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Research Papers

Issue date: 
Category: 
Seismology & geology
Paper number: 
3805

Potential shallow seismic sources in the Hamilton Basin

Vicki Moon, Willem de Lange, University of Waikato (EQC funded project 16/717)

Non-Technical Abstract (A Snapshot)

Finding the needle in the haystack

Previous studies have found evidence for deep faults within the Hamilton Basin. However, no evidence of faulting at the surface had been found, so it was assumed that the buried faults were inactive and the seismic risk for Hamilton resulted from younger active faults outside the basin. However, the discovery of faulting within an excavation in northern Hamilton showed that the deep faults could still be active. Locating active faults within the basin is difficult due to burial by soft sediments and land development that disguise the effects of past earthquakes.

This project examined the bed of the Waikato River between Cambridge and Taupiri using specialised echo sounders to see if any faults could be located. This approach meant that the effects hiding the faults on land would be removed. The geology and geomorphology of the riverbanks was also mapped.

The riverbed survey located 25 possible fault zones, including the previously inferred Taupiri Fault near Ngaruawahia. Several zones were then examined in more detail to see if they could be traced away from the river. Having a starting point made locating traces of faulting easier, and some faults could be linked to low grade geothermal activity between Templeview and Orini and at Horotiu, and a patch of basalt volcanoes at Koromatua.

While most of the faults identified do not appear to disturb sediments less than 340,000 years old, some have displaced the youngest sediments at the surface suggesting that they are still active.

The initial results indicate that while the maximum earthquake magnitude for Hamilton is similar to that assumed for the present seismic risk used for building codes, the shaking intensity will be higher due to the faults being closer to the city. Due to the soft sediments at the surface, the ground deformation caused by any earthquake is likely to be spread over a wide area, but will be less severe than if the same earthquake occurred in harder materials. However this suggests that disruption of services is likely to be more extensive.

Several of the fault zones identified by this project have subsequently been located in excavations for the Waikato Expressway. Data are being collected from these sites that will lead to a better assessment of the magnitude and frequency of earthquakes near Hamilton.

Technical Abstract

Excavation of a fault in NE Hamilton City, led to an examination of LiDAR data that suggested the presence of 4 fault zones within the Hamilton Basin. The aim of this project was to refine the locations of four potentially active faults within the Hamilton Basin. Two main phases of geophysical surveying were undertaken to achieve this aim: a high resolution CHIRP seismic reflection survey combined with existing multibeam and sidescan data collected between Lake Karapiro and Taupiri; and 2D and 3D resistivity surveys, combined with CPT and borehole data, at locations where faulting was indicated by LIDAR data. In addition a literature review was undertaken to identify potential fault zones; mapping of the geology of the riverbanks; and measurements obtained from excavations for the Waikato Expressway provided additional evidence to characterise the faults located.

The literature review uncovered previous data indicating the presence of faults within the basin including a small volcanic field, zones of low grade geothermal activity, and deep seismic reflection data and test bores indicating possible block faulting of the basement and Pliocene-Early Pleistocene rocks beneath the Basin.

The riverbed survey combined with the multibeam data identified at least 25 potential fault zones crossing the Waikato River, with 4 of these coinciding with the zones previously identified. Mapping of the geology and geomorphology found features consistent with faulting on the riverbanks at the zones identified within the riverbed. This study focussed on 9 of the zones (from north to south): Taupiri Fault, a boundary fault for the Hamilton Basin indicated on QMap sheet 4; Horotiu Fault zone; Osborne Rd - Kay Rd zone; Kukutaruhe Fault zone near the western city boundary; Te Kourahi Fault zone through the central city; Hamilton Gardens - Silva Ave fault zone; Te Tatua o Wairere Fault zone near the eastern city boundary; and Mystery Creek fault zone.

Reanalysis of the deep seismic reflection data obtained during oil and gas exploration within the Hamilton Basin showed the fault zones identified in the surficial sediments correlated with deeper faults in the Pliocene and Early Pleistocene rocks. A few also could also be traced into the basement greywacke.

A resistivity survey was undertaken on the Kerepehi Fault close to a previously trenched site to confirm the technique could identify faulting within the surficial sediments of the Hinuera Formation. This was successful and surveys were undertaken on the Osborne Rd - Kay Rd fault zone; and the Te Tatua o Wairere Fault zone at Stubbs Rd and close to the University of Waikato. The last survey was at the site of an inland port development, so the resistivity survey data could be correlated with CPT and borehole data. Both sites were selected due to evidence of displacement of the Hinuera Formation, indicating the faults are still active. Faulting was evident in all the resistivity surveys.

The study has confirmed the presence of multiple fault zones in the western Hamilton Basin, with gravity and deep seismic reflection data suggesting that the structure in the eastern Basin differs from that in the west, with a shallow basement ridge separating the Basin into two sub-basins. This study focussed on the western sub-basin, which shows a pattern of major southwards dipping normal listric faults, forming a series of half grabens of tilted Walton Sub-group blocks (Hamilton Hills). Numerous smaller synthetic and antithetic faults occur between the main fault zones, creating a complex pattern of small horsts and grabens. These smaller faults cluster at the northern end of the half-grabens, representing fracturing which allows accommodation space against the steeply dipping sections of the main faults. The half grabens have been partially buried by the sediments of the Hinuera Formation.

An initial assessment suggests the maximum credible earthquake is associated with an ~25 km long fault segment corresponding to Mw 6.6, with a maximum MM Intensity of 9-10. These data suggest that the shaking intensity within Hamilton City will be significantly greater due to a local earthquake than predicted for the Kerepehi Fault.

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