|dc.description.abstract||The Te Puninga Fault is a newly discovered active fault on the western side of the Hauraki Plains, northern North Island, New Zealand. Preliminary evidence in a 2016 study suggests that it may generate earthquakes Mw>6.8. Therefore, the fault presents a hazard and risk to the population and infrastructure of the Hauraki Plains region, including the population centres of Morrinsville and Te Aroha, and beyond. Several consequences from rupture of the Te Puninga Fault are likely: strong ground motion from earthquakes (possibly amplified by basin effect in the Hamilton Basin and Hauraki Plains); permanent surface deformation by fault surface rupture; and secondary effects such as liquefaction and landslides. Thus, it is important to better understand the seismicity of this fault to better understand the hazard and risk to the wider region.
Paleoseismic trenching was undertaken in February 2021 along two fault traces at Ryland Trench and Arnold Trench. Exposures in the trenches enabled a stratigraphy to be established of the near-surface deposits (within 3 m) that were deformed by the fault. The deposits were described, and samples of organic material and tephra were collected. The organic material was submitted to the Waikato Radiocarbon Dating Laboratory for radiocarbon (¹⁴C) dating, and major element analysis of volcanic glass was undertaken at the electron probe microanalysis (EPMA) facility, Victoria University of Wellington. The bulk of the deposits exposed in each trench comprised secondary volcaniclastic sediments of the Hinuera Formation. An additional alluvial unit, deposited after 11,000 cal yr BP, was identified in the Ryland trench and correlated with the Waitoa Formation. The ¹⁴C dating indicated that the Hinuera Surface at each trench site was abandoned at approximately 24,000 cal yr BP. At both trenches, the Hinuera Surface was overlain by a cover bed of composite tephras deposited incrementally whilst being transformed pedologically (via pedogenic upbuilding) to form Allophanic Soils on well to moderately well drained higher landscape positions. Major element analysis of glass shards isolated from the base of the composite tephra column suggests that the Okareka (23,500 cal yr BP) or Rerewhakaaitu (17,600 cal yr BP), or both, (crypto)tephras were present, generally supporting the age ascribed to the Hinuera Surface (c. 24.0 cal ka) based on the ¹⁴C age modelling. The Taupo (1718 ± 10 cal yr BP) and Kaharoa (636 ± 12 cal yr BP) tephras occurred in the upper parts of both trenches as visible, thin, discontinuous deposits. They were identified by their stratigraphic superpositioning and physical properties, confirmed by their diagnostic ferromagnesian minerals and glass-shard major element compositions.
A site near Quine Road where the fault scarp dipped westwards, in the opposite direction to the eastward slope of the general land surface, and intersected stream terraces, was identified using high resolution LiDAR data. Using a digital elevation model, a hillshade model, and field work including geomorphic mapping, the terraces formed by the streams were identified and labelled from 1 (youngest) to 8 (oldest). Because terraces became progressively younger as they descended from the c. 24 cal ka Hinuera Surface, they can be used as chronological markers for fault activity. If the scarps formed by fault surface ruptures represent different surface displacement of different terraces, an analysis can be done to assess numbers and timing of fault ruptures. Elevation profiles were drawn parallel and perpendicular to the fault and it was found that the vertical fault displacement decreased as the terraces got younger. By comparing two models for incision rates, terrace ages were modelled and thus possible ages of displacement were obtained. The oldest terraces had an average vertical displacement of 2.6 ± 0.15 m, whereas Terrace 3 had a vertical displacement of 1.0 ± 0.15 m, and the youngest two had no vertical displacement. Therefore, it is likely that the penultimate earthquake that generated the displacement occurred between the formation of Terrace 4 (19,500 ± 1,400 yrs BP) and Terrace 3 (18,600 ± 600 yrs BP), and the most recent earthquake occurred between the formation of Terraces 3 (18,600 ± 600 yrs BP) and 2 (15,000 ± 1,900 yrs BP). Using the age of the youngest terrace that was displaced in two events (19,500 yrs BP), and the average vertical displacement of the other terraces displaced twice (2.64 ± 0.15 m), a slip rate of 0.135 ± 0.01 mm/yr was calculated. As this is one of at least nine strands of the Te Puninga Fault at this latitude, is a possible the 2016 published slip rate of 0.160 ± 0.03 mm/yr is understated.
A trial investigation of ground penetrating radar (GPR) at three locations on the Te Puninga Fault was conducted. The first two applications were at each of the two paleoseismic trench sites (Arnold and Ryland trenches), and hence the reflectors could be compared to the known stratigraphy, and the third at the Quine Road study site. The Quine Road site is a possible future trench location. The GPR investigation found: a) that a suitable depth of stratigraphy (<2.5 m) could be obtained; b) the displaced surfaces at all three sites were formed by seismic deformation rather than fluvial erosional processes because the reflectors in the radargrams ran parallel to the land surface, reflecting folding; and c) at Quine Road, in contrast, the sediments potentially show clear faulting rather than folding which could provide key information if trenching were to be undertaken at the site. ||