Coffey GL, Langridge RM, Van Dissen RJ, Clark KJ, Litchfield NJ, Morgenstern R, Palmer AS. 2022. Paleoseismology of the northern Ōhāriu Fault, a study into earthquake recurrence and slip rate. Lower Hutt (NZ): GNS Science. 38 p. (GNS Science report; 2022/52). doi:10.21420/TJK0-MY13.
Abstract
The Kāpiti Coast and Horowhenua districts are areas of rapid growth and development with expansion of housing and construction of major traffic infrastructure. Extending through these regions is the 60 km-long Northern Ōhāriu Fault, which is thought to be the northern extension of the Ōhāriu Fault. Little work on the Northern Ōhāriu Fault has been done since the early 2000s and given the potential for continuous ruptures along the Ōhāriu and Northern Ōhāriu faults, as well as increasing development in the Wellington-Levin corridor, further study is warranted. In this report we present the results of two paleoseismic trenches and a slip rate pit excavated across a branch of the Northern Ōhāriu Fault and an offset alluvial terrace, respectively. The work has been funded by the It’s Our Fault programme. Evidence of earthquakes spanning the Holocene and into the Late Pleistocene is identified from two paleoseismic trenches. Radiocarbon dating in one trench constrains the ages of the two most recent earthquakes to 120–273 cal yr BP and 1063–1271 cal yr BP and these are used to develop a preliminary recurrence interval of 400–2,300 years. Individual earthquakes in the other trench cannot be dated but suggest the recurrence interval is closer to the longer end of this range. At this stage, a slip rate cannot be derived from the pit excavated in an offset terrace due to uncertainty in when displacement began. The earthquake timings and recurrence interval of the Northern Ōhāriu Fault determined here are in good agreement with that of the Ōhāriu Fault. The results of this work suggest that either the Northern Ōhāriu and Ōhāriu faults form a single continuous structure along which earthquakes can rupture (with a preliminary maximum magnitude of Mw 7.9), or that earthquake activity on each fault is highly influenced by the other causing both faults to fail in earthquakes together or very close in time. Furthermore, earthquakes on the Northern Ōhāriu and Ōhāriu faults overlap in time with earthquakes on the Wellington and Rangatira faults, confirming earlier suggestions that earthquakes in the lower North Island may not be restricted to single faults and instead may occur as multi-fault ruptures. Results described in this report are preliminary and will be developed further with additional analysis and age modelling. However, these results demonstrate that the Northern Ōhāriu Fault is more active than previously thought and have implications for multi-fault rupture in the lower North Island, highlighting the importance of further research to understand the earthquake timing of this and other faults in the area. (The authors)
