Seismogenic structures and stress state of the Puysegur Subduction Zone (Fiordland, New Zealand) from detailed earthquake observations.

The formation of new subduction zones, termed Subduction Zone Initiation (SZI), has a large influence on global plate tectonics. However, how stresses within the plate boundary region evolve throughout the evolution of nascent subduction zones remains unresolved. The Puysegur Subduction Zone in Fiordland, near the southern tip of New Zealand’s South Island, is a young, steeply dipping subduction zone and a key site for studying such incipient stages of subduction. Despite the global significance of the Puysegur Subduction Zone, it has received relatively little attention, mostly due to its remote location. Few passive seismic studies have been carried out in the region, and the continuous GeoNet network is too sparse to detect and accurately resolve seismicity around the Puysegur Subduction Zone. Because of this, the present-day structure and stress state of the Puysegur Subduction Zone remain poorly resolved.

We aim to study these two unresolved characteristics by using a combination of temporary seismic networks and the permanent GeoNet network to increase station coverage in the region. We have developed a Puysegur-appropriate workflow consisting of automated earthquake detection and association, manual event evaluation and P-wave polarity determination. Highly accurate earthquake locations are obtained using NonLinLoc and a 3D velocity model. Focal mechanism analyses and stress inversion are conducted using Bayesian approaches. Currently, we have obtained a preliminary catalogue for the period between 02/2018 and 10/2018, which shows that the developed methodology is capable of producing more complete earthquake catalogues compared to the national GeoNet catalogue. Preliminary precise hypocentral locations and well- constrained focal mechanisms for moderate-to-large magnitude events are used to constrain the region’s seismogenic structures, such as the subduction interface and major active faults, as well as provide preliminary constraints on the region’s stress state.