Rattenbury, M.S. 2022 Regional fault orientation and length analysis, Aotearoa New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2022/13. 49 p.; doi: 10.21420/V2MV-R640
Abstract
Over the past few decades, several faults have been identified in Aotearoa New Zealand only because they ruptured in recent damaging earthquakes. The recognition of these previously unidentified earthquake-generating faults has heightened awareness of seismic hazard that extends beyond Aotearoa New Zealand’s large, but still fragmentary, record of known active faults. This has reinforced how important estimating distributed ‘off-fault’ seismicity is for characterising earthquake hazard. To help quantify aspects of distributed seismicity for the National Seismic Hazard Model (NSHM 2022) revision project, a region by region characterisation of both fault orientation and fault length has been undertaken using neotectonic domains recently re-defined in the New Zealand Community Fault Model (NZ CFM v1.0). The completeness and appropriateness of several mapped fault digital datasets of national extent have been assessed for fault orientation and length analysis suitability. Faults from an upgraded version of the 1:1M Geological Map of New Zealand (1M GMNZ+) provide a relatively consistent portrayal of onshore fault traces in moderately good detail. The dataset has been improved through the addition of concealed faults and comprehensive unique naming of related features that are not necessarily connected at the surface. Faults from the 1M GMNZ+ have been combined with the simplified fault zones of the NZ CFM v1.0, a dataset that provides onshore and offshore representations of active and potentially seismogenic faults. Low-angle faults that are unlikely to persist to seismogenic depths (such as the Northland Allochthon thrusts) and annealed ductile shear zones (such as the Caples–Rakaia terrane boundary) have been excluded from the combined dataset. The analysis determined the orientations of 1761 faults and summarised them according to fault length classes for26 neotectonic domains. As fault length is, to a first order, related to earthquake size, the range of fault orientations in each neotectonic domain has been assessed in three fault-length classes that roughly correspond to earthquakes of >M 6, >M 7 and >M 7.5. The orientations of longer faults generally reflect an accentuated tectonic grain in the local neotectonic domain. Away from the faster slipping parts of the plate tectonic boundary, there is more variety in orientation, particularly for shorter faults (auth)
