Plate boundary behaviour, recent uplift, and seismic hazard along the Central Alpine Fault near the Whataroa River, South Island, New Zealand

Abstract:

Understanding plate boundary behaviour is a major objective in seismotectonics to better understand and mitigate seismic hazards. Field- and light detection and ranging (lidar)-derived topographic mapping, geological characterisation, and optically stimulated luminescence (OSL) dating of on-fault sediments were used to better constrain rangefront deformation of the Southern Alps at the Alpine Fault near the Whataroa River. The Alpine Fault, which forms the plate boundary in the South Island of New Zealand, is thought to rupture in large to great earthquakes (most recently in 1717 AD). Here the fault is dextral-reverse, although primarily strike-slip with clear fault traces cutting across older surfaces of varying elevations and ages. Deformational bulges are observed along these traces that are likely thrust-bounded. A terrace of Whataroa River sediments on the hanging wall of the fault approximately ~ 55-75 m (when considering uncertainties) above the floodplain of the Whataroa. OSL ages for hanging wall sediments of ~ 11 ka in this terrace, ~ 2.8 ka for Whataroa River terrace deposits in a deformational bulge, and ~ 11.1 ka for a rangefront-derived fan and aggradation along the rangefront and Holocene hanging wall uplift rates of ~6.0 + or - 1 mm/yr at the fault. These Whataroa River-sourced terrace deposits suggest that the adjacent bounding faults are steeply-dipping, with no geometries in the shallow subsurface that would tend to cause rotation and tilting. Because GPS-derived “interseismic” vertical uplift rates are < 1 mm/yr here, the majority of rock uplift at the rangefront happens during episodic major Alpine Fault earthquakes. Additionally, our recent data on fault behaviour based on mapping and field measurements indicate that the fault may not exhibit characteristic rupture behaviour. We suggest instead ‘bimodal behavior’ where the AF exhibits both ‘partial’ (rupture length <300 km; moment magnitude, M_w, 6.5 to 7.8) and ‘full’ (rupture length ≥300 km; M_w ≥ 7.9) ruptures. This further questions the validity of the characteristic earthquake model for seismic hazard assessments. Finally, we present a appraisal of the active South Westland Fault Zone, including new outcrop mapping that demonstrates other important sources of seismic hazard exist near this plate boundary.