Horizontal and vertical velocity fields for Central South Island, New Zealand, from Sentinel-1 InSAR Timeseries

Abstract:

The South Island of New Zealand is formed in the continental transpression between the Australian and Pacific plates. Oblique convergence of ~ 40 mm/yr has resulted in the formation of the Alpine Fault and the ongoing uplift of the Southern Alps. Existing horizontal velocity fields created across New Zealand have been based on comprehensive GNSS campaigns, with data points spaced on average 10-20 km apart, though this can decrease to as little as 2 km in some areas, such as Arthur’s Pass. However, the launch of the Sentinel constellation will allow the measurement of displacements, and therefore the production of velocity products, at potentially much higher temporal and spatial resolutions, in addition to providing a sensitivity to vertical motion that is often absent in GNSS studies. By doing this, we will be able to more accurately characterise the current spatial variations in deformation to infer major patterns of mountain growth during the formation of this orogeny.

The Southern Alps are a particularly difficult target for InSAR time series analysis, due to the many sources of incoherence that they contain. Coherence has been found to be strongly seasonal, with prolonged periods where the average coherence over the Alps is less than 0.15, preventing a simple small-baseline approach from being effective. Instead, we generate a coherence matrix using every possible combination of interferograms, weeding out incoherent and isolated pairs prior to further processing. This results in seasonal clusters of interferograms with short-temporal baselines connected by interferograms with multi-year baselines. The StaMPS/MTI processor is then used for a combined persistent scatter and small-baseline approach.

As well as the density of observations that Sentinel-1 provides, we take advantage of another benefit over previous InSAR missions such as Envisat, in that New Zealand is covered by both ascending and descending tracks. This allows us to carry out a joint inversion of multiple LOS and GNSS velocities to decompose the Central South Island velocity field into horizontal and vertical components. Initial results indicate a consistent uplift pattern across the Southern Alps comparable to both those measured by limited existing GNSS transects and exhumation rates calculated using thermochronology and seismicity.