Abstract
More than 10% of the world’s population (~600 million people) live in the coastal zone that is within 10 metres of sea level. The combination of global sea level rise (SLR) of 2-3 mm/yr, and late 20th century sea level acceleration, makes coastal communities in many parts of the world vulnerable to gradual inundation that will, in the future, require either adaptation or retreat from affected coastlines. Although the measurement and monitoring of SLR is well established through a combination of globally distributed tide gauge sites and altimeter measurements; what is not well understood is local and regional geodynamical processes that results in vertical land motion (VLM) that have the potential to increase coastal hazard. By way of example, New Zealand straddles the Australian/Pacific plate boundary and we are just starting to realise the spatial and temporal complexities associated VLM. GPS/GNSS measurements have been used to monitor the Hikurangi subduction zone (east coast of the North Island) that has resulted in a combination of subsidence (2-5 mm/yr) and slow slip events (1 mm/yr uplift). The recent Kaikōura 2016 earthquake resulted in spatially coherent coseismic displacements that caused subsidence in the Wellington region of 30 mm followed by ongoing postseismic deformation that has uplifted the region by up to 50 mm. In seismically activate regions and especially coastal zones close to tectonic plate boundaries, VLM needs to be monitored and included in SLR studies to understand the geodynamics that are affecting coastal regions and by implication, hazard and risk assessments.