The Rangipo (Desert Road) Fault: the most hazardous fault in the Taupo Volcanic Zone?

Abstract

The Rangipo and Shawcroft Road faults are located at the southern end of the Taupo Volcanic Zone in the vicinity of Ruapehu Volcano. The Rangipo Fault is not the fastest slipping fault in the Taupo Volcanic Zone, as was supposed prior to this study. We have determined two well constrained estimates of slip rate of c. 1.5 mm/yr and c. 0.2 mm/yr over two different time intervals (the past c. 25.6 ka and the past c. 13.8 ka respectively) for the Rangip Fault. This is much less than the earlier determined estimates of 3 mm/yr. The long term slip rate of 1.5 mm/yr is still comparable with other high slip rate faults in the Taupo Volcanic Zone (TVZ), but the Holocene slip rate of c. 0.2 mm/yr suggests the fault is a low slip rate fault compared with other TVZ faults.

The abrupt decrease in slip rate estimated for the Rangipo and Shawcroft Road faults coincides with an overall decrease in the eruptive activity from nearby Ruapehu Volcano from c. 15 ka to the present. We infer that there is a correlation between fast slipping periods on the Rangipo Fault and large eruptive volumes from Ruapehu Volcano. This relationship suggests that clustering of earthquakes on the Rangipo Fault is likely, but it is unclear whether the clustering of earthquakes is triggering the eruptions or vice versa. We do not favour an alternative model whereby the Rangip Fault had increased displacement prior to c. 15 ka caused by ground surface response to emptying of the magma chamber (ie, Rangip Fault acting as a passive structure with aseismic displacement). Further studies are needed to confirm one of these models.

Our results indicate that although rupture of the Rangipo Fault occurs less frequently than previously assessed, it is associated with large earthquakes with magnitudes between 6.3 and 7.1. Earthquake recurrence intervals of moderate M6 earthquakes are in the order of 3000 years and recurrence intervals of large M7 earthquakes are assessed to occur less frequently (≥ 11,200-14,000 years). Although our new characterisation of the fault activity indicates larger recurrence intervals and lower slip rates than previously assessed, rupture of the Rangipo Fault still poses a relatively high hazard to the area due to the large earthquake magnitudes with up to M 7.1 associated with it. Rupture of the Rangipo Fault will affect the major lifelines in the region, as well as the ski fields, townships, forest processing plants and hydroelectric power schemes.