Surface Rupture Hazard Zonation: Lessons from Recent New Zealand Earthquakes

Engineered structures crossing active faults are vulnerable to damage during surface faulting earthquakes. The design and location of mitigation measures to counteract fault rupture requires detailed knowledge of the location of the active fault traces, fault geometry, including the width of the fault zone at the surface, and the distribution of strain within the fault zone. The current understanding of fault geometry and displacement profiles is based on predominantly subsurface data through essentially isotropic ground conditions. Although empirical relationships among fault parameters, such as rupture length, earthquake magnitude and average or maximum displacement, can be used to characterize potential surface rupture hazard for an entire fault zone, the behavior of a fault at a specific location, as is required for engineering design, can be harder to forecast. For hazard planning and front-end engineering design, rupture zonation is a useful approach. To produce meaningful fault rupture zonation maps requires an integration of data on tectonic geomorphology, paleoseismology, and both crustal and near-surface fault geometry. The results of detailed surface rupture mapping, LiDAR image interpretation and shallow geophysical investigations following the 2016 Kaikōura earthquake are used to highlight some of the problems in determining potential fault rupture hazard zones. Existing zonation approaches are evaluated in light of this complex, multi-fault rupture. Rather than define narrow prescriptive fault avoidance zones, a better approach is to develop a broader zonation that highlight areas where there is the need for detailed fault rupture mitigation studies to be performed for all significant developments.