues a further 60 km northwards as the Northern Ohariu Fault to just south of Palmerston North (eg, Van Dissen et al. 1999, Palmer and Van Dissen. 2002). The Gibbs Fault is less constrained than the Ohariu and Northern Ohariu faults, but is thought to branch off the Ohariu Fault near MacKays Crossing and extend 30km north north-east to within 3-4 kms of the Otaki Forks Fault which passes through Kapiti Coast District hill country to the east for about 10-15 kms. Little is known about the Southeast Reikorangi Fault which most likely extends from the Gibbs Faults about 20km in the hills east of Kapiti Coast (Van Dissen et al. 2003).
12.1.1 Fault Rupture in the Region
In the Wellington region, the Wairarapa fault is the only fault that has ruptured in historical times (during the 1855 magnitude 8 Wairarapa earthquake). The most known recent surface fault rupture on the Wellington Fault occurred about 400 years ago (Van Dissen and Berryman, 1996) and on the Ohariu Fault about 1000 years ago (Litchfield et al. 2004).
It is estimated that the Wellington Fault is capable of generating earthquakes in the order of magnitude 7.5 with a 10 percent probability of it rupturing in the next 50 years. Such a rupture could move the ground along the fault horizontally by 4-5 metres and vertically by about 1 metre (Froggatt & Rhodes1996, Van Dissen & Berryman 1996).
The Ohariu fault is capable of an earthquake about magnitude 7.5 with expected fault rupture of 3-5 metres of right-lateral displacement at the ground surface with lesser and more variable vertical displacement (Heron et al. 1998). The Northern Ohariu Fault, Gibbs Fault and Otaki Forks Fault are all capable of generating earthquakes magnitude 7+ and metre-scale surface rupture displacements (Litchfield et al. 2004, Van Dissen et al, 2003).
The region's most active faults (Wellington, Wairarapa and Ohariu) all have varying fault complexity at stages along the fault meaning that while parts of these faults are well-defined, other parts are distributed or the location is uncertain. Finding the fault location can be difficult in some areas due to two key reasons: fault traces have been removed by natural processes (landslide, weather, and coastal); and/or the intensity of urban development has obscured the fault trace.
12.2 The Wellington City Council
Wellington City Council's District Plan Change 22 amended the Hazard (Fault Line) Area for the Wellington Fault on district plan maps, and amended a number of district plan provisions relating to the fault hazard.
In 2001, the Wellington Emergency Management Office (WEMO) engaged the Institute of Geological and Nuclear Sciences (GNS) to assess the impact on property from an earthquake along the Wellington fault. The work by GNS uncovered the fact that the Wellington City district plan maps depicting the Wellington Fault did not reflect GNS's understanding of the fault location.
The district plan team engaged the Institute of Geological and Nuclear Sciences to undertake a Wellington Fault location review to provide up to date information on the location of the urban section of the Wellington Fault from Aotea Quay to the lower Karori Reservoir (including the port, railway yards and the parts of the suburbs of Thorndon, Northland, Kelburn and Karori). Wellington City Council decided to concentrate the fault location investigation solely on the Wellington Fault (although they were also aware of the other active faults in the district these were not considered as high risk as the Wellington Fault). The findings of the Institute of Geological and Nuclear Sciences report highlighted inaccuracies in the existing Hazard (Fault Line) Area as shown on district plan maps and as a result identified two new updated fault hazard zones:
- Likely fault rupture hazard zone: The area containing the likely position of the Wellington Fault, and the zone within which the fault is likely to rupture (but not across its entire width). The width of the zone varies from approximately 10 to 50 metres.
- Recommended fault rupture hazard zone: The width of this zone ranges from 50 to 90 metres as it includes the recommended (as per the Guidelines) 20 metre buffer zone either side of the likely fault rupture hazard zone. In its report, the Institute of Geological and Nuclear Sciences recommended that thisrecommended fault rupture hazard zone be used for district planning purposes as it accommodates uncertainties in the location and width of the likely fault rupture hazard zone.
12.2.2 Properties Affected
The Wellington Fault location review identified 665 properties within the new recommended fault rupture hazard zone (some properties straddle both the likely fault rupture zone and the recommended fault rupture hazard zone or buffer zone). Of these 665 properties, there were 244 more properties than currently identified on the planning maps. Approximately 35 properties were removed from the fault rupture hazard zone.
12.2.3 Justification for Plan Change
In light of the new information from the Wellington Fault location review, the Wellington City Council decided to look at whether a district plan change was justified to reflect the findings.
In addition to learning that the planning maps depicted the Wellington Fault in the wrong location, the district plan team recognised that the current district plan fault hazard zone provisions were not proving to be effective. A review of the existing plan provisions (which has been developed as part of the district plan review in 1999) showed that they were not achieving their intention (eg, multiple unit developments had been approved and built in areas identified in the district plan as active fault zones). Although the district plan policies reflected the intention to limit development in these areas, the rules were not explicit enough and the planning team decided they were in need of updating.
Clearer information requirements for developers were also needed and planners needed to have better assessment criteria to use when assessing resource consent applications for development in the fault rupture hazard zone.
12.2.4 Public Information Process
Prior to initiating Plan Change 22, the Wellington City Council undertook an extensive public consultation campaign to clearly communicate the findings of the Wellington Fault location review. Affected property owners and occupiers were targeted to gauge initial responses. Less than two weeks after receiving the final Institute of Geological and Nuclear Sciences report Wellington City Council undertook the following:
- letters were sent to over 700 property owners affected by the fault rupture hazard zones
- an information centre was established on Tinakori Road (ie, close to the affected properties)
- a public meeting was held.
Over 70 people visited the information centre during its three days of opening. An estimated 65 people attended the public meeting. The Institute of Geological and Nuclear Sciences scientists who worked on the Wellington Fault location review attended the public meeting along with Wellington City Council staff. Institute of Geological and Nuclear Sciences's role was to explain the science behind the hazard zones, and Wellington City Council staff outlined the plan change process. A facilitator was used to help manage the questions that followed the main presentations.
Key issues raised by the public at the information centre and public meeting related to:
- the 20m buffer zone and whether there was scope to change this
- the nature of information included on Land Information Memoranda
- requests that no new significant buildings be built in the fault hazard area, whereas others were concerned about the level of existing regulation in the Plan
- the impact on house values, insurance premiums and council rates
- expectations about compensation where the fault hazard zone now covered a property
- whether or not property owners were now required to strengthen their homes.
12.2.5 The Plan Change
A number of options were considered when recommending the final Plan Change 22 which included "do nothing" and reducing the buffer zone around the likely fault rupture hazard zone. The final recommendations included:
- Amend the existing planning maps to re-align the Hazard (Fault Line) Area to reflect the Institute of Geological and Nuclear Sciences recommendations which suggested a 20 metre buffer area either side of the likely fault rupture hazard zone
- Delete reference to NZS4203:1992 and replace with definitions of 'light roof' and 'light wall cladding' (from NZS 3604:19999 (Timber Framed Buildings)
- Allow for only one residential unit as a Permitted Activity in the Hazard (Fault Line) Area
- Provide for multi-unit developments to be assessed as a Discretionary (Unrestricted) Activity (this would have the effect of allowing appropriate assessment criteria to be developed for use by resource consent planners unlike a Non-Complying Activity status)
- Amend the explanation of the hazard policies to include specific reference to earthquake hazards, and that the damage caused by such hazards can be reduced with mitigation measures
- Provide assessment criteria to give planners more scope when determining the effects to a specific site from fault rupture including the opportunity to obtain geotechnical and engineering information
- Provide for geotechnical reports and engineering design reports to be supplied as part of any resource consent in the hazard area.
- Changes to other associated rules in the plan.
12.2.6 Issues raised by submitters
Following notification, Plan Change 22 received eleven submissions and four further submissions. The majority of the submitters opposed aspects of the Plan Change or sought amendments. Issues raised by submitters included:
a) The width of the 20m buffer zone
b) Whether a whole property was affected by the hazard zone rules, or only land within the Hazard (Fault Line) Area
c) The requirement to provide geotechnical and engineering design reports with any resource consent in the Hazard (Fault Line) Area
d) The proposed change to reduce the number of permitted residential units to one per site
e) The impact of this information on property values, insurance premiums and compensation.
Of these, the first two points were considered the most significant but all are discussed below:
a) The width of the 20m buffer zone
Both the Guidelines and the Institute of Geological and Nuclear Sciences report recommend a minimum 20 metre buffer zone. Public concerns were mostly related to this additional 20 metre zone rather than the narrower likely fault rupture hazard zone - suggesting that residents accepted the risk of living on the fault. Those residents not within the likely fault rupture hazard zone however, questioned the necessity of their inclusion within the buffer zone.
It was decided, that if a smaller buffer zone (ie, less than 20 metres) was put in place it would not resolve the fundamental problem that there would always be some properties just within the zone that would argue to be taken out of the zone. Wellington City Council acknowledged that the science of accurately locating fault rupture areas will continue to improve new technology, and better understanding of the hazard itself. If relevant information became known as site specific geotechnical investigations were carried out this may allow Wellington City Council to narrow the fault rupture hazard zone even further.
b) Whether a whole property was affected by the hazard zone rules, or only land within the Hazard (Fault Line) Area.
As with any type of zoning that does not strictly adhere to property boundaries, issues arose over interpretation of properties that:
i) had a boundary aligned with a line of the hazard zone
ii) were partially within the hazard zone
iii) had a right of way or similar within the fault rupture hazard zone.
The Wellington City Council was required to make decisions on these situations in relation to whether or not the hazard information would be included in a Land Information Memoranda (LIM) report; however the interpretations could easily apply to resource consent decisions. In scenario (i) planning staff assessed this property as being out of the hazard zone. In scenario (ii) the hazard information had to be included in a Land Information Memoranda (LIM), but the rules in the plan only apply to that portion of the land covered by the hazard area. Likewise with scenario (iii), the information had to be included in a Land Information Memoranda (LIM), but an extra note was included on that Land Information Memoranda (LIM) explaining it was only the Right of Way (ROW) affected by the hazard area and not the building itself.
Notes were put on property files for those properties where interpretation of the fault rupture hazard zone lines was unclear (as in the scenarios above) to provide clarity for property owners and planners assessing development proposals. In most cases, the planner will be able to interpret whether or not a property is in the hazard zone from the planning maps.
c) Requirement to provide geotechnical & engineering reports
The requirement for geotechnical and engineering reports as part of a resource consent application was objected to by a utility company on the grounds that:
- such structures were designed to withstand ground-shaking events
- that the structures are small in comparison to other structures (such as houses)
- the potential environmental impacts are minor.
The requirement for geotechnical and engineering reports were part of Plan Change 22 as they allow for ground conditions (which can vary from site to site) to be assessed and also provide Wellington City Council with information about how a fault rupture event may affect a certain development. It was agreed that as the focus of the rules was on structures where people live, work and play. Therefore there no need for utility structures to be subject to the requirement to provide geotechnical and engineering reports.
d) Limiting residential units to one per site
Although the district plan already permitted only one residential unit per site in most of the area covered by the hazard zone (ie, Thorndon), other areas of Wellington where two units per site were currently permitted, were affected by a rule in the Plan Change.
The rule does not prevent landowners from building more than one dwelling on a site but outlines what is permitted as of right without requiring resource consent. The assessment criteria, geotechnical and engineering requirements, developed as part of Plan Change 22, will allow Wellington City Council the opportunity to gather the information needed to assess any proposals in the hazard area that require a resource consent.
e) Property values, Insurance Premiums and Compensation
While some property owners accepted the hazard risk by living in the area, others were concerned about the impact of a hazard zone on property values and insurance premiums.
Although difficult to accurately confirm, there has been no evidence to suggest that the fault hazard zone has affected property prices in the past; similarly insurance premiums have not reflected any increase due to the risk identified in the fault rupture hazard zone. Even if it had been proven that property values decreased as a direct result of the fault hazard zone, Wellington City Council had not prohibited any development along the fault allowing people to still make reasonable use of their land. No compensation would be required.
12.2.7 Council hearing and decision-making process
The hearing for Plan Change 22 was held in February 2004 and attended by three submitters. The hearing was notable for the level of detail that the Hearings Committee went into in order to establish the appropriateness of the hazard zone in areas that were contested by submitters. One submitter bought along their own geotechnical advisor, which helped to raise the level of the debate about the accuracy of the hazard zones. The Committee found itself in a position of weighing the evidence from its District Planning Team geotechnical advisors against the expert bought in by the submitter. As a consequence of this debate between the experts, the Committee decided that there was enough evidence to narrow the fault rupture hazard area at two specific locations as argued by the submitter's expert. The Committee considered that it was ultimately better to narrow the fault rupture hazard area based on good quality information, rather than to reduce the 20m buffer area to appease submitters. Upon reflection, these changes were agreeable to Institute of Geological and Nuclear Sciences also, and consequently the hazard zones were revised for the decision.
Plan Change 22 received no appeals.
Plan Change 22 resulted in planning map inaccuracies being fixed with properties that were no longer within the fault rupture hazard zone removed from the zone and no longer be subject to the rules for the Hazard (Fault Line) Area. Similarly, properties not currently within the fault rupture hazard area, but included in the fault rupture hazard zone recommended by Institute of Geological and Nuclear Sciences became subject to the Hazard (Fault Line) Area rules.
12.2.8 Key lessons
- Once Wellington City Council had the findings of the Institute of Geological and Nuclear Sciences report theyacted quickly by initiating anextensive public consultation campaign that included the information centre, a public meeting and media liaison. A lot of questions the public had related to science and geotechnical issues which were able to be answered by the Institute of Geological and Nuclear Sciences staff who attended the meeting, and who had written the Wellington Fault location review report. As a result, very few written submissions were received on the proposed Plan Change 22. Of those that were received, they were all very focused and did not generally cover issues that could not be resolved in the plan change process. Wellington City Council considered that because of their well executed public campaign the submissions received were far more manageable than anticipated.
- The information requirements, developed as part of the plan change for inclusion within the district plan, needed to be explained clearly for both the planner (to request the right information) and the developer (to provide the right information). The cost of these requirements needed to be considered and should be met by the developer.
- If a council requires geotechnical and engineering information then it is important to have staff who can explain what is needed and interpret the information when it is received. The Wellington City Council now has a staff member who has expertise in geotechnical matters.
- It is important for assessment criteria to be very clear as it gives the consent planner a good basis when assessing an application and reasoning to refuse consent if necessary.
12.3 The Kapiti Coast District Council
The Kapiti Coast District is the fastest growing area in the Wellington Region ( with a population growth rate of approximately 2% per annum) and is traversed by five known active faults - the Ohariu, Northern Ohairu, Gibbs, Otaki Forks and South East Reikorangi. The Ohairu and Northern Ohariu faults are two of the more significant earthquake generating faults in the Wellington Region, and they both pass through areas of urban, semi-urban and rural development.
Following a comprehensive review of all the known fault traces in the district, the Kapiti Coast District Council is now in the process of reviewing and updating its district plan provisions for the development and subdivision of land on or close to active faults.
Plan Change 64 (Fault traces), while not yet complete, will seek to update the GIS and District Plan maps by more accurately depicting the locations of faults traces, as well as amending the supporting package of objectives, policies, rules and standards in the district plan.
In November 2000, Kapiti Coast District Council notified a proposed plan change that sought better planning and management of development on or close to the active faults in the district. The plan change however, was withdrawn after submissions highlighted that further research was needed to more accurately define the fault trace locations in the district.
In 2003 Kapiti Coast District Council, along with Greater Wellington - the Regional Council, commissioned Institute of Geological and Nuclear Sciences to carry out a comprehensive study of the known active fault traces in the Kapiti Coast District.
Although Kapiti Coast District Council already had some data regarding the location and type of fault generated features for some parts of the district, the information had been gathered in a piecemeal and site specific manner, and was basically confined to small sections of the Ohariu and Gibbs faults only. In addition, the accuracy of the information was in some cases limited to +/- 100 metres. A fault trace study was therefore necessary to improve the existing information held by Kapiti Coast District Council and improve the detail and accuracy of fault trace locations on the district plan maps.
12.3.2 Current planning for fault rupture
The Kapiti Coast District Plan currently contains provisions in the rural and residential zones restricting the construction of buildings within 20 metres of an earthquake fault trace shown on district plan maps. Any building proposal falling within 20 metres of a fault trace requires Controlled Activity resource consent and conditions are usually applied to ensure appropriate engineering requirements are included in the building design in order to avoid, remedy or mitigate any adverse effects resulting from ground rupture.
The Institute of Geological and Nuclear Sciences report presented a comprehensive study of all known active fault traces in Kapiti. The locations were mapped into GIS to allow for incorporation into the Council's GIS system and onto the district plan planning maps. The findings were presented in a way compatible with the process set out in these guidelines.
Institute of Geological and Nuclear Sciences established Fault Avoidance Zones (this is the same as the terminology in these guidelines, whereas Wellington City Council used the term fault rupture hazard zone) based on fault locations and complexity (well defined, distributed, and uncertain). A Fault Avoidance Zone includes the fault rupture hazard zone, and the buffer zone.
Due to the particular fault trace complexities in Kapiti, Institute of Geological and Nuclear Sciences found it necessary to expand upon these categories to include:
- Well defined: fault rupture is well defined and of limited geographic width
- Well defined - extended: a well defined fault had either been buried or eroded over short distances but its position is tightly constrained
- Distributed: fault rupture can be constrained to lie within a relatively board geographic width (tens to hundreds of metres) typically as multiple fault traces and/or folds
- Uncertain - constrained: areas where the location of the fault rupture is uncertain because evidence has been eroded or buried but where the location can be constrained to within a reasonable geographic extent (eg, within 300 metres)
- Uncertain: poorly constrained where the fault trace was too uncertain to be within 300 metres, usually because deformation has been buried or eroded or the fault features are widely spaced and/or very broad.
Fault Avoidance Zones are defined along all the faults based on the rupture complexity of the particular fault, and the precision to which its location can be constrained. The Fault Avoidance Zones identified range in width from about 40m (well defined) to greater than 300m (uncertain-poorly constrained).
The Institute of Geological and Nuclear Sciences report also provided examples of resource consent activity classes appropriate to different Fault Avoidance Zones based on the fault recurrence interval, fault complexity and building importance category. This approach is consistent with the Guidelines and was included in order to provide assistance in drafting the district plan rules relating to fault traces.
12.3.4 Public consultation
As soon as Kapiti Coast District Council received the Institute of Geological and Nuclear Sciences report and considered its findings, planning staff set about putting into action a public consultation process that would advise landowners affected by the report findings and seek feedback to assist the council with preparing a plan change.
Letters were sent to all landowners in September 2003, along with an Information Sheet summarising the fault trace study results and the implications. A large number of responses were received, including 32 written comments, which raised a raft of concerns including:
- The effect of the new information on property value, insurance premiums and insurance policy coverage
- The nature and extent of fault trace information included on Land Information Memoranda
- Expectations for compensation where the fault trace hazard now covers a property, as well as a reduction in council rates
- Concerns regarding existing houses built on or very close to a fault - what can landowners do to reduce risk and damage? Should owners be strengthening their homes?
- Greenfield areas should not be treated any differently to areas that are already developed
- The approach proposed is overly conservative and risk adverse, especially in areas where risk is uncertain (ie,uncertain-unconstrained areas)
- The building importance categories identified are defective (no provision for 2-3 story timber framed houses within scope of NZS 3604)
- Concerns regarding the accuracy of information - How was it gathered? How accurate is it? Why did Kapiti Coast District Council not already have accurate information for the whole of the district?
12.3.5 Towards a Plan Change
Kapiti Coast District Council is currently dealing with the concerns raised by submitters and deciding on the scope and content of Plan Change 64. District plan maps will be updated with the new fault trace information supplied by Institute of Geological and Nuclear Sciences and amendments made to the supporting objectives, policies, rules and standards in the district plan.
- amending the relevant objectives and policies within the Natural Hazards chapter to include specific reference to earthquake fault trace hazards
- including the opportunity within the rules and standards to obtain geotechnical and engineering information as part of any resource consent within a Fault Avoidance Zone
- amending other relevant rules and standards in the plan.
The plan change will reflect the Institute of Geological and Nuclear Sciences report findings and the approach set out in the Guidelines, but will be adapted to the Kapiti Coast situation, and to the District Plan structure. The comments already received from landowners will also be taken into account in the drafting of new provisions.
The complexity of the nature of faults in Kapiti raises issues in terms of the provisions to be included in the District Plan. The challenge includes drafting provisions which cover:
- five different faults, all with slightly different faulting characteristics
- five different Fault Avoidance Zones reflecting different levels of certainty
- greenfield versus already developed land
- the different types of structure/building that could be erected (temporary structures, single or multiple-storied timber dwellings, through to more significant structures and buildings)
- and because of these differences, the potential for several different categories of resource consent.
The emphasis is on making the district plan provisions, particularly the rules and standards, as straightforward as possible to aid understanding by landowners, developers and decision makers.
In order to facilitate robust decision-making whilst the plan change is being developed, and to ensure the Council meets its obligations in terms of providing the most up to date information available, the GIS layer supplied by Institute of Geological and Nuclear Sciences as part of the study has been incorporated into the Council's GIS system.