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 other smaller related faults.
The Wellington Fault is a dextral strike-slip fault that runs from the Cook Strait on the southern coast of North Island up to near Woodville, where the fault branches into the Mohaka and Ruahine Faults. No historical earthquakes have been recorded along this fault although a significant event is estimated to have occurred within the last 1,000 years. The recurrence interval for large earthquakes on this fault is estimated to be less than 2,000 years. Three main segments have been identified, the Wellington-Hutt section, the Tararua section and the Pahiatua section.
The Wairarapa Fault extends from near the coast just southwest of Lake Wairarapa, running along the lake's northwestern edge. The 1855 Wairarapa earthquake was caused by movement along this fault. The recurrence interval for large earthquakes on this fault is less than 2,000 years.
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 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.
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