R. Van Dissen

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.

Estimates of the time-varying hazard of rupture of the Alpine Fault, New Zealand, allowing for uncertainties

Abstract The time‐varying hazard of rupture of the Alpine Fault is estimated using a renewal process model and a statistical method that takes account of uncertainties in data and parameter values. Four different recurrence‐time distributions are considered. The central and southern sections of the fault are treated separately. Data inputs are based on estimates of the long‐term slip rate, the average single‐event displacement, and the dates of earthquakes that have occurred in the last 1000 yr from previous studies of fault traces, landslide and terrace records, and forest ages and times of disturbance. Using these data and associated uncertainties, the current hazard of rupture on the central section of the fault is estimated to be 0.0051, 0.010, 0.012, and 0.0073 events per year under the exponential, lognormal, Weibull, and inverse Gaussian recurrence‐time distributions, respectively. The corresponding probabilities of rupture in the next 20 yr are 10, 18, 21, and 14%, respectively. The current hazard on the southern section of the fault is estimated to be 0.0033, 0.0075, 0.0070, and 0.0053 events per year for the four models, and the 20 yr probabilities 6, 14, 13, and 10%, respectively. Increased precision in the date of the second to last event on the southern section of the fault would result in only small changes to these rates and probabilities. The indicated hazard under the lognormal model is about double the long‐term average rate but less than half of that estimated in previous studies that did not take account of all the uncertainties. Dating additional prehistoric ruptures is likely to have a greater effect on the hazard estimates than improved precision in the existing data.

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.

The New Zealand Active Faults Database

ABSTRACT The New Zealand Active Faults Database (NZAFD) is a national geospatial database of active faults – including their locations, names and degrees of activity – that have deformed the ground surface of New Zealand within the last 125,000 years. The NZAFD is used for geological research, hazard modelling and infrastructure planning and is an underlying dataset for other nationally significant hazard applications such as the National Seismic Hazard Model. Recent refinements to the data structure have improved the accuracy of active fault locations and characteristics. A subset of active fault information from the NZAFD, generalised for portrayal and use at a scale of 1:250,000 (and referred to as NZAFD250), is freely available online and can be downloaded in several different formats to suit the needs of a range of users including scientists, governmental authorities and the general public. To achieve a uniform spatial scale of 1:250,000 a simplification of detailed fault locational data was required in some areas, while in other areas new mapping was necessary to provide a consistent level of coverage. Future improvements to the NZAFD will include the incorporation of data on active folds and offshore active faults.

Previously Unknown Fault Shakes New Zealand's South Island

At 4:35 A.M. local time on 4 September (1635 UTC, 3 September), a previously unrecognized fault system ruptured in the Canterbury region of New Zealands South Island, producing a moment magnitude (Mw) 7.1 earthquake that caused widespread damage throughout the area. In stark contrast to the 2010 Mw 7.0 Haiti earthquake, no deaths occurred and only two injuries were reported despite the epicenters location about 40 kilometers west of Christchurch (population ˜386,000). The Canterbury region now faces a rebuilding estimated to cost more than NZ

Late Quaternary paleoseismic history and surface rupture characteristics of the eastern Awatere strike-slip fault, New Zealand

4 billion (US

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

2.95 billion). On the positive side, this earthquake has provided an opportunity to document the dynamics and effects of a major strike-slip fault rupture in the absence of death or serious injury. The low-relief and well-maintained agricultural landscape of the Canterbury Plains helped scientists characterize very subtle earthquake-related ground deformation at high resolution, helping to classify the earthquakes basic geological features [Quigley et al., 2010]. The prompt mobilization of collaborating scientific teams allowed for rapid data capture immediately after the earthquake, and new scientific programs directed at developing a greater understanding of this event are under way.

On the handling of uncertainties in estimating the hazard of rupture on a fault segment

The recurrence and magnitude of paleo- earthquakes on the eastern section of the dextral strike-slip Awatere fault (>105 km long) are determined from stratigraphic evidence in a fault trench and measurement of fault-displaced geomorphic features. We identify at least six ground-rupturing events younger than 8330–8610 yr, the youngest being a historically recorded Mw ∼ 7.5 event in A.D. 1848 that ruptured the entire eastern section. The maximum mean recurrence interval is ∼1.4 k.y., although repeat intervals range from 605 ± 235 to 2500 ± 600 yr. The smallest fault offsets on the easternmost 27 km of the onshore part of the fault indicate a mean strike-slip displacement of 4.9 m during the 1848 event. The second-smallest offsets are approximately double the smallest displacements, suggesting that the last two events were ruptures of comparable dimensions (>105 km) and magnitude. Because the Awatere fault terminates <20 km northeast of the study area, the penultimate rupture probably extended inland, perhaps rupturing the entire eastern section. Further evidence of characteristic earthquake behavior is preserved in the fault trench, where a regular event subsidence of ∼50 cm per event is indicated. During the last two events a slip maximum of 7 ± 1 m occurred on a 5.5-km-long fault subsection consisting of adjacent restoring and releasing fault bends. These bends define a 1-km-wide side step in the fault that, although of short wavelength, apparently strongly influences the amount of coseismic slip and moment release. New data on the age of late Quaternary alluvial surfaces in the Awatere Valley, when combined with observed horizontal displacements, indicate that strike slip has accrued on the Awatere fault at a constant rate of 6 mm/yr for at least the past 20 k.y.

Active faults, paleoseismology, and historical fault rupture in northern Wairarapa, 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....

Paleo-equilibrium line altitude estimates from late Quaternary glacial features in the inland Kaikoura Range, South Island, New Zealand

Uncertainties in data and parameter values have often been ignored in hazard estimates based on historic and prehistoric records of rupture on fault segments. A mixture of distributions approach is appropriate to handle uncertainties in parameters of recurrence time distributions estimated from the geological and historical earthquake record of a fault segment, and a mixture of hazards approach is appropriate for data uncertainties and for uncertainties in parameters estimated from a set of similar faults. The former approach admits updating of the distributions for uncertainty as time passes. The aim is to present the hazard as a single value which takes account of both data and parameter uncertainties, conditional only on modeling assumptions. The proposed methods are described in detail for the exponential and lognormal recurrence time models for fault-rupturing earthquakes and applied, by way of illustration, to selected fault segments, namely, the Mojave segment of the San Andreas fault, California, and the Wellington-Hutt Valley segment of the Wellington fault, New Zealand.