North Island Fault System

The North Island Fault System or North Island Dextral Fault Belt is a set of southwest-northeast trending seismically-active faults in the North Island of New Zealand that carry most of the dextral (right lateral) strike-slip component of the oblique convergence of the Pacific Plate with the Australian Plate. They include the Wairarapa Fault and Wellington Fault to the southwest, the Ruahine and Mohaka Faults in the central section and the Waimana, Waiotahi, Whakatane and Waiohau Faults to the northeast. Most of the fault system consists of dextral strike-slip faults, although towards its northeastern end the trend swings to more S-N trend and the faults become mainly oblique normal in sense as the zone intersects with the Taupo rift zone. This fault zone accommodates up to 10 mm/yr of strike-slip displacement.

The North Island Fault System consists of eight main fault strands and many smaller related faults.

The more southeasterly branch of the Wellington Fault is known as the Mohaka fault. The fault splays to the north, onto the Waimana Fault, which itself branches into the Waiotahi and Waioeka Faults, the main segment of the Mohaka Fault eventually passes into the Whakatane Fault.

As the Wellington Fault branches near Woodville, the more northwesterly branch is known as the Ruahine Fault. Results from trenching over this fault suggest an earthquake recurrence interval of 400–500 years, with typical offsets in the range 3.0–5.5 m. At its northern end this fault becomes the Waiohau Fault.

The Waiohau Fault extends from the end of the Ruahine Fault north towards the Bay of Plenty. It lies roughly parallel with, and to the west of, the Whakatane, Waimana, and Waiotahi Faults, and to the east of the Taupo Rift. At its southern end it is a dextral strike-slip fault, becoming a normal dip-slip fault for the northern part of its length. The valley of the Rangitaiki River approximately follows the line of the fault. The fault is believed to be responsible for an earthquake in 1866 which was centred near Te Mahoe, east of Kawerau.

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