A7 Makaroro River dam site Phase 1C: Field characterisation of possible secondary fault displacement

Abstract

GNS Science has undertaken a field study to investigate the possibility of active secondary faulting in the vicinity of the proposed A7 dam site on the Makaroro River, central Hawke’s Bay. The A7 site is located c. 750 m east of the primary active Mohaka Fault which has a short earthquake recurrence interval (average c. 1125 yr) and poses a credible shaking hazard to the dam site. Prior studies for the A7 dam site commissioned to GNS Science addressed the tectonic setting and characteristics of nearby active faults, as well as a literature review of the potential for secondary faulting at the dam site as a consequence of primary faulting along the Mohaka Fault. This current study focusses on site specific fieldwork undertaken to further evaluate the possibility of recent (late Quaternary) secondary faulting at, or near the proposed A7 dam site, and to define secondary faulting parameters such as possible displacement size, sense of movement, and recurrence. Based on our brief and previous investigations, we selected likely candidate sites for excavation to bedrock on the true left side of the valley on Smedley Station. The three trench sites were located to: 1) investigate the bedrock within the A7 dam footprint; 2) to intercept a NNE-striking mapped fault/shear zone; and 3) test whether evident linear hillslope geomorphology was related to recent faulting near the dam site. To assess recent displacement on bedrock exposed in the trenches we have: 1) mapped the bedrock structure (bedding and defects) in detail to identify faults/shear zones that could have potentially moved with fault displacements; 2) assessed whether bedrock faults had displaced the late Quaternary cover deposits or the strath surface (bedrock/cover contact); and, 3) assessed if fault rocks have characteristics of recent movement (i.e., non-cohesive materials such as fault breccias and gouge or clays. The surface fault rupture history of an active fault, the Gwavas Fault, located 5 km to the north of the A7 dam site and its relevance to the potential for faulting at the dam site have also been investigated through paleoseismic trenching.