Northern Ohariu Fault: earthquake hazard assessment of a newly discovered active strike-slip fault in Horowhenua

Abstract

The Northern Ohariu fault is a newly discovered active fault that extends for some 60 km from between about Otaki to near Palmerston North. It appears to be the northern extension of the better known active Ohariu fault, and collectively these two faults define a major earthquake generating strike-slip fault that has a total length in excess of 130 km.

The Northern Ohariu fault is capable of generating large damaging earthquakes (magnitude Mw 7.2-7.5) once every few thousand years. Even larger, more damaging, earthquakes (Mw 7.7) could result if the Northern Ohariu fault were to rupture simultaneously with the Ohariu fault. The most recent large earthquake on the Northern Ohariu fault occurred between several hundred and about one thousand years ago, and resulted in 3-4 metres of surface rupture displacement of the ground surface.

Fault rupture of the ground surface, strong ground shaking, liquefaction, and landsliding will be the principle earthquake hazards associated with a large, surface-rupture earthquake on the Northern Ohariu fault. Houses, transmission lines, pipe lines, and roads cross the fault, and 3-4 metres of surface rupture displacement will adversely affect these structures. Damaging levels of strong ground shaking will result from a large Northern Ohariu fault earthquake, and will be felt as far south as Petone, as far north as Marton, and as far east as Dannevirke and Mastertron, with the most extreme levels of shaking occurring closest to the fault and encompassing the communities of Te Horo, Otaki, Manakau, Kuku, Ohau, Muhunoa East, Levin, Shannon, Tokomaru, and Linton.

Parts of the Horowhenua and Manawatu are known to be extremely susceptible to liquefaction. As such, we anticipate that liquefaction will be a significant feature of a Northern Ohariu fault earthquake, and will affect most severely the young, low-lying, sand country, river terraces, and flood plains of the Kapiti, Horowhenua, Manawatu, and Wairarapa lowlands from Paraparaumu in the south, Palmerston North in the north, and Foxton Beach, and Ekatahuna in the west, and east, respectively. Earthquake-induced landslides will also be generated, and these will be most extensive and severe along the western flank of the Tararua Range between the Otaki River in the south, and Kahuterawa Stream in the north. The steep slopes of the Manawatu gorge are also susceptible to failure during such an earthquake. Consequent with earthquake-induced landsliding is river aggradation, and the possible impacts this may have on flood control measures, and potential flooding resulting from catastrophic failure of landslide dams.

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Technical Abstract

The Ohariu fault, discovered about 100 years ago, extends northeastward from offshore of the Wellington south coast near Tongue Point through Porirua towards Paraparaumu. It has long been thought that the Ohariu fault “dies out” to the north in the Waikanae/Otaki area as a series of splays. However, in 1996 we discovered that the pronounced, 60 km long, NNE- to NE-trending topographic lineation that runs just inland of, and parallel to, the western flank of the northern portion of the Tararua Range between Otaki and Palmerston North is an active, right-lateral strike-slip fault. We name this newly discovered fault the Northern Ohariu fault because it appears to be the northern, along-strike, continuation of the Ohariu fault. Collectively the Ohariu and Northern Ohariu faults define a major active strike-slip fault that has a total length in excess of 130 km.

Offset geomorphic features such as alluvial terrace risers and valley margins, and estimated ages based on soil and loess stratigraphy are used to constrain the timing, amount, and rate of movement on the fault. The most recent surface rupture displacement on the fault is younger than about one thousand years, and older than several hundred years. The net surface rupture displacement that occurred during this event, measured at only two localities, was about 3-4 m. The fault has a lateral slip rate of 1-3 mm/yr, similar to that of the Ohariu fault to the south, a steep dip, and a subordinate up to the NW sense of displacement. The Northern Ohariu fault is considered capable of generating Mw 7.2-7.5 earthquakes once every one thousand to several thousand years. Larger earthquakes, ca Mw 7.7, would result if the Northern Ohariu fault were to rupture simultaneously with the Ohariu fault.

Fault rupture of the ground surface, strong ground shaking, liquefaction, and landsliding will be the principle earthquake hazards associated with a large (≥Mw 7.2) surface-rupture earthquake on the Northern Ohariu fault. Houses, transmission lines, pipe lines, and roads cross the fault, and 3-4 m of the net surface-rupture displacement will adversely affect these structures. Damaging levels of strong ground shaking (≥Modified Mercalli intensity 7) will result from a large Northern Ohariu fault earthquake, and will be felt as far south as Petone, as far north as Marton, and as far east as Dannevirke and Masterton. The highest and most damaging levels of shaking (≥Modified Mercalli intensity 9) will occur closest to the fault and will encompass the communities of Te Horo, Otaki, Manakau, Kuku, Ohau, Muhunoa East, Levin, Shannon, Tokomaru, and Linton. Parts of the Horowhenua and Mnawatu are known to be extremely susceptible to liquefaction. As such, we anticipate that liquefaction will be a significant feature of a Northern Ohariu fault earthquake, and will affect most severely the late Holocene sand country, low level river terraces, and flood plains of the Kapiti, Horowhenua, Manawatu, and Wairarapa lowlands from Paraparaumu in the south, Palmerston North in the north, and Foxton Beach, and Ekatahuna in the west, and east, respectively. Earthquake-induced landslides will also be generated, and these will be the most extensive along the western flank of the Tararua Range between the Otaki River in the south, and Kahuterawa Stream in the north.