Thingbaijam KKS, Gerstenberger MC, Rollins C, Christophersen A, Williams CA, Ristau J, Rastin SJ, Fraser J, Van Dissen RJ. 2022. A seismogenic slab source model for New Zealand. Lower Hutt (NZ): GNS Science. 31 p. (GNS Science report; 2021/50). doi:10.21420/CDMK-3F30.
Abstract
We present an advanced model for intraslab seismicity in the Hikurangi and Puysegur subduction zones. To develop the model, we use improved datasets: a geospatial configuration of the subduction geometry, an updated earthquake catalogue and a regional moment tensor solution catalogue. We adopt two different strategies, areal zonation of uniform seismicity and smooth-seismicity distribution. The areal zonation is implemented by demarcating shallower (≤40 km) and deeper (>40 km) regions of the slabs. We find that the overall seismicity in both Hikurangi and Puysegur slabs agrees with Gutenberg-Richter b-value ~1.0. However, the shallower regions in these zones have lower b-values. On the other hand, the Hikurangi slab exhibits a seismicity rate, with a larger a-value. The smooth-seismicity distribution is realised through a quasi-3D approach using a declustered catalogue. The key steps in this approach are (1) delineation of mid-slab, (2) orthogonal projections of hypocentres onto the mid-slab, (3) generation of a uniform grid of 0.1° on the mid-slab and (4) application of gaussian smoothing. The maximum magnitude is conjectured to range between MW 8.0 and 8.3, based on the historical largest intraslab earthquakes across the globe. Lastly, we analyse the focal mechanism dataset to construct a model for finite-rupture geometry. This model has average strike angles sub-parallel to subduction trenches and median dip angles ≥60°. The distribution of rake angles suggests that the Hikurangi slab has an extensional regime in the shallower parts, while the deeper parts exhibit a compressional regime, indicative of slab flexuring. On other hand, the Puysegur slab predominantly exhibits a compressional regime. (The authors)
