Jarg R. Pettinga

National Seismic Hazard Model for New Zealand: 2010 Update

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

Structure of a growing accretionary prism, Hikurangi margin, New Zealand

The Hikurangi margin of eastern North Island, New Zealand, represents the feather edge of the Indian plate at its convergent boundary with the subducting Pacific plate. A migrated seismic reflection profile across this margin clearly displays the structural evolution of an accretionary prism. A 25-km-wide band of “protothrusts” is delineated between the toe of the slope and a converging seamount; this illustrates an early stage in the seaward propagation of a deformation front. Landward-tilted trench-slope basins are separated by ridges that have clearly defined thrusts, which appear to sole out at a decollement. The decollement continues at an angle of only 3° beneath the 150-km-wide margin to a depth of 14 km near the coast where it coincides with an onshore zone of high seismicity.

Complex multifault rupture during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand

An earthquake with a dozen faults The 2016 moment magnitude (Mw) 7.8 Kaikōura earthquake was one of the largest ever to hit New Zealand. Hamling et al. show with a new slip model that it was an incredibly complex event. Unlike most earthquakes, multiple faults ruptured to generate the ground shaking. A remarkable 12 faults ruptured overall, with the rupture jumping between faults located up to 15 km away from each other. The earthquake should motivate rethinking of certain seismic hazard models, which do not presently allow for this unusual complex rupture pattern. Science, this issue p. eaam7194 At least 12 faults spaced up to 15 kilometers apart ruptured during the magnitude 7.8 Kaikōura earthquake. INTRODUCTION On 14 November 2016 (local time), northeastern South Island of New Zealand was struck by a major moment magnitude (Mw) 7.8 earthquake. The Kaikōura earthquake was the most powerful experienced in the region in more than 150 years. The whole of New Zealand reported shaking, with widespread damage across much of northern South Island and in the capital city, Wellington. The earthquake straddled two distinct seismotectonic domains, breaking multiple faults in the contractional North Canterbury fault zone and the dominantly strike-slip Marlborough fault system. RATIONALE Earthquakes are conceptually thought to occur along a single fault. Although this is often the case, the need to account for multiple segment ruptures challenges seismic hazard assessments and potential maximum earthquake magnitudes. Field observations from many past earthquakes and numerical models suggest that a rupture will halt if it has to step over a distance as small as 5 km to continue on a different fault. The Kaikōura earthquake’s complexity defies many conventional assumptions about the degree to which earthquake ruptures are controlled by fault segmentation and provides additional motivation to rethink these issues in seismic hazard models. RESULTS Field observations, in conjunction with interferometric synthetic aperture radar (InSAR), Global Positioning System (GPS), and seismology data, reveal the Kaikōura earthquake to be one of the most complex earthquakes ever recorded with modern instrumental techniques. The rupture propagated northward for more than 170 km along both mapped and unmapped faults before continuing offshore at the island’s northeastern extent. A tsunami of up to 3 m in height was detected at Kaikōura and at three other tide gauges along the east coast of both the North and South Islands. Geodetic and geological field observations reveal surface ruptures along at least 12 major crustal faults and extensive uplift along much of the coastline. Surface displacements measured by GPS and satellite radar data show horizontal offsets of ~6 m. In addition, a fault-bounded block (the Papatea block) was uplifted by up to 8 m and translated south by 4 to 5 m. Modeling suggests that some of the faults slipped by more than 20 m, at depths of 10 to 15 km, with surface slip of ~10 m consistent with field observations of offset roads and fences. Although we can explain most of the deformation by crustal faulting alone, global moment tensors show a larger thrust component, indicating that the earthquake also involved some slip along the southern end of the Hikurangi subduction interface, which lies ~20 km beneath Kaikōura. Including this as a fault source in the inversion suggests that up to 4 m of predominantly reverse slip may have occurred on the subduction zone beneath the crustal faults, contributing ~10 to 30% of the total moment. CONCLUSION Although the unusual multifault rupture observed in the Kaikōura earthquake may be partly related to the geometrically complex nature of the faults in this region, this event emphasizes the importance of reevaluating how rupture scenarios are defined for seismic hazard models in plate boundary zones worldwide. Observed ground deformation from the 2016 Kaikōura, New Zealand, earthquake. (A and B) Photos showing the coastal uplift of 2 to 3 m associated with the Papatea block [labeled in (C)]. The inset in (A) shows an aerial view of New Zealand. Red lines denote the location of known active faults. The black box indicates the Marlborough fault system

Upper Cenozoic structural history, coastal Southern Hawke's Bay, New Zealand

Abstract The structural high (Coastal High) trending northeast along coastal Southern Hawkes Bay comprises Upper Cretaceous to Miocene successions that have been complexly folded and thrust-faulted. Major tectonic melange and crushed zones are associated with thrusts. The high developed in Late Oligocene time in conjunction with a slope basin to the west. Continued deformation led to westward growth of the high by incorporating. older basin-fill sequences. A thick wedge of flysch subsequently accumulated within the landward-migrating slope basin. The progressively tilted margins shed cohesive debris flows which interdigitate with the flysch. On the high, sedimentation is typified by a greatly condensed mudstone succession with local unconformities and onlaps which disappear basinwards. An earlier Oligocene deformation resulted from northward movement of an allochthonous gravity slide sheet, recognised by local east-west-trending structures, inverted sequences, and northward transposition. Overprinting by...

Visualization of active faults using geometric attributes of 3D GPR data: An example from the Alpine Fault Zone, New Zealand

Three-dimensional ground-penetrating radar GPR data are routinely acquired for diverse geologic, hydrogeologic, archeological, and civil engineering purposes. Interpretations of these dataareinvariablybasedonsubjectiveanalysesofreflectionpatterns. Such analyses are heavily dependent on interpreter expertiseandexperience.UsingdataacquiredacrossgravelunitsoverlyingtheAlpineFaultZoneinNewZealand,wedemonstratethe utilityofvariousgeometricattributesinreducingthesubjectivity of3DGPRdataanalysis.Weuseacoherence-basedtechniqueto compute the coherency, azimuth, and dip attributes and a graylevel co-occurrence matrixGLCMmethod to compute the texture-basedenergy,entropy,homogeneity,andcontrastattributes. A selection of the GPR attribute volumes allows us to highlight key aspects of the fault zone and observe important features not apparent in the standard images. This selection also provides information that improves our understanding of gravel deposition andtectonicstructuresatthestudysite.Anewdepositional/structuralmodellargelybasedontheresultsofouranalysisofGPRattributes includes four distinct gravel units deposited in three phases and a well-defined fault trace. This fault trace coincides with a zone of stratal disruption and shearing bound on one side by upward-tilted to synclinally folded stratified gravels and on the other side by moderately dipping stratified alluvial-fan gravelsthatcouldhavebeenaffectedbylateralfaultdrag.Whenused in tandem, the coherence- and texture-based attribute volumes can significantly improve the efficiency and quality of 3D GPR interpretation, especially for complex data collected across activefaultzones.

A model of active faulting in New Zealand

Active fault traces are a surface expression of permanent deformation that accommodates the motion within and between adjacent tectonic plates. We present an updated national-scale model for active faulting in New Zealand, summarize the current understanding of fault kinematics in 15 tectonic domains, and undertake some brief kinematic analysis including comparison of fault slip rates with GPS velocities. The model contains 635 simplified faults with tabulated parameters of their attitude (dip and dip-direction) and kinematics (sense of movement and rake of slip vector), net slip rate and a quality code. Fault density and slip rates are, as expected, highest along the central plate boundary zone, but the model is undoubtedly incomplete, particularly in rapidly eroding mountainous areas and submarine areas with limited data. The active fault data presented are of value to a range of kinematic, active fault and seismic hazard studies.

Structure of the Hanmer strike-slip basin, Hope fault, New Zealand

Hanmer basin (10 x 20 km), located in northern South Island, New Zealand, is evolving where two major segments of the dextral strike-slip Hope fault are projected to converge across a 6- to 7-km-wide releasing step-over. The structural geometry and development of Hanmer basin does not conform to traditional pull-apart basin models. The respective fault segments do not overlap but are indirectly linked along the southwest margin of the basin by an oblique normal fault. The Hope River segment terminates in an array of oblique normal faults along the northwestern basin range front, and east-west-striking normal faults on the west Hanmer Plain. Faulted Holocene alluvial-fan surfaces indicate west Hanmer basin is actively subsiding evolving under north-south extension. The Conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces, small pop-up ridges, and graben depressions. Early basin-fill sediments of Pleistocene age are being folded, elevated, and dissected as the eastern part of Hanmer basin is progressively inverted and destroyed by north-south contraction. The north margin of the basin is defined by a series of topographic steps caused by normal faulting outside of the area of the releasing step-over. These normal faults we interpret to reflect large-scale upper crustal collapse of the hanging-wall side of the Hope fault. New seismic reflection data and geologic mapping reveal a persistent longitudinal and lateral asymmetry to basin development. Four seismic stratigraphic sequences identified in the eastern sector of the basin thicken and are tilted southward, with insequence lateral onlaps occurring to the north and east, and also onto basement near the fault-controlled basin margins. The basin depocenter currently contains >1000 m of sediment adjacent to the south margin and is disrupted by faulting only at depth. In the western part of the basin, the sediment fill is thinner ( Today the rate of basin deepening under transtension at the western end is matched by its progressive inversion and destruction under transpression in the eastern sector, with the oldest basin fill now being recycled. We propose a hybrid model for Hanmer strike-slip basin, one in which geometric elements of a fault-wedge basin (downward and upward tipped, spindle-shaped ends) are combined with those of a pull-apart basin (step-over region between the major fault segments). We also conclude that changes in fault geometry (releasing and restraining bends and step-overs) at a variety of scales and over short distances control the development of the extensile and contractile parts of the basin and three-dimensional basin asymmetry. Strain partitioning is complex and cannot be related simply to local reorientation of the regional stress field.

A model of the Cenozoic evolution of northern New Zealand and adjacent areas of the southwest Pacific

Abstract New information from the southwest Pacific indicates that earlier attempts at formulating the evolution of the area assuming a single Upper Cenozoic magmatic arc are untenable. It now appears that there were two arcs during the Miocene and Pliocene, a western Northland/Three Kings Rise arc, and an eastern Tonga-Lau/Kermadec-Colville arc. Both appear to have developed above west-dipping subduction zones. It is suggested that the Norfolk- and Reinga basins formed as back-arc basins to the western arc, and that eastern North Island lay adjacent to Northland and formed the accretionary prism to that arc. Upper Cenozoic evolution of the region involved the simultaneous opening of the Norfolk/Reinga basins, consumption of the western portion of the Oligocene South Fiji Basin by subduction beneath the western arc, and eastwards movement of New Zealand/Three Kings Rise towards the Tonga/Kermadec arc. When the Kermadec and Hikurangi trenches came into line late in the Pliocene, the Tonga/Kermadec arc was able to propagate rapidly southwards into North Island; simultaneously the western arc became extinct, and the tectonic tempo and strike-slip faulting accelerated markedly throughout New Zealand. Eastern North Island was moved dextrally an uncertain distance relative to the western North Island, and rotated 25°–30° clockwise. This accounts for the paradox of a 22-m.y. old accretionary margin lying adjacent to a 2-m.y. old arc (Taupo Volcanic Zone) at the present day.

Holocene paleoearthquakes on the strike‐slip Porters Pass Fault, Canterbury, New Zealand

Abstract The Porters Pass Fault comprises a series of discontinuous Holocene active traces which extend for c. 40 km between the Rakaia and Waimakariri Rivers in the foothills of the Southern Alps. There have been no historical earthquakes on the Porters Pass Fault (i.e., within the last 150 yr), and the purpose of this paper is to establish the timing and magnitudes of displacements on the fault at the ground surface during Holocene paleoearthquakes. Displaced geomorphic features (e.g., relict streams, stream channels, and ridge crests), measured using either tape measure (n = 20) or surveying equipment (n = 5), range from 5.5 to 33 m right lateral strike slip and are consistent with six earthquakes characterised by slip per event of c. 5–7 m. The timing of these earthquakes is constrained by radiocarbon dates from four trenches excavated across the fault and two auger sites from within swamps produced by ponding of drainage along the fault scarp. These data indicate markedly different Holocene earthquake histories along the fault length separated by a behavioural segment boundary near Lake Coleridge. On the eastern segment at least six Holocene earthquakes were identified at 8400–9000, 5700–6700, 4500–6000, 2300–2500, 800–1100, and 500–600 yr BP, producing an average recurrence interval of c. 1500 yr. On the western segment of the fault in the Rakaia River valley, a single surface‐rupturing earthquake displaced Acheron Advance glacial deposits (c. 10 000–14 000 yr in age) and may represent the southward continuation of the 2300–2500 yr event identified on the eastern segment. These data suggest Holocene slip rates of 3.2–4.1 mm/yr and 0.3–0.9 mm/yr on the eastern and western sections of the fault, respectively. Displacement and timing data suggest that earthquakes ruptured the western segment of the fault in no more than one‐sixth of cases and that for a sample period of 10 000 yr the recurrence intervals were not characteristic.

Paleoseismicity of the Wellington ‐ Hutt Valley Segment of the Wellington Fault, North Island, New Zealand

Abstract The Wellington Fault is one of the major active right‐lateral strike‐slip faults of the southern North Island and represents a significant seismic hazard to the greater Wellington region. Trench excavations across the fault in the Long Gully/Karori Reservoir area and near Kaitoke, along with Quaternary stratigraphic and soil studies at Te Mania, indicate that the most recent surface rupture event along the southern portion of the Wellington Fault was 300–450 cal B .P. (calendar years before A.D. 1950) and the next oldest event was 670–830 cal B.P. The elapsed time between these two events is 220–530 years. Based on the previously reported 6.0–7.6 mm/yr, long‐term (c. 140 ka), average, horizontal slip rate calculated at Emerald Hill, and the 3.2–4.7 m single‐event offsets (the five most recent events) measured at Te Marua, the average recurrence interval for this portion of the Wellington Fault is 420–780 years. At the Long Gully trench site, two stream channels are laterally displaced by c. 50 m....