Taupō Rift termination faults
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.[3] In a multi-fault rupture event there is the potential for the earthquake being of Mw7.1 magnitude.[2] The faults can be classified as belonging to three normal fault sets:[4]
Ohakune Fault This most northerly of the cross faults extends 24 km (15 mi) from northwest of Tohunga Junction[5] 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 65 m (213 ft) of vertical displacement and to the east there is only 55 m (180 ft).[3] This downthrown to the south normal fault has been active for over 18,000 years and has a dip-slip displacement rate of 3.5 mm (0.14 in)/year.[4] 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.[6] Raetihi faultsThe E - W trending 6 km (3.7 mi) of the Raetihi North Fault is downthrown to the south while the 2.5 km (1.6 mi) Raetihi South Fault is downthrown to the north so they have a small graben between them.[4] The Raetihi South Fault possibly continues along the Mangahowhi Stream for up to 5 km (3.1 mi). Displacement rates based on an age of 18,000 years for the scarps are 0.5 mm (0.020 in)/year for the Raetihi North Fault and 0.4 mm (0.016 in)/year for the Raetihi South Fault.[4] Waipuna FaultSouth of the Raetihi faults, the most western of the faults discussed, the Waipuna Fault is downthrown to the south and extends 16 km (9.9 mi) from the Waimarino Forest to intersept the Karioi Fault.[4] The displacement rate might be 0.4 mm (0.016 in)/year but this has an error of the same order.[4] Oruakukuru FaultThe Oruakukuru Fault to the south of the Waipuna Fault commences at State Highway 4 (New Zealand) close to the upper Mangawhero River extending 9 km (5.6 mi) also intersects the Karioi Fault and has a displacement rate of 0.7 mm (0.028 in)/year.[4] Snowgrass FaultThe south east Taupō Rift is associated with a 35 km (22 mi) 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 0.55 mm (0.022 in)/year.[4] Shawcroft Road FaultThe 7 km (4.3 mi) Shawcroft Road Fault cuts across the Rangipo Fault line just to the north east of the Waiouru Military Camp and is displacing at 0.7 mm (0.028 in)/year.[4] Karioi FaultThe normal Karioi Fault is parallel and to the north of the Shawcroft Road Fault and is 22 km (14 mi) long with a summed displacement for its two strands of 0.55 mm (0.022 in)/year.[4] Wahianoa FaultThe Wahianoa Fault is a northeast-striking presumed normal fault across the southeast flank of Mount Ruapehu extending at least 10 km (6.2 mi) from the Ohakune Fault towards the Rangipo Fault and the Upper Waikato Stream Fault with a displacement rate of 0.3 mm (0.012 in)/year.[4] 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 Mw 7.1 event rupturing 43 km (27 mi) by an average of 2.4 m (7 ft 10 in)[2]
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