Vertical land movement

Local sea-level rise around the coast of Aotearoa is affected by up and down movements of our land. We tend to think most about the vertical land movement that occurs in large jumps during earthquakes, but also important are the smaller shifts that occur continuously in between large seismic events. These small but continuous changes add up, and in areas that are going down (subsiding) the annual rate of sea-level rise can double. The NZ SeaRise Programme connected this vertical land movement data with climate driven sea-level rise to provide locally relevant sea-level projections (“The Significance of Interseismic Vertical Land Movement at Convergent Plate Boundaries in Probabilistic Sea-Level Projections for AR6 Scenarios: The New Zealand Case”, Naish et al.)

Earthquakes near the coast can cause the land to instantaneously move up or down. Earthquakes that cause regional uplift can provide some mitigation against the effects of climate driven sea-level rise. However, they cause other problems, such as shifting port and marina facilities out of the sea, drying out estuaries, and mass die-off of kai moana in the intertidal zone. Earthquakes that cause sudden land subsidence exacerbate the effects of climate driven sea-level rise by moving the shoreline landwards and putting new assets, environments, and people into harm’s way.

Te Ao Hurihuri: Te Ao Hou aims to add to this knowledge by measuring and modelling vertical land movement during and between earthquake events to produce estimates of vertical land movement today and into the future.

Both vertical land movement in between earthquakes and vertical land movement during earthquakes will be integrated into a model of evolving land surface elevation for Aotearoa New Zealand’s coastal zones. This model will go out to 2100 and beyond and will include uncertainty. Values along the seaward boundary of our time evolving land surface model will be used for the vertical land movement component in local relative sea-level rise projections produced using the Framework for Assessing Changes To Sea-level (FACTS) at high spatial resolution (FACTS paper). We hope this data will help people make better decisions about how to manage the consequences of rising seas in New Zealand.

Estimate rates of vertical land movement from satellite derived time series

Use high resolution interferometric Synthetic Aperture Radar (InSAR) combined with Global Navigation Satellite Systems (GNSS) data to provide an improved up to date estimate to coastal VLM. Previous SLR Projections from NZ SeaRise used Envisat InSAR data collected between 2003 and 2011. We will extend the time series to include enhanced observations from the Sentinel platform. These analyses will reduce uncertainty in VLM projections and provide better temporal sampling along Aotearoa’s coast.

Develop a probabilistic model of coseismic VLM based on the NZ National Seismic Hazard Model

Integrate annual rates of earthquake rupture from the Aotearoa New Zealand National Seismic Hazard Model with established dislocation modelling techniques to create a probabilistic model of coseismic VLM during future earthquakes (with uncertainties). This modelling approach uses the best available data on NZ faults and is informed by empirical observations of earthquake behaviour from global catalogues; it represents the current state of the art in the form of ongoing revisions to the National Seismic Hazard Model.

Produce an earthquake simulator-based probabilistic model of coseismic coastal deformation

Generate million-year catalogues of synthetic earthquakes using the same input data as the National Seismic Hazard Model (fault geometries and slip rates) and earthquake simulator RSQSim. RSQSim relies on simple physical assumptions in place of the empirical relations employed by the National Seismic Hazard Model. Calculate coastal VLM and annual probabilities from the synthetic earthquake catalogue. Compare results with those from the coseismic probabilistic model to test the sensitivity of results to the modelling approach.

Calibrate models using historic earthquakes and geological data

Create a new database of earthquake timing, single-event VLM and long-term rates of tectonically driven VLM around the coast of Aotearoa New Zealand. These observations will be used to inform and validate the National Seismic Hazard Model derived and physics-based models of coseismic VLM.

Produce probabilistic estimates of ‘rapid/instantaneous’ changes in relative sea level related to coseismic VLM over timescales of 50, 100, and 150 years into the future

Combine results from the two modelling approaches using a logic-tree approach, producing projections of future VLM during earthquakes at sites around the coastline of Aotearoa New Zealand. The results and associated uncertainties can then be integrated with projections of sea level change.

Our Team

Experts in both seismic-hazard analysis and coastal earthquake science form the team that will incorporate vertical land movement during and between earthquakes into our sea-level rise projections. Co-leader, Ian Hamling, Earth Sciences New Zealand (formally GNS Science), is experienced in space-based remote sensing for land deformation. Co-leader Andy Howell, University of Canterbury and Earth Sciences New Zealand (formally GNS Science), is an expert on coastal tectonics and earthquake simulators. Matt Gerstenberger and Chris Rollins are world leaders in seismic hazard modelling and run the National Seismic Hazard Model at Earth Sciences New Zealand (formally GNS Science). Sigrun Hreinsdottir, Earth Sciences New Zealand (formally GNS Science), has extensive experience in precise measurement of land movement using GNSS. Kate Clark and Nicola Litchfield, Earth Sciences New Zealand (formally GNS Science), are experts in coastal earthquake geology and have studied the effects of past earthquakes on the coastline of Aotearoa.