The Investigation into the Geologically Recent Activity of the Mangatangi Fault in the Hunua Range

Abstract

Normal fault systems in regions of low extension in New Zealand are comparatively understudied. A series of normal faults are present in the South Auckland Region as a part of the regional block faulting reactivated in the Late Miocene. These faults represent significant seismic risk for New Zealand's most populated city, yet currently have poor constraints on slip rates and earthquake recurrence intervals. This study addresses this issue through an investigation of the Mangatangi Fault. The Mangatangi Fault was a relatively understudied fault in the Auckland region prior to this study, striking NE-SW along the southeastern flanks of the Hunua Range. We used LiDARbased geomorphic analysis along with Ground Penetrating Radar, hand auguring, and vibracoring to investigate the Mangatangi Fault. The presence and morphology of the range front indicates that the uplift rate is very slow relative to the erosion rate. The western segments of the Mangatangi Fault show higher rates of activity and a change in orientation towards N/S direction, most likely due to the long-term deformation of the area being dominated by the Wairoa South Fault. A piedmont scarp, manifesting ~ 60 m south of the main range front, is the focal field site of this study. Field investigations of the scarp reveal ~ 4 m of vertical offset accumulating through multiple rupture events. Bulk organic carbon dating of sediments deposited at the base of the scarp constrains the minimum age of scarp formation to 12013± 51 calBP, and the last activity within the past 11,100 ± 108 calBP and is reflected in the structurally deformed near-surface sediments. A vertical displacement rate range of 0.06 – 0.51 mm y-1 is established and is consistent with slow slip rate faults elsewhere in New Zealand. The maximum magnitude of Mw 6.8 – 6.9 is determined and a recurrence interval range of 39,000 – 4,000 years is established. The Mangatangi Fault has the potential to pose significant seismic risk to the Auckland Region with ground shaking up to MMI 7 – 9 in the city.