Tracking New Zealand’s Shifting Ground From Space with InSAR Radar

New Zealand straddles the volatile boundary between the Pacific and Australian plates. This tectonic marriage means our mountains, valleys and coastlines are always on the move – sometimes slowly, sometimes in sudden lurches. For decades, scientists relied on ground‑based instruments to measure these shifts.

Today, we are using a combination of radar satellites and clever mathematics, to monitor the land movements... even from hundreds of kilometres above.

InSAR land-movement data for the Cromwell Gorge landslide just north of the Clyde Dam. Source: SatSense

What is InSAR?

Interferometric Synthetic Aperture Radar (InSAR) is not only fun to say, it’s also an exciting radar‑based technology for measuring ground deformation with mm-precision. InSAR is not like a camera, which passively records reflected sunlight, a synthetic aperture radar satellite is an active sensor. It transmits pulses of microwave energy towards Earth and records the returning echoes. Because it provides its own illumination, it can collect data day or night and through cloud or rain.

InSAR combines this radar imaging with the concept of interferometry, where at least two radar images of the same location taken at different times are compared. By analysing subtle differences in the returning signal, scientists can detect changes in the distance between the satellite and the ground to within millimetres.

A single radar scene acts as the baseline. On a later pass the satellite acquires another scene. If a patch of ground has moved – perhaps uplifted during an earthquake or subsided due to groundwater withdrawal – the travel path of the radar wave changes slightly. That tiny change shifts the phase of the returning wave, which is the precise point in its cycle when it reaches the sensor. Subtracting the phase values from the two passes yields an interferogram – a map not of the landscape itself but of how it has changed.

How InSAR detects ground movement: by measuring changes in radar wave phase between satellite passes. Source: SatSense

Real‑world applications of InSAR Data for us in Aotearoa

Earthquakes

When the 14 November 2016 Kaikōura earthquake struck, it ruptured multiple faults and triggered over 10,000 landslides. Interferograms generated from Sentinel‑1 radar data collected on 3 November and 15 November showed ground uplift of 8–10 m and lateral offsets up to 12 m, highlighting the complexity of the rupture. Ground survey teams could never have mapped such widespread deformation so quickly.

Volcanoes

Active volcanoes often “breathe” as magma moves underground. InSAR can detect slow inflation or deflation of a caldera long before any eruption. Continuous monitoring of volcanoes in the Taupō Volcanic Zone uses stacks of Sentinel‑1 scenes to watch for changes in ground level, providing an early warning system for unrest.

Slow‑moving hazards

Not all hazards are dramatic. InSAR excels at revealing gradual processes such as subsidence in reclaimed land, settlement of new infrastructure, or the creeping movement of large landslides. Its ability to measure deformation across urban areas with centimetre‑ to millimetre‑scale precision allows planners to prioritise maintenance and mitigation. Studies in Gisborne have shown how persistent scatterer interferometry can map mm/year velocities across the city, helping to identify slopes at risk and to guide land‑use planning.

Building a time‑series from InSar Data

By stacking tens of images, InSAR can be used to turn snapshots into a comprehensive movie of surface deformation. This time-series approach reveals both gradual, long-term displacement trends as well as sudden movement events. 

Over 10 years of Sentinel-1 time series data is now available for New Zealand, with images acquired every 6-12 days, allowing users to track both long-term subsidence or uplift patterns and episodic deformation events such as landslide acceleration, volcanic unrest, or post-earthquake adjustment. This temporal density enables detection of seasonal variations, identification of movement thresholds, and early warning capabilities for hazard monitoring.

Land displacement timeline from InSAR satellite radar data in New Zealand. Source: SatSense.

Understanding the limits of InSAR

InSAR isn’t magic. Several factors can degrade the signal.

• Decorrelation: Vegetation growth, ploughing, snow and construction can change the surface between radar passes and scramble the returning signal. L‑band radar penetrates vegetation better than C‑ or X‑band, so it is preferred in forested areas.

• Revisit time: Typically the satellites revisit the same area every 6-12 days. In the case of rapidly changing events, or for highly sensitive structures near active construction, this may not be frequent enough to capture the complexity of movements.

• Geometric limitations:SAR satellites are in polar orbits and hence are insensitive to movements in the north-south direction the satellite flies in. Additionally, InSAR measures movement along the radar line of sight, not true vertical displacement. Two orbits (ascending and descending) may be required to isolate vertical and horizontal east-west displacement components. In areas with steep slopes, parts of the slope may be in the radar shadow.

InSAR now offers us all a bird’s‑eye view of ground deformation with centimetre‑ to millimetre‑scale precision, revealing data patterns that would be invisible from the ground.

When you work with the right radar band and processing method, users can track everything from sudden fault ruptures to the slow settling of a harbour wharf. While some challenges remain, time‑series analysis and the continual improvement of satellites are making InSAR an indispensable tool for keeping tabs on New Zealand’s shifting ground.

Avant partners with SatSense to bring InSAR land stability reports for the residential market in New Zealand. Check out our LandSure Geo-Reports Here.