New information on the activity of the Wellington-Hutt Valley segment of the Wellington Fault, New Zealand, has become available from geological and modelling studies undertaken in the last several years as part of the “It’s Our Fault” project. There are now revised estimates of: 1) the timing of the most recent rupture, and the previous four older ruptures; 2) the size of single-event displacements; 3) the Holocene dextral slip rate; and 4) rupture statistics of the Wellington-Wairarapa fault-pair, as deduced from synthetic seismicity modelling. The conditional probability of rupture of this segment over the next 100 years is re-evaluated in light of this new information, assuming a renewal process framework. Four recurrence-time distributions (exponential, lognormal, Weibull and Brownian passage-time) are explored. The probability estimates take account of both data and parameter uncertainties. A sensitivity analysis is conducted, entertaining different bounds and shapes of the probability distributions of important fault rupture data and parameters. Important findings and conclusions include: The estimated probability of rupture of the Wellington-Hutt Valley segment of the Wellington Fault in the next 100 years is ~11% (with sensitivity results ranging from 4% to 15%), and the probability of rupture in the next 50 years is about half of that (~5%). In all cases, the inclusion of the new data has reduced the estimated probability of rupture of the Wellington Fault by ~50%, or more, compared to previous estimates.
Theoretical studies of the seismic cycle at convergent plate boundaries anticipate that most coseismic deformation is recovered, yet significant permanent vertical displacement of the overriding plate is observed at many subduction margins. To understand the mechanisms driving permanent vertical displacement, we investigate tectonic uplift across the southern Hikurangi subduction margin, Aotearoa New Zealand, in the last ∼200 ka. Marine terraces preserved along the Wellington south coast have recently been dated as Marine Isotope Stage (MIS) 5a (∼82 ka), 5c (∼96 ka), 5e (∼123 ka) and 7a (∼196 ka) in age. We use these ages, together with new reconstructions of shoreline angle elevations, to calculate uplift rates across the margin and to examine the processes responsible for their elevation. The highest uplift rate—1.7 ± 0.1 mm/yr–and maximum tilting—2.9° to the west–are observed near Cape Palliser, the closest site to (∼50 km from) the Hikurangi Trough. Uplift rates decrease monotonically westward along the Palliser Bay coast, to 0.2 ± 0.1 mm/yr at Wharekauhau (∼70 km from the trough), defining a gently west-tilted subaerial forearc domain. Locally, active oblique-slip upper-plate faults cause obvious vertical offsets of the marine terraces in the axial ranges (>70 km from the trough). Uplift rates at Baring Head, on the upthrown side of the Wairarapa-Wharekauhau fault system, are ∼0.7–1.6 mm/yr. At Tongue Point, uplift on the upthrown side of the Ōhāriu Fault is 0.6 ± 0.1 mm/yr. Dislocation and flexural-isostatic modelling shows that slip on faults within the overriding plate—specifically the Palliser-Kaiwhata Fault and the Wairarapa-Wharekauhau fault system—may dominate uplift in their immediate hanging walls. Depending on their slip rate and geometry, slip on these two upper-plate fault systems could plausibly cause >80% of late Pleistocene uplift everywhere along the south coast of North Island. Our modelling suggests that subduction of the buoyant Hikurangi Plateau contributes uplift of 0.1–0.2 mm/yr and uplift due to sediment underplating at Tongue Point and Wharekauhau is likely ≤0.6 mm/yr but could be significantly lower. Earthquakes on the subduction interface probably contribute ≤0.4 mm/yr of late Pleistocene uplift, with ≤10% of uplift due to each earthquake being stored permanently, similar to other subduction zones. These results indicate a significant contribution of slip on upper-plate faults to permanent uplift and tilting across the subduction margin and suggest that in regions where upper-plate faults are prevalent, strong constraints on fault geometry and slip rate are necessary to disentangle contributions of deeper-seated processes to uplift.
<p>At the southern Hikurangi margin, the subduction interface between the Australian and Pacific plates, beneath the southern North Island of New Zealand, is ‘locked’. It has previously been estimated that sudden slip on this locked portion of the interface could result in a subduction zone or ‘megathrust’ earthquake of Mw 8.0-8.5 or larger. Historically, however, no significant (>Mw 7.2) subduction interface earthquake has occurred at the southern Hikurangi margin, and the hazard from subduction earthquakes to this region, which includes New Zealand’s capital city of Wellington, remains largely unknown. Patterns of uplift at active margins can provide insight into subduction processes, including megathrust earthquakes. With the objectives to i) contribute to the understanding of partitioning of margin-parallel plate motion on to upper plate faults, and ii) provide insight into the relationship of permanent vertical deformation to subduction processes at the southern end of the Hikurangi margin, I investigate flights of late Pleistocene fluvial and marine terraces preserved across the lower North Island. Such geomorphic features, when constrained by numerical dating, provide a valuable set of data with which to quantify tectonic deformation - be they locally offset by a fault, or collectively uplifted across the margin. Fault-offset fluvial terraces along the Hutt River, near Wellington, record dextral slip for the southern part of the Wellington Fault. From re-evaluated fault displacement measurements and new Optically Stimulated Luminescence (OSL) data, I estimate an average slip rate of 6.3 ± 1.9/1.2 mm/yr (2σ) during the last ~100 ka. However, slip on the Wellington Fault has not been steady throughout this time. During the Holocene, there was a phase of heightened ground rupture activity between ~8 and 10 ka, a period of relative quiescence between ~4.5 and 8 ka, and another period of heightened activity during the last ≤ 4.5 ka. Moreover, these results agree with independent paleoseismological evidence from other sites along the Wellington Fault for the timing of ground rupture events. The time-varying activity observed on the Wellington Fault may be regulated by stress interactions with other nearby upper plate active faults. Net tectonic uplift of the southern Hikurangi margin is recorded by ancient emergent shore platforms preserved along the south coast of the North Island. I provide a new evaluation of the distribution and age of the Pleistocene marine terraces. Shore platform altitudes are accurately surveyed for the first time using Global Navigational Satellite Systems (GNSS). From these data I have determine the shore platform attitudes where they are preserved along the coast. The terraces are also dated, most for the first time, using OSL techniques. The most extensive Pleistocene terraces formed during Marine Isotope Stages (MIS) 5a, 5c, 5e and 7a. Because the ancient shorelines are now obscured by coverbed deposits, I use shore platform attitudes to reconstruct strandline elevations. These strandline elevations, corrected for sea level during their formative highstands, have been used to quantify rates of uplift across the southern Hikurangi margin. In the forearc region of the Hikurangi margin, within ~70 km of the trough, uplift observed on the marine terraces along the Palliser Bay coast monotonically decreases away from the trough. The highest uplift rate of 1.7 ± 0.1 mm/yr is observed at the easternmost preserved terrace, near Cape Palliser, about 40 km from Hikurangi Trough. Further to the west, at Lake Ferry, uplift is 0.8 ± 0.1 mm/yr. The lowest rate of uplift, 0.2 ± 0.1 mm/yr, is observed at Wharekauhau, the westernmost marine terrace preserved on the Palliser Bay coast. Overall, the terraces are tilted towards the west, away from the trough, with older terraces exhibiting the most tilting. This long-wavelength pattern of uplift suggests that, in this forearc region of the margin, deep-seated processes, most likely subduction of a buoyant slab in combination with megathrust earthquakes, are the main contributors to permanent vertical deformation. West of Palliser Bay, at a distance of >70 km from the Hikurangi Trough, vertical offsets on the marine terraces are evident across upper plate faults, most notably the Wairarapa and Ohariu Faults. The uplift rate at Baring Head, west and on the upthrown side of the Wairarapa Fault, is as much as 1.6 ± 0.1 mm/yr. At Tongue Point, where the Ohariu Fault offsets the marine terraces preserved there, uplift calculated from the western, upthrown side of the fault is 0.6 ± 0.1 mm/yr. These uplift rates suggest that, in the Axial Ranges, in addition to sediment underplating, movement on the major active upper plate faults also contributes to rock uplift.</p>
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