Beavan, R.J.; Litchfield, N.J. 2012 Vertical land movement around the New Zealand coastline : implications for sea-level rise. Lower Hutt, N.Z.: GNS Science. GNS Science report 2012/29 41 p.
Abstract: Vertical motion of coastal lands may be an important element in assessing the impact of sea level rise on coastal communities and infrastructure. If the land is subsiding the effects of sea level rise are augmented, while if the land is rising the effects are diminished. Long-term (thousands of years and longer) vertical motion can be assessed by geological methods, while short-term (years to tens of years) vertical motion can be measured with space-geodetic methods (Global Navigational Satellite Systems or “GNSS”, of which GPS is the original example). Geological estimates of vertical rates can achieve uncertainties of 0.1 – 0.2 mm/yr, while geodetic methods are currently limited to uncertainties of 0.2 – 1 mm/yr. Along those parts of the coastline away from present-day earthquake-cycle deformation, geological estimates of vertical motion are the most relevant for predicting future motion. In regions such as the Hikurangi subduction margin, where earthquake-cycle effects are strong, geodetic estimates of vertical motion are the most relevant for predicting motion over the next several decades. This applies to the east coast of the North Island, the southern North Island and the northeastern South Island. In the absence of large earthquakes, present-day GNSS measurements determine the interseismic (between large earthquakes) rates of vertical motion. Above subduction zones in particular, these rates are generally much larger than geological rates, and they may have the same or opposite sense. The occurrence of a large earthquake can have a major effect on coastal elevations in the region of the earthquake. If such earthquakes occur, their effects will dominate over the interseismic rates. The occurrence of such an earthquake is unlikely at any particular location over the next 50-100 years, except along the Alpine Fault where earthquake probabilities as high as 30% over the next 50 years can be inferred. In this report, we do not account for land level changes due to future large earthquakes. We find present-day land subsidence rates averaging ~2 mm/yr, with a maximum up to ~4 mm/yr, along the east coast of the North Island south of Hawke Bay, and on the south and west coasts of the North Island as far north as Bulls. This land subsidence will exacerbate the effects of sea level rise in these regions. Some of these rates may be biased high by the effects of long-term slow-slip events, but in general they are likely to persist at least until the next major earthquake on the Hikurangi subduction interface. (auth)
