InSAR as a volcano monitoring tool for Auckland City, New Zealand.

Auckland City, New Zealand, with a population exceeding 1 million, is built directly on top of an active monogenetic volcanic field, which last erupted several hundred years ago. The field contains around 50 discrete volcanoes that have erupted over the last 250ka. The eruptions are typically small volume (<0.1km3), although the most recent event produced a total volume of 2.4km3 or 59% of the total volume of the field as a whole. It is important therefore to have an effective and robust early warning system in place to detect geodetic changes that may be precursors to such eruptions. However, because each new eruption forms a new vent anywhere within the 25 by 15 km volcanic field, it is extremely difficult to use field-based techniques to monitor the entire area at an adequate spatial and temporal density. We therefore looked to orbital InSAR measurements of deformation as a potential solution to this problem. The viability of using C-band InSAR as a monitoring tool was established using ERS data during the 1990s, and the largely urbanised nature of the Auckland region means that coherence between acquisitions persists for years, allowing long-term baselines to be established. Auckland magma is thought to ascend exceptionally rapidly. We undertook Mogi modelling of ascending volumes at different depths in the crust to simulate a rising diapir, and ascertained whether the precursive deformation signals are likely to be (a) within the detection limits of C-band InSAR, and (b) discernible above any atmospheric noise. As a result of our research, a program of regular radar data collection from the Envisat satellite has been established for monitoring purposes, with data collected every 3 months, ensuring that an adequate archive of suitable data is available for inter-comparison with future Envisat acquisitions, should a volcanic crisis occur. Once near-real-time data supply issues are totally overcome, the InSAR monitoring capability at Auckland will be fully operational.