| Taupō Rift termination faults | |
| Other Name: | Ohakune Fault, Raetihi North Fault, Raetihi South Fault, Waipuna Fault, Oruakukuru Fault, Rangiahu Fault, Maketu Stream Fault (Maketu Trace), Karioi Fault, Wahianoa Fault, Snowgrass Fault, Shawcroft Road Fault, Moawhango Fault |
| Named For: | Ohakune, Raetihi, Wahianoa River, Moawhango |
| Pushpin Relief: | 1 |
| Country: | New Zealand |
| Region: | Manawatu-Wanganui and Waikato Regions |
| Strike: | mainly E-W |
| Plate: | Indo-Australian |
| Status: | Active |
| Type: | Normal fault |
| Movement: | potential |
| Age: | Quaternary |
| Volcanic Arc/Belt: | Taupō Volcanic Zone |
| Embed: | New Zealand Active Fault database |
The seismically active southern end of the Taupō Rift beyond Mount Ruapehu has a number of mainly east to west orientated termination faults where the western wall Raurimu Fault and eastern wall Rangipo Fault (Desert Road Fault) terminate in the Ruapehu Graben, of the central North Island of New Zealand.[1] In a multi-fault rupture event there is the potential for the earthquake being of magnitude.
The faults can be classified as belonging to three normal fault sets:[2]
This most northerly of the cross faults extends 24km (15miles) from northwest of Tohunga Junction[3] into the Rangataua Forest. At Tohunga Junction it crosses the Raurimu Fault with a complex series of faults. The fault to the west of this junction region has 65m (213feet) of vertical displacement and to the east there is only 55m (180feet).[1] This downthrown to the south normal fault has been active for over 18,000 years and has a dip-slip displacement rate of 3.5mm/year.[2]
The Ohakune volcanic complex (Rochfort ejecta ring and craters) which erupted about 31,500 years ago is situated on and aligned with the fault just north of the town.[4]
The E - W trending 6km (04miles) of the Raetihi North Fault is downthrown to the south while the 2.5km (01.6miles) Raetihi South Fault is downthrown to the north so they have a small graben between them.[2] The Raetihi South Fault possibly continues along the Mangahowhi Stream for up to 5km (03miles). Displacement rates based on an age of 18,000 years for the scarps are 0.5mm/year for the Raetihi North Fault and 0.4mm/year for the Raetihi South Fault.[2]
South of the Raetihi faults, the most western of the faults discussed, the Waipuna Fault is downthrown to the south and extends 16km (10miles) from the Waimarino Forest to intersept the Karioi Fault.[2] The displacement rate might be 0.4mm/year but this has an error of the same order.[2]
The Oruakukuru Fault to the south of the Waipuna Fault commences at State Highway 4 (New Zealand) close to the upper Mangawhero River extending 9km (06miles) also intersects the Karioi Fault and has a displacement rate of 0.7mm/year.[2]
The south east Taupō Rift is associated with a 35km (22miles) northeast trending dome south of the Ngamatea Swamp where the Rangipo Fault terminates. The Snowgrass Fault on the northern side of this dome is a downthrown to the south normal fault that is displacing at an estimated 0.55mm/year.[2]
The 7km (04miles) Shawcroft Road Fault cuts across the Rangipo Fault line just to the north east of the Waiouru Military Camp and is displacing at 0.7mm/year.[2]
The normal Karioi Fault is parallel and to the north of the Shawcroft Road Fault and is 22km (14miles) long with a summed displacement for its two strands of 0.55mm/year.[2]
The Wahianoa Fault is a northeast-striking presumed normal fault across the southeast flank of Mount Ruapehu extending at least 10km (10miles) from the Ohakune Fault towards the Rangipo Fault and the Upper Waikato Stream Fault with a displacement rate of 0.3mm/year.[2] It has been postulated that it has the potential to do a whole fault rupture at the same time as whole fault rupture of the Rangipo and Upper Waikato Stream faults which would result in a event rupturing 43km (27miles) by an average of 2.4m (07.9feet)[5]
| Fault event Number | date of last marker before event | Max Displacement | Note | |
|---|---|---|---|---|
| 9 | 11,200 years BP | 6m (20feet) | Minor rupture Upper Waikato Stream Fault may have occurred | |
| 8 | 11,200 + years BP | 5.5m (18feet) | Minor rupture Upper Waikato Stream Fault may have occurred, this is about time of Pahoka–Mangamate eruption sequence (vents Ruapehu's northern summit and between Ruapehu and Tongariro).[6] | |
| 7 | 11,770 years BP | 1.76m (05.77feet) | ||
| 6 | 15,000 + years BP | 12m (39feet) | ||
| 5 | 17,700 years BP | 7m (23feet) | ||
| 4 | 17,700 + years BP | 2.22m (07.28feet) | ||
| 3 | 36,100 + years BP | 2m (07feet) | ||
| 2 | 2m (07feet) | |||
| 1 | 133,000 + years BP | 1.21m (03.97feet) |