Philip M. Barnes

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.

Strain partitioning in the transition area between oblique subduction and continental collision, Hikurangi margin, New Zealand

Spatial differences in late Quaternary structural style and deformation rates indicate a complex pattern of strain partitioning within the transition area from oblique subduction beneath southern North Island to oblique continental collision in northern South Island, New Zealand. The late Quaternary structure of the offshore southern Hikurangi margin is determined here using seismic reflection profiles, MR1 side-scan sonar swath images, EM12Dual multibeam swath bathymetry and backscatter images, and sediment core and dredge samples. Where oblique (50°) subduction of the oceanic Hikurangi Plateau is occurring beneath southern Wairarapa region, the steep, dissected, upper margin consists of a wedge of rocks that are being deformed by strike-slip and reverse faults. Beneath the lower margin an accretionary wedge of Quaternary age has developed rapidly by frontal accretion of a thick succession of turbidites on the Pacific Plate. Estimates of late Quaternary geological strain rates have been made on two margin-perpendicular transects across southern North Island and the offshore margin to budget the predicted NUVEL-1A plate motion. More than 33–55 % of the total plate motion is expressed in the offshore part of the upper plate above the subduction detachment in the central Hikurangi margin between 40°S and 41°S. This offshore deformation could potentially include margin-parallel fault slip of up to 14–15 mm/yr. Across the upper plate, strain is partitioned into zones of folding and thrust faulting and possibly more than one zone of strike-slip faulting, but the partitioning may be incomplete with oblique-slip thrusting an important component of the deformation. The wide distribution of deformation within the upper plate partially reflects strong coupling between the subducting oceanic plateau and the leading edge of the overriding plate. This coupling increases toward the southwest, where the continental part of the subduction zone beneath Marlborough may now be inactive or very strongly coupled. Toward the southwest the transpressive deformation style of the upper Wairarapa margin continues into the upper Marlborough margin and onshore into the strike-slip Marlborough Fault System of northern South Island. In contrast, the accretionary wedge narrows to the southwest and terminates in southeastern Cook Strait, approximately above the transition from subducted oceanic crust to subducted continental crust of the Pacific Plate. Farther southwest beneath the lower Marlborough margin, low-strain folding and thrust faulting is occurring, but the style of deformation is not typical of the accretionary wedge. This shortening results either from a small amount of subduction slip beneath Marlborough and/or from contractional strain partitioned to the southeast of the continental transpression zone. Whereas the initial onset of continental collision occurs in Marlborough, the surface expression of deformation of the Pacific Plate occurs south of 42°30′S. Normal faulting of the northwestern margin of the continental Chatham Rise possibly reflects flexural extension of the Pacific Plate crust at the southern end of the subduction zone, whereas folding and thrust faulting of the north Canterbury coastal region results from NW-SE contraction of the delaminated upper crust entering the Southern Alps collision zone.

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

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrusts seismogenic zone.

Rates and mechanics of rapid frontal accretion along the very obliquely convergent southern Hikurangi margin, New Zealand

Analysis of seismic reflection profiles, swath bathymetry, side-scan sonar imagery, and sediment samples reveal the three-dimensional structure, morphology, and stratigraphic evolution of the central to southern Hikurangi margin accretionary wedge, which is developing in response to thick trench fill sediment and oblique convergence between the Australian and Pacific plates. A seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby, by coccolith and foraminifera biostratigraphy of three core and dredge samples, and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment. Structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds, on the growth and mechanics of frontal accretion under variable convergence obliquity, and on the amounts and rates of horizontal shortening. The data place constraints on the partitioning of geological strain across the entire southern Hikurangi margin. The principal deformation front at the toe of the wedge is discontinuous and represented by right-stepping thrust faulted and folded ridges up to 1 km high, which develop initially from discontinuous protothrusts. In the central part of the margin near 41°S, where the convergence obliquity is 50°, orthogonal convergence rate is slow (27 mm/yr), and about 75% of the total 4 km of sediment on the Pacific Plate is accreted frontally, the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening. At least 80% of this shortening has occurred within the last 0.4±0.1 m.y. at an average rate of 12±3 mm/yr. This rate indicates that the frontal 30 km of the wedge accounts for about 33–55% of the predicted orthogonal contraction across the entire plate boundary zone. Despite plate convergence obliquity of 50°, rapid frontal accretion has occurred during the late Quaternary with the principal deformation front migrating seaward up to 50 km within the last 0.5 m.y. (i.e., at a rate of 100 km/m.y.). The structural response to this accretion rate has been a reduction in wedge taper and, consequently, internal deformation behind the present deformation front. Near the southwestern termination of the wedge, where there is an along-the-margin transition to continental transpressional tectonics, the convergence obliquity increases to >56°, and the orthogonal convergence rate decreases to 22 mm/yr, the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward-verging folds. These structures have accommodated not more than 0.5 km of horizontal shortening at a rate of < 1 mm/yr, which represents < 5% of the predicted orthogonal shortening across the entire plate boundary in southern North Island. The landward-vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin, to the northern South Island region where the upper and lower plates are locked or at least very strongly coupled.

Late Cenozoic evolution and earthquake potential of an active listric thrust complex above the Hikurangi subduction zone, New Zealand

In the center of the frontal wedge of the Hikurangi subduction zone, New Zealand, Mahia Peninsula and its submarine continuation, Lachlan Ridge, are being uplifted and folded above an active landward- dipping thrust-fault complex that is 80 km long. High-quality marine seismic reflection profiles reveal complex deformation of a Cretaceous to Holocene sedimentary section and enable a detailed analysis of the stratigraphy, structural evolution, deformation rates, and future earthquake potential. The structural analysis is facilitated by uplifted marine terraces on Mahia Peninsula and by 14 submarine unconformities in the hanging-wall sequence, five of which are correlated across the eroded crest of Lachlan Ridge and into the footwall basin. The ages of the unconformities are determined by seismic ties to an offshore exploration well, onshore outcrops on the peninsula, and seabed samples dated by pollen, coccolith nannoflora, and foraminifera biostratigraphy. Nine regional Quaternary unconformities, which developed in response to eustatic fluctuations in sea level and are not older than ca. 1 Ma, are correlated with oxygen isotope stages in equatorial Pacific Ocean Drilling Project core 677. The ages of fault-growth strata and progressive restorations of deformed stratigraphy indicate that Lachlan Ridge developed during three phases of deformation since subduction of the Pacific plate commenced beneath the Australian plate in the early Miocene. These include an initial phase of thrust faulting, a subsequent phase dominated by extensional faulting, and the current, mainly Pleistocene to Holocene phase of structural inversion, reactivated listric thrust faulting, and folding. Early to middle Miocene thrusts in the deeper core of the complex developed out of sequence, by sequentially stepping up into the hanging-wall section, creating an imbricate fan with emerging thrust tips now buried within the forelimb basin. In the middle Miocene–early Pliocene, listric extensional faults developed in the active thrust wedge—possibly as a result of substantial relief on the subducted Pacific plate—and controlled the development of Lachlan Basin to the west of the ridge. The principal active thrust is the Lachlan fault, a listric extensional detachment reactivated to accommodate thrust movement and consisting of at least three right-stepping segments. Depth-converted seismic profiles indicate that the fault dips westward at 15°–20°, 6–8 km beneath the western flank of Lachlan Ridge, and steepens to 55°–70° or even steeper in the upper 1–2 km of section beneath the eastern flank. Syninversion Pleistocene fault-growth strata on both flanks of the associated anticline provide an exceptional record of progressive fold-limb rotation resulting from the listric-fault geometry. A geometric analysis of the fault-growth strata and deformed terraces was used to derive a maximum dip-slip displacement rate of as fast as 3.0–6.5 mm/yr. The implied shortening rate of 2.6–6.3 mm/yr represents ∼8%–20% of the total 31 mm/yr of orthogonal plate convergence across this part of the upper plate of the Hikurangi subduction zone. The top of the pre–fault-growth Paleogene section reveals as much as 5.8 ± 1.5 km of vertical separation in the north, decreasing to ∼30%–50% of this value in the south. Temporal (10 3 –10 6 yr) and spatial (10 3 –10 4 m of strike length) variations in vertical-deformation rate have occurred during the past 1 m.y.; maximum rates occurred in the Holocene and middle Quaternary. A long-term increase in vertical- separation rate on all segments during the Pleistocene largely reflects a change in thrust kinematics associated with structural inversion. The relatively greater increase in uplift rate on the northern part of the fault during the past 1 m.y. could be related to the possibility that a subducted seamount lies >10 km beneath the peninsula. Estimates of earthquake source parameters, incorporating paleoseismic uplift data from Mahia Peninsula, indicate a potential moment magnitude of up to M w 7.6–8.0 for an earthquake that ruptures all three segments of the Lachlan fault. The average recurrence interval for such events is estimated to be 615–2333 yr, which is consistent with a mean recurrence interval of 1062 yr for four late Holocene earthquakes. Thus, the uplift and folding of Mahia Peninsula and Lachlan Ridge results from coseismic displacements on a major listric thrust fault that ruptures the upper plate frequently in association with large-magnitude earthquakes.

Kaikoura Canyon, New Zealand: active conduit from near-shore sediment zones to trench-axis channel

Kaikoura Canyon is one of a few major, active conduits between a near-shore sediment transport system and a deep-ocean channel. It is the sink for mobile zones of gravel, sand and mud that migrate northwards off northeastern South Island, New Zealand. It is presently the primary source for the 1500 km long Hikurangi Channel, which supplies overbank turbidites to a filled trench, an ocean-plateau basin and a distal fan-drift. Swath data, seismic profiles, side-scan sonographs, aerial photographs, cores, grab samples and current meter data are used to define the shape and texture of the canyon and adjacent shelf in order to better understand how coastal processes and canyon interact to supply sediment to the deep-ocean. Kaikoura Canyon is 60 km long, up to 1200 m deep and generally U-shaped in profile. Its head is within 500 m of the shore, and within 200 m of rocky projections from the shore-platform, in a mountain-backed bay without large rivers. The canyon head incises the 18 m depth contour and boulders, pebble gravel and megarippled coarse sand reach the canyon rim. Fine sand migrating northward along the shelf under the influence of waves and currents is trapped in a southward-projecting, canyon-head gully, which incises the thickest part of the Holocene sediment prism. It is estimated that about 1.5×106 m3 of sediment falls into the canyon head each year. Tensional fractures around the canyon rim suggest that sediments in the canyon-head gully are unstable. Gravel turbidites, with post last glacial age shells, are at or near the seabed in the lower canyon but are blanketed by many thin, silt and sand, possibly storm-generated, turbidites in the upper canyon. The top gravel contains a twig that is about 170 years old, suggesting that the last major collapse in the canyon head coincides with many onshore rockfalls triggered by rupture of a major, strike-slip, plate-boundary fault in about 1833. An underlying gravel is about 300 years old and may again coincide with fault rupture. Most of the large, earthquake-triggered, failures may “ignite” to form self-perpetuating, autosuspension flows, that feed a 1500 km long, deep-sea, turbidite channel.

Faulting and Extension Rate over the last 20,000 Years in the Offshore Whakatane Graben, New Zealand Continental Shelf.

Oblique rifting in the offshore Taupo Volcanic Zone, New Zealand, is expressed in widely distributed active normal faulting in the 20 km-wide Whakatane Graben. Active faults are identified along seafloor scarps and displacements of the post-last glacial transgressive ravinement surface ( 400 faults. We derive an extension rate at seismogenic depths (6-10 km) across the graben of 13 ± 6 mm/yr, by summing surface measurements, assuming an average crustal fault dip of 45±15o, and correcting for the discrepancies between surface and deep crustal extension estimates. Structural and kinematic data implies an extension direction 20o oblique to the rift axis, resulting in up to 4.6 ± 2.1 mm/yr of dextral motion parallel to the rift axis. The strike-slip motion is accommodated by dip-slip displacements on oblique faults in the centre of the graben, and oblique-slip faulting along the rift margins. Pure dip-slip in the graben centre represents >50% of the total slip, with the Rangitaiki Fault accommodating 25% of the total extension in the graben.

Strike-slip structure and sedimentary basins of the southern Alpine Fault, Fiordland, New Zealand

The Alpine Fault is an 850-km-long, continental dextral strike-slip fault that accommodates some 60%–90% (∼20–30 mm/yr) of the obliquely convergent motion between the Pacific and Australian Plates in South Island, New Zealand. The southern 230 km of the fault traverses the continental margin off Fiordland and intersects the subduction thrust at the northern end of the Puysegur Trench. Marine seismic reflection profiles and bathymetric data are used to evaluate the late Quaternary structure and 3 m.y. evolution of the fault and five sedimentary basins. The fault offshore is more complexly segmented than its onshore counterpart, and at sedimentary basins has substantial (>1 km) bathymetric relief. Three right-stepping and overlapping active sections are identified on the basis of structural continuity and geomorphic expression. The northern, Milford-Caswell section, is 90 km long and continuous with the southern Westland section on land. The southern, Resolution section is 150 km long and developed from initially discontinuous, right-stepping segments. These sections overlap by 35 km at the large Secretary-Nancy Basin, which is currently being dissected longitudinally by the Nancy section, resulting in a straighter, almost fully linked principal displacement zone. The average strike of the fault differs from the azimuth of Pacific-Australian Plate motion by 11°–25°. Nevertheless, on the whole, the structure, tectonic geomorphology, and lateral displacements indicate predominantly dextral strike-slip displacement. The basins have the structural characteristics of pull-apart and releasing-bend basins. They commonly initiated at step-overs in the fault and evolved by development of very oblique cross- and along-basin faults that linked the principal displacement zone, resulting in longer, through-going surface traces. Localized transpressional features are developing contemporaneously with the basins and may relate to constrictional fault geometries and/or a 10° rotation of part of the fault within the last 3 m.y., rather than to changes in plate motions. The absence of significant regional contraction across the fault indicates that obliquely convergent plate motion is strongly partitioned. The convergence normal to the boundary is accommodated largely on thrust and reverse faults offshore and onshore, to the northwest and southeast of the Alpine Fault, respectively. The position of the Alpine Fault is clearly associated with the position of inherited Eocene rift structures carried in the subducting Australian Plate. Our observations and kinematic model imply a spatial-temporal evolution where the position of the southwestern end of the fault trace is transitory on a time scale of 10 5 –10 6 years, and the fault matures toward the northeast. The data are consistent with models invoking dextral reactivation of the subducting rift structures and tearing of the Australian Plate in the approximate direction of plate convergence, and/or the loading of shear stresses and strike-slip deformation on the Alpine Fault in response to spatial differences in surface roughness and interplate coupling. Several potential earthquake scenarios on the Alpine Fault are considered, ranging from rupture of individual structural sections associated with earthquakes of magnitude M 7.0–7.8, to larger, onshore-offshore composite ruptures of up to M 8.1.

Recognition of active strike-slip faulting from high-resolution marine seismic reflection profiles: Eastern Marlborough fault system, New Zealand

Major strike-slip faults within the eastern part of the Marlborough fault system of New Zealand extend offshore beneath the continental shelf, into the southern end of the Hikurangi margin. Six major submarine faults, each tens of kilometers in length and exhibiting the three-dimensional structural characteristics typical of strike-slip deformation zones, are mapped in detail using closely spaced high-resolution seismic-reflection profiles. Each fault displaces late Quaternary sediments that have been interpreted within the framework of the sequence stratigraphic model and the established history of glacio-eustatic sea-level cyclicity. Some of the faults are associated with strike-slip microearthquake focal mechanisms. The faults are within a complex structural high that underlies much of the outer continental shelf, and within the Flaxbourne basin between the structural high and the coast. The plan-view pattern of faulting recorded in the Quaternary sediments reflects only the major surface traces that have propagated upsection as contemporaneous sedimentation has blanketed most of the evolving structures. Regionally, the late Quaternary faults compose two groups that bound blocks with rhomboid surface areas on a scale of tens of square kilometers to hundreds of square kilometers. One group strikes typically between 023° and 057°, i.e., 22°–56° from the plate motion vector (079°), and includes dextral, oblique-slip thrust faults as well as inherited, steeply dipping, pre-Pliocene strike-slip faults that bound thick sedimentary basins. The second group comprises possibly young (<1 Ma), steeply dipping strike-slip faults that commonly exhibit normal-slip separation, and strike between 067° and 085°, subparallel to the azimuth of the instantaneous Pacific-Australian plate motion vector.