Geodetic investigation of triggered slip below Fiordland

<p>Secretary Island, at the head of Doubtful Sound in Fiordland, has been seismically active in past 30 years, with earthquakes larger than M w 6.5: the 1989 Doubtful Sound, 1993 Secretary Island, and 2003 Fiordland earthquakes. These events were approximately coincident with the 17° bend in the strike of the young, obliquely-converging, and steeply dipping Puysegur Subduction Zone. This section of the plate interface also has a history of triggered slip: the 1989 earthquake is inferred to have triggered the 1993 earthquake and, further north at George Sound, triggered afterslip was reported following the 2009 Dusky Sound earthquake. We have used L-band (23.6 cm-wavelength) Synthetic Aperture Radar (SAR) data from the ALOS1 and ALOS2 satellites, and C-band (5.5 cm-wavelength) SAR data from Sentinel 1A/B satellites, to test the hypothesis that triggered slip also occurred in the vicinity of Secretary Island following the 2007 George Sound, 2009 Dusky Sound and 2016 Kaikōura earthquakes. SAR images were aligned, interfered, filtered, and unwrapped using GMTSAR processing tools. Long-wavelength ionosphere noise was removed by inverting for the best-fitting linear plane, and we assumed a linear function of height to remove short-wavelength atmospheric noise. Small Baseline Subset (SBAS) timeseries analysis indicated a localised deformation signal centred on Secretary Island following the Dusky Sound earthquake. A re-analysis was undertaken of the co- and post-seismic deformation caused by the Dusky Sound earthquake so that any surface deformation centred on Secretary Island could be isolated. Campaign and continuous Global Positioning System (GPS) data were simultaneously inverted with co- and post-seismic interferograms using a statistical Bayesian modelling approach to determine the optimal Dusky Sound earthquake source parameters. Limitations arising from orbital drift, the frequency of SAR acquisitions and the observation geometry hindered our ability to constrain the timing, magnitude and location of reactivated slip from a source similar to the 2003 Secretary Island earthquake. Our findings indicate that slip was not triggered following either the 2007 George Sound earthquake or 2016 Kaikōura earthquake. However, we cannot rule out triggered slip near Secretary Island following the 2009 Dusky Sound earthquake. Any such slip likely occurred on an area of c. 350 km² (c. 15 km updip of the Secretary Island epicentre) with an average slip of 1–3 m, producing motion away from the satellite of c. 25 mm at Secretary Island.</p>