Paleoearthquakes at the junction of the Tokomaru and Northern Ōhāriu faults, implications for fault interactions in the southern North Island, New Zealand

ABSTRACT

The Tokomaru Fault extends along the west side of the southern North Island of New Zealand. Given the fault’s proximity to major and expanding population centres, new work was undertaken to understand its contribution to seismic hazard in the Wellington-Levin corridor. Two paleoseismic trenches excavated across the southern end of the Tokomaru Fault, close to its junction with the Northern Ōhāriu Fault, show that it and the Northern Ōhāriu Fault are more active than previously thought. Ages of the two youngest events on the Tokomaru Fault are 93–220 and 1179–1254 cal yr BP, and result in a recurrence interval of 470–2300 years. These ages agree with the two youngest events on the Ōhāriu Fault and the timing of some events on the Wellington Fault. This suggests that the Tokomaru, Northern Ōhāriu, Ōhāriu, and Wellington faults may have ruptured together in the past, have triggered earthquakes on each other, or host slip that is triggered by larger nearby earthquakes. The results of this work illustrate the importance of fault interactions in the southern North Island and show that even if faults here do not rupture together, they likely play a key role in influencing the earthquake behaviour of each other.

KEYWORDS:

• Northern Ōhāriu Fault
• Tokomaru Fault
• Horowhenua
• Kāpiti Coast
• earthquake
• paleoseismic trenching
• recurrence interval
• fault interactions

Acknowledgements

We would like to thank Daniel and Rebecca Kilsby-Halliday, and Blair and Anna Robinson for access and allowing us to excavate on their land. This work was undertaken as part of the It’s Our Fault programme and we are grateful to Toka Tū Ake EQC, the Wellington Regional Management Group and Wellington City Council for funding this work. This project was supported by the New Zealand Ministry of Business, Innovation and Employment (MBIE) through the Hazards and Risk Management programme (Strategic Science Investment Fund, contract C05X1702). We would also like to thank, Andy Dodd for his archaeological assessment, Danielle Willis for help with the UAV survey at Makahika, Dr Ningsheng Wang for processing the OSL samples, and the team in the Rafter Laboratory at GNS for the analysis of radiocarbon samples. We thank Pilar Villamor and Dougal Townsend for their useful feedback on this manuscript. We also thank the anonymous reviewer and Jack Williams for their useful edits and comments on this manuscript during the review process.

Data availability statement

The data supporting the findings of this study are available within the article and its supplementary materials, which have been uploaded on Figshare (dx.doi.org/10.6084/m9.figshare.6025748).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Ministry of Business, Innovation and Employment and also supported by “Its Our Fault”.