Subsidence on the predominantly strike‐slip Alpine Fault, Aotearoa New Zealand
Journal Article
Overview
abstract
Abstract; There has been considerable research both on‐fault and in adjacent wetland environments aimed at understanding the nature and timing of movements on the predominantly strike‐slip Alpine Fault in southern Aotearoa New Zealand. Findings have documented significant horizontal displacement on its western side and uplift to the east but have as yet not reported any vertical land displacement to the west. Here, we report on a recent study of drowned forests, subsided soils, exposed tree stumps, and sedimentary evidence from Saltwater Lagoon, western South Island, which suggest multiple subsidence/compaction events. Radiocarbon dating of drowned trees revealed multiple tree ages with at least two buried soils overlain by apparent tsunami and/or slope failure deposits. The overall trend appears to be one of net lagoon subsidence/compaction. The recognition of co‐seismic subsidence/compaction events in this area may help explain a geomorphological conundrum where coastal wetlands and lagoons are maintained in a region where substantial sediment supply to the coast should cause infilling and progradation. It is important to gain a far better understanding of the nature and extent of such co‐seismic subsidence/compaction on the western side of the Alpine fault in order to determine the possible risks posed to coastal infrastructure. Equally, this applies internationally where there is potential to recognise seismic linkages between regional coastal geomorphologies such as lagoons, wetlands, drowned forests, and uplifted or isolated benches. This work highlights the significance of looking beyond the predominant nature of a fault’s movement in order to fully understand a region’s seismicity.
