Wairarapa Fault Research Articles

Tsunami or storm deposit? A late Holocene sedimentary record from Swamp Bay, Rangitoto ki te Tonga/D’Urville Island, Aotearoa – New Zealand

ABSTRACT Informed by Māori oral histories that refer to past catastrophic marine inundations, multi-proxy analysis of stratigraphic records from Swamp Bay, Rangitoto ki te Tonga (D’Urville Island) shows evidence of an anomalous deposit extending some 160 m inland. The deposit includes two distinct lithofacies. The lower sand unit is inferred to have been transported from the marine environment, with corresponding increases in the percentages of benthic marine and brackish–marine diatoms, and geochemical properties indicative of sudden changes in environmental conditions. Radiocarbon dating indicates the deposit formation is less than 402 yrs BP, and pollen indicates it is unlikely to be younger than 1870 CE. Core stratigraphy age models and co-seismic chronologies point to the marine unit most likely being emplaced by tsunami transport associated with rupture of the Wairarapa Fault in 1855 CE. The overlying unit of gravel and silt is inferred to be fluvial deposit and slope-wash from the surrounding hills, loosened by ground-shaking following the earthquake. These findings indicate the 1855 CE earthquake may have been more complex than previously thought and, or, available tsunami modelling does not fully capture the local complexities in bathymetry and topography that can cause hazardous and localized tsunami amplification in embayments like Swamp Bay.

Pleistocene marine terraces of the Wellington south coast – their distribution across multiple active faults at the southern Hikurangi subduction margin, Aotearoa New Zealand

ABSTRACT Along the south coast of the North Island of New Zealand, elevated Pleistocene marine terraces provide evidence for vertical deformation associated with active crustal faults and the westward subduction of the Pacific Plate at the southern Hikurangi margin. We have reassessed the age and elevation of these terraces. Optically Stimulated Luminescence (OSL) data of their cover bed sediments (20 new ages) provide the first numerical ages for most of these terraces. Shore platform elevations have been surveyed using differential Global Navigation Satellite System (GNSS) measurements of the wave-cut bedrock straths underlying these terraces. These new data allow the terraces to be temporally correlated across the margin. Seven different-aged terraces were identified and mapped along the Wellington south coast; these are preserved discontinuously between the westernmost site at Tongue Point, and Ngawi near Cape Palliser, to the east. The OSL data indicate that most of these terraces formed at MIS 5a, 5c, 5e (Last Interglacial) and 7a. The terraces are best preserved within the Hikurangi margin forearc, where they decrease in altitude towards the west, indicating long wavelength, westward tectonic tilting. The terraces are locally offset by a number of active crustal faults, most notably the Wairarapa and Ohariu faults.

Repeated giant earthquakes on the Wairarapa fault, New Zealand, revealed by Lidar-based paleoseismology

The Mw 7.8 2016 Kaikoura earthquake ruptured the Kekerengu-Needle fault resulting in the loading of its eastern continuation, the Wairarapa fault. Since the most recent earthquake on Wairarapa occurred in 1855 and is one of the strongest continental earthquakes ever observed, it is critical to assess the seismic potential of the Wairarapa fault, which might be prone to break. Using Lidar data, we examine its bare-earth morphology and reveal ~650 mostly undiscovered offset geomorphic markers. Using a code we developed in earlier work, we automatically measure the lateral and vertical offsets of these markers providing more than 7000 well constrained measurements. The data document the lateral and vertical slip profiles of the 1855 earthquake for the first time and show its total slip reached ~20 m at surface. Modeling the entire offset dataset reveals 7 prior earthquakes ruptured the entire fault, each similarly producing 16.9 ± 1.4 m dextral slip and ~0.6 m vertical slip at surface in the same central bend zone of the fault. Thus, the Wairarapa fault repeatedly produced giant earthquakes and is likely able to produce a similarly strong forthcoming event. The extreme large size of the Wairarapa earthquakes questions our understanding of earthquake physics.

Triple junction kinematics accounts for the 2016 Mw 7.8 Kaikoura earthquake rupture complexity

The 2016, moment magnitude (Mw) 7.8, Kaikoura earthquake generated the most complex surface ruptures ever observed. Although likely linked with kinematic changes in central New Zealand, the driving mechanisms of such complexity remain unclear. Here, we propose an interpretation accounting for the most puzzling aspects of the 2016 rupture. We examine the partitioning of plate motion and coseismic slip during the 2016 event in and around Kaikoura and the large-scale fault kinematics, volcanism, seismicity, and slab geometry in the broader Tonga-Kermadec region. We find that the plate motion partitioning near Kaikoura is comparable to the coseismic partitioning between strike-slip motion on the Kekerengu fault and subperpendicular thrusting along the offshore West-Hikurangi megathrust. Together with measured slip rates and paleoseismological results along the Hope, Kekerengu, and Wairarapa faults, this observation suggests that the West-Hikurangi thrust and Kekerengu faults bound the southernmost tip of the Tonga-Kermadec sliver plate. The narrow region, around Kaikoura, where the 3 fastest-slipping faults of New Zealand meet, thus hosts a fault-fault-trench (FFT) triple junction, which accounts for the particularly convoluted 2016 coseismic deformation. That triple junction appears to have migrated southward since the birth of the sliver plate (around 5 to 7 million years ago). This likely drove southward stepping of strike-slip shear within the Marlborough fault system and propagation of volcanism in the North Island. Hence, on a multimillennial time scale, the apparently distributed faulting across southern New Zealand may reflect classic plate-tectonic triple-junction migration rather than diffuse deformation of the continental lithosphere.

A 2000 Yr Paleoearthquake Record along the Conway Segment of the Hope Fault: Implications for Patterns of Earthquake Occurrence in Northern South Island and Southern North Island, New Zealand

Abstract Paleoseismic trenches excavated at two sites reveal ages of late Holocene earthquakes along the Conway segment of the Hope fault, the fastest-slipping fault within the Marlborough fault system in northern South Island, New Zealand. At the Green Burn East (GBE) site, a fault-perpendicular trench exposed gravel colluvial wedges, fissure fills, and upward fault terminations associated with five paleo-surface ruptures. Radiocarbon age constraints indicate that these five earthquakes occurred after 36 B.C.E., with the four most recent surface ruptures occurring during a relatively brief period (550 yr) between about 1290 C.E. and the beginning of the historical earthquake record about 1840 C.E. Additional trenches at the Green Burn West (GBW) site 1.4 km west of GBE reveal four likely coseismically generated landslides that occurred at approximately the same times as the four most recent GBE paleoearthquakes, independently overlapping with age ranges of events GB1, GB2, and GB3 from GBE. Combining age constraints from both trench sites indicates that the most recent event (GB1) occurred between 1731 and 1840 C.E., the penultimate event GB2 occurred between 1657 and 1797 C.E., GB3 occurred between 1495 and 1611 C.E., GB4 occurred between 1290 and 1420 C.E., and GB5 occurred between 36 B.C.E. and 1275 C.E. These new data facilitate comparisons with similar paleoearthquake records from other faults within the Alpine–Hope–Jordan–Kekerengu–Needles–Wairarapa (Al-Hp-JKN-Wr) fault system of throughgoing, fast-slip-rate (≥10 mm/yr) reverse-dextral faults that accommodate a majority of Pacific–Australia relative plate boundary motion. These comparisons indicate that combinations of the faults of the Al-Hp-JKN-Wr system may commonly rupture within relatively brief, ≤100-year-long sequences, but that full “wall-to-wall” rupture sequences involving all faults in the system are rare over the span of our paleoearthquake data. Rather, the data suggest that the Al-Hp-JKN-Wr system may commonly rupture in subsequences that do not involve the entire system, and potentially, at least sometimes, in isolated events.

Reciting the layers: Evidence for past tsunamis at Mataora-Wairau Lagoon, Aotearoa-New Zealand

Sedimentary, geochemical, microfossil and geochronological analyses were carried out on 10 sediment cores across the south-eastern corner of Mataora-Wairau Lagoon (M-WL), South Island, Aotearoa-New Zealand. This multi-proxy approach provides strong evidence for three late Holocene palaeotsunamis (two previously unreported) and a historical tsunami generated by rupture of the Wairarapa Fault in 1855CE. The main scientific evidence for these events consists of anomalous, high-energy marine sediment layers that overlie material of contrasting composition. These layers coincide with peaks in planktonic and benthic marine to brackish-marine diatoms and geochemical signatures indicative of sudden changes in environmental conditions. Palaeotsunami I (maximum date 2095cal.BP) and Palaeotsunami II (maximum date 915cal.BP - previously identified by Clark et al., 2015) appear to represent large events contemporaneous with major earthquakes inducing tectonic uplift and coseismic subsidence around the Cook Strait region. Palaeotsunami II is close to the date of the earliest settlement of the M-WL area by Māori, and a traditional narrative that references loss of life following waves over-washing the Wairau boulder bank. Given the boulder bank was likely to have been seaward of its current position during events I and II, we surmise that the inundation distances would have been greater than the most landward extent (340m) inferred from the deposits found in this study. Palaeotsunami III (maximum date 525cal.BP) was associated with tectonic subsidence across the study site, most likely associated with rupture of the southern Hikurangi subduction zone. We expect the ongoing refinement of the methods used in this study, and the results from future studies of analogue sites and offshore fault systems, will improve our understanding of the significant tsunami hazard facing this region.

SAHKE seismic‐scatter imaging of subduction beneath Wellington, North Island, New Zealand

AbstractScattering reflectivity analysis of onshore seismic data (Seismic Array HiKurangi Experiment) images slab geometry and crustal structure at a geodetically locked subduction boundary. A broad seismic scattering zone (BSZ) up to 10 km thick above the subduction interface at depths of 16–30 km has seismic properties (Vp, Vp/Vs, and Q) that imply high fluid pressures, probably associated with underplated sediments. The listric strike‐slip Wairarapa Fault and other splay faults that sole into the BSZ imply long‐term weakness. High rates of intraslab seismicity with low rates above the slab imply stress decoupling across the BSZ. Association of high fluid pressures and long‐term weakness with geodetic locking is in contrast to similar observations at zones of slow slip at the northern Hikurangi margin and suggest that either (1) pore fluid pressure or locking behavior may cycle over times longer than two decades or (2) local material properties play a significant role in determining if slow slip or locking occurs.

Representative ground-motion ensembles for several major earthquake scenarios in New Zealand

In this paper, representative ground motion ensembles for several major earthquake scenarios in New Zealand are developed. Cases considered include representative ground motions for the occurrence of Alpine, Hope and Porters Pass earthquakes in Christchurch city, and the occurrence of Wellington, Wairarapa and Ohariu fault ruptures in Wellington city. For each considered scenario rupture, ensembles of 20 and 7 ground motions are selected using the generalized conditional intensity measure (GCIM) approach, ensuring that the ground motion ensembles represent both the mean and distribution of ground motion intensity which such scenarios could impose. These scenario-based ground motion sets can be used to complement ground motions which are often selected in conjunction with probabilistic seismic hazard analysis, in order to understand the performance of structures for the question “what if this fault ruptures?”

Wairau Basin and fault connections across Cook Strait, New Zealand: seismic and geological evidence

Cook Strait separates North Island and South Island of New Zealand. It contains the Wairau and other sedimentary basins, which have sedimentary thicknesses of up to 4 km. The Strait overlies part of the Australia–Pacific plate boundary where subduction below the North Island changes to strike-slip motion through the South Island. Strike-slip faults are well documented in the South Island and less so in the North Island. Cook Strait has been surveyed by a variety of shallow penetration seismic in the past, and this has resulted in various models and uncertainty about how major faults join across the strait. The shallow-penetration data image numerous fault traces close to the sea bed, most of which can be traced for only a short distance. Gaps between individual fault traces are common. In recent years, a growing body of research, government and oil company seismic data has imaged structures much more deeply and is the subject of this paper. The Wairau Basin structural style is one dominated by normal faulting, with expansion of basin area and progressive eastwards shift in depocentres with time. A steep structural gradient or faulted margin to basement subcrop and outcrop on the North Island margin indicates a significant north-northwest to northwest component of fault control in this area of Neogene development. The seismic interpretation of this deeper data suggests there are continuous seismogenic zones underlying the discontinuous young seafloor fault scarps. Such discontinuities have led to a perception that Cook Strait faults are not connecting through, that fault rupturing across Cook Strait is improbable. Fault connections across Cook Strait along seismogenic zones have recently been interpreted from historical records. These indicate that the October 1848 earthquakes involved mainshock (Mw 7.4–7.7) displacement on the Awatere Fault in the South Island and probable aftershock displacement on the Ohariu and Wellington faults in the North Island. The January–February 1855 earthquakes (mainshock Mw ∼8.2–8.4) occurred on the Wairarapa Fault in southern North Island, Wharekauhau, Nicholson Bank faults in Cook Strait, Vernon and Awatere faults in the South Island, and Needles Fault off the NE Marlborough coast. These interpretations are supported by recent seismic data that indicate underlying basin-defining seismogenic zones connect across Cook Strait, with movements that date back at least to the top Paleogene.

SAHKE geophysical transect reveals crustal and subduction zone structure at the southern Hikurangi margin, New Zealand

The Seismic Array Hikurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath the southern North Island along a 350 km transect. Tomographic inversion of first‐arrival travel times was used to derive a well‐resolved 15–20 km deep P wave image of the crust. The refracted phases and migrated reflection events image subducting slab geometry and crustal structure. In the west, Australian Plate Moho depth decreases westward across the Taranaki Fault system from 35 to ∼28–30 km. In the east, subducted Pacific Plate oceanic crust is recognized to have a positive velocity gradient, but becomes less distinct beneath the Tararua Ranges, where the interface increases in dip at about 15 km depth from <5° to >15°. This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching, and low‐velocity high‐attenuation material that we interpret to be an underplated subduction sedimentary channel. We infer that a step down in the decollément transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the Tararua Ranges. Reflections from the Wairarapa Fault show that it is listric and soles into the top of underplated sediments, which in turn abut the Moho of the overriding plate at ∼32 km depth, near the downdip end of the strongly locked zone. The change in dip of the Hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface.

Conditional probability of rupture of the Wairarapa and Ōhariu faults, New Zealand

New information on the timing of past ruptures, size of single-event displacements and Holocene dextral slip rate of the Wairarapa and Ōhariu faults has become available from recent geological studies. This information is used to evaluate the conditional probability of rupture over the next 100 yr using four different recurrence–time models, allowing for data and parameter uncertainties. The sensitivity of estimates to data and distributional assumptions is examined. The southern Wairarapa Fault has a probability of rupture in the next 100 yr of c. 7.7, 1.3, 2.3 and 4.3% under the exponential, lognormal, Weibull and inverse Gaussian models, respectively, based on preferred data inputs. Corresponding estimates for the Ōhariu Fault are c. 4.3, 5.1, 3.9 and 5.1%. A logic tree with subjectively assigned weights is suggested to combine such differing time-dependent estimates. This gives a 100-yr conditional probability of rupture of c. 3.0% for the southern Wairarapa Fault and c. 4.9% for the Ōhariu Fault.

Making Wellington: earthquakes, survivors and creating heritage

Landing at Te Whanganui a Tara in 1840, New Zealand Company settlers lost no time to construct the "England of the South" using familiar building materials of brick, stone, clay and mortar. Within months of settling at Pito-one (Petone), the newly arrived people not only experienced earthquakes, but also flooding of Te Awa kai Rangi (Hutt River). Consequently, the original plan to build the City of Britannia at Pito-one was transferred to Lambton Harbour at Pipitea and Te Aro.
 The construction of Wellington was severely disrupted by the first visitation occurring on 16 October 1848 when the Awatere fault ruptured releasing an earthquake of Mw 7.8. The earthquake sequence, lasting until October 1849, damaged nearly all masonry buildings in Wellington, including newly constructed Paremata Barracks. This event was soon followed by the 2nd visitation of 23 January 1855. This time it was a rupture of the Wairarapa fault and a huge 8.2 Mw earthquake lasting until 10 October 1855.
 Perceptions of buildings as "permanent" symbols of progress and English heritage were fundamentally challenged as a result of the earthquakes. Instead, the settlers looked to the survivors – small timber-framed buildings as markers of security and continued occupation. A small number of survivors will be explored in detail – Taylor-Stace Cottage, Porirua, and Homewood, Karori, both buildings of 1847 and both still in existence today. Also the ruins of Paremata Barracks as the only remnant of a masonry structure pre-dating 1848 in the Wellington region. There are also a few survivors of 1855 earthquake including Christ Church, Taita (1854) and St Joseph's Providence Porch, St Mary's College, Thorndon (1852). There are also the post-1855 timber-framed legacies of Old St Paul's Cathedral (1866), Government Buildings (1876) and St Peter's Church (1879). Improved knowledge about the historical evolution of perceptions of heritage in Wellington as a result of past earthquake visitations can help inform public education about heritage values, how to build today and strengthen existing buildings in readiness for future earthquake visitations.

Geometry and scale of fault segmentation and deformational bulging along an active oblique-slip fault (Wairarapa fault, New Zealand)

The active dextral-reverse Wairarapa fault in the North Island, New Zealand, cuts ca. 12 ka, post–Last Glacial Maximum (post-LGM) fluvial gravels with a highly segmented fault trace. This

Using displaced river terraces to determine Late Quaternary slip rate for the central Wairarapa Fault at Waiohine River, New Zealand

We analyse a flight of post-Last Glacial Maximum terraces at Waiohine River, New Zealand that are progressively displaced by the dextral-slip Wairarapa Fault. The Waiohine River is shown to have smoothed its faulted river banks after each earthquake so that riser displacements are only recorded after abandonment of their lower bounding terrace tread. Digital elevation models produced from newly collected high-precision topographic data allowed us to precisely measure the cumulative dextral displacement of five risers and two palaeochannels, and the vertical displacement of six treads. The ratio of horizontal to vertical slip at the terraces ranges from 5.0 to 10.2 (average of 6.9±3.5 (2σ)). Combining our new displacements for the ‘Waiohine’ aggradation terrace and next younger terrace with new and previous optically stimulated luminescence (OSL) ages for Waiohine terrace silts, we calculate Late Quaternary dextral slip rates for this central part of the Wairarapa Fault of 11.9±2.9 mm a−1 (2σ) and >9.2±1.3 mm a−1 (2σ). Based on the smallest and next smallest observed offsets, the magnitudes of the inferred 1855 (smallest) and penultimate (next-smallest) single-event displacements are 12.4±0.8 m (2σ) and 9.7±1.7 m (2σ), respectively.

Displaying Seismic Deaggregation: The Importance of the Various Sources

Seismic hazard deaggregation has become a standard part of probabilistic seismic hazard assessment (PSHA). The first product of PSHA is calculation of the likely severity of ground motion at a given range of annual probability levels, and this is extremely important for seismic design of structures to be built at the site under examination. However, for full analysis of proposed structural designs, engineers also need to examine scenario events to produce detailed time histories. To select such scenarios, a deaggregation of the hazard is performed, whereby the details of sources that contribute to the annual frequency of exceeding specified levels of ground motion, or P exc, are identified. A common format for such a deaggregation is shown in Figure 1. This relates to the 475-year peak ground acceleration (pga) at Wellington, New Zealand (41.28°S 174.77°E), and shows the distribution in magnitude and distance of sources that contribute to P exc. Return period is approximately the reciprocal of P exc. Stiff soil site conditions (Standards New Zealand 2004) were assumed. The analysis in Figure 1 used the interim version of the updated seismic hazard model for New Zealand (Stirling et al. 2007), with the attenuation function developed by McVerry et al. (2007). Based on a Poisson time dependence model, a return period of 475 years corresponds to a 10% probability of exceedance in 50 years. From Figure 1, it is apparent that for this site the main contribution to ground motion of this severity is from earthquakes of magnitude about 7.6 less than 10 km from the site (blue), and there is another strong contribution from larger events in the distance range 10 to 20 km (red). These correspond to the Wellington and Wairarapa faults, respectively (see Table 1). There are other events less than 10 …

Post c. 300 year rupture of the Ohariu Fault in Ohariu Valley, New Zealand

An important component of quantifying seismic hazard and risk in regions such as Wellington is to characterise both the long-term rate of occurrence of the region's major earthquake-generating active faults, as well as potential interactions between faults (e.g., the potential for earthquake triggering). This paper describes paleoseismic data from two trenches (Ohariu Valley) and a natural streambank exposure (Horokiri valley) c. 24 km apart, which constrain the timing and size of recent surface rupture events on the northeast striking Ohariu Fault c. 6 km northwest of Wellington City. One event is recorded at all three sites and radiocarbon age constraints indicate it correlates with the 1050–1000 cal. years BP event previously identified elsewhere along the fault. A younger, smaller (decimetre-scale) event is recorded in one of the Ohariu Valley trenches, and the timing is constrained by two radiocarbon ages to post-310 cal. years BP. This event may either be a small, primary Ohariu Fault rupture, or a triggered event associated with a large earthquake on a nearby fault. If it is a triggered rupture, then a possible mechanism is dynamic triggering associated with one of the recent large-great earthquakes on the Wellington (post-300 yr), Wairarapa (AD1855), or Awatere (AD1848) Faults. Small rupture events do not necessarily contribute to the recurrence interval classification of the Ministry for the Environment Active Fault Guidelines, but they could be important for sensitive structures crossing the fault and for lifelines crossing multiple active faults.

Possible reduction of earthquake hazard on the Wellington Fault, New Zealand, after the nearby 1855, M 8.2 Wairarapa earthquake and implication for interpreting paleoearthquake intervals

Based on the indicative modelling, the changes in Coulomb failure function (?CFS) suggest that the W-HV segment and the T-P segment could be stable in at least the future 300 years and 190 years respectively, for these periods should be needed to accumulate the stress released by the M 8.2 Wairarapa earthquake, assuming that there is no influence from other sources, the earthquake did not alter the failure threshold, and that failure is a fairly deterministic process. The results also show that the influence on the W-HV segment and T-P segment of the Wellington Fault caused by the 1855, M 8.2 Wairarapa earthquake is significant considering that the average fault rupture recurrence interval on the Wellington Fault is about 500-770 years. With our present understanding of the Wellington and Wairarapa faults, it can be concluded that the 1855 Wairarapa earthquake retarded earthquake occurrence on the W-HV segment and the T-P segment of the Wellington Fault. Thus the seismic hazard in the Wellington region may be over-estimated.

Quaternary deformation along the Wharekauhau fault system, North Island, New Zealand: Implications for an unstable linkage between active strike‐slip and thrust faults

The southern Wairarapa region of the North Island of New Zealand preserves a variably deformed late Quaternary stratigraphic sequence that provides insight into the temporal variability in the partitioning of contraction onto faults in the upper plate of an obliquely convergent margin. Detailed mapping, stratigraphic data, and new radiocarbon and optically stimulated luminescence ages from Quaternary units reveal the interaction between tectonics and sedimentation from ∼125 ka to <9 ka along the Wharekauhau thrust and related faults (i.e., Wharekauhau fault system) at the southern end of the Wairarapa fault zone, a major oblique‐slip fault in the upper plate of the Hikurangi Margin. The Wharekauhau thrust accommodated a minimum of 280 ± 60 m of horizontal shortening from ∼70 to 20 ka. The inferred shortening rate, 3.5–8.4 mm/yr, may have accounted for 11–30% of margin‐normal plate motion. By ∼20 ka, the thrust was abandoned. Subsequent deformation at shallow levels occurred on a segmented fault system that accommodated <1 mm/yr shortening. Active deformation in the region is partitioned between slip on (1) the more western Wairarapa‐Mukamuka fault system (dominantly dextral slip but also causing local uplift of the coast); (2) a series of discontinuously expressed strike‐slip faults and linking blind oblique‐reverse thrusts near the trace of the inactive Wharekauhau thrust; and (3) a blind thrust fault farther to the east. The spatial and temporal complexity of the Wharekauhau fault system and the importance it has had in accommodating upper plate deformation argue for an unsteady linkage between upper plate faults and between these faults and the plate interface.

Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: Link between earthquakes and the uplifting of beach ridges on a rocky coast

The Holocene beach ridges at Turakirae Head, New Zealand, are remarkable because the fault that caused their uplift is accessible to paleoseismic trenching. Based on 40 14 C samples from eight trenches, we identify five surface-rupturing earthquakes since ca. 5.2 ka (mean earthquake recurrence of 1230 ± 190 yr). The paleoearthquake record includes two more events than were recorded by the uplift and stranding of beach ridges at Turakirae Head. We conclude that beach ridges may provide an incomplete record of paleoearthquakes on oblique-reverse faults. The southern end of the Wairarapa fault includes several splays in the near surface at variable distances from Turakirae Head. Variable partitioning of slip between these splays (and perhaps the subduction interface down-dip of them) is inferred to have caused variable magnitudes of coseismic uplift at the coast, where at least one 14 C data support the view that a widespread post–Last Glacial Maximum aggradational terrace in southern North Island, New Zealand, was abandoned soon after 12.1 cal yr B.P. From this, we infer that the Wairarapa fault has a late Quaternary slip rate of 11 ± 3 mm/yr.

Implementation of Routine Regional Moment Tensor Analysis in New Zealand

The active tectonics of New Zealand are dominated by three main features (figure 1). Beneath the North Island there is active subduction of the Hikurangi trough where the Pacific plate is undergoing oblique subduction beneath the Australian plate at ∼ 45 mm/yr ( e.g. , Demets et al. 1990; Demets et al. 1994). The Hikurangi trough extends from the Kermadec trench in the north and terminates in the upper South Island. Large subduction zone earthquakes of around Mw 8.0 in the lower North Island and around Mw 6.9 farther north are expected (Reyners 1998). Slow slip events have also been observed on the Hikurangi subduction interface ( e.g. , Wallace and Beavan 2006) similar to those documented at other Pacific Rim subduction margins ( e.g. , Dragert et al. 2001; Obara et al. 2004; Larson et al. 2004). A number of dextral strike-slip faults are also present throughout the North Island and the upper South Island and have produced a number of M 6.0–7.5 earthquakes in the past century (Doser and Webb 2003). The Wellington region is cut by five active dextral faults that have average recurrence intervals of meter-scale surface rupture that range from ∼ 500 to 5,000 years (Van Dissen and Berryman 1996). The Wairarapa fault is the only one of these to have ruptured in historic times, in 1855, in an event estimated at Mw 8.2 (Darby and Beanland 1992). ▴ Figure 1. Overview of the tectonic setting of New Zealand, which is dominated by the interaction between the Pacific plate and the Australian plate. In the North Island the Pacific plate is subducting beneath the Australian plate at the Hikurangi trough and Kermadec trench. The Alpine fault delineates a dextral strike-slip fault through most of the South Island, and south of the South Island the …