A Seismogenic Slab Source Model for Aotearoa New Zealand

Abstract

Abstract Intraslab seismicity within the Hikurangi and Puysegur subduction zones constitutes >50% of recorded (Mw≥4.0 events) earthquakes in Aotearoa New Zealand. Here, we develop a source model for intraslab seismicity using recently augmented datasets including models of subduction interface geometries, an earthquake catalog, and a regional moment tensor catalog. For the areal zones of uniform seismicity, we consider the whole of each slab, as well as demarcations between shallower (depth ≤40 km) and deeper regions. Thereafter, we evaluate the magnitude–frequency distributions in each zone. To compute smoothed seismicity distributions, we apply a novel quasi-3D approach that involve: (1) delineation of midslab surfaces (defined by regions of maximum earthquake density), (2) orthogonal projections of hypocenters onto the midslab profiles, (3) uniform gridding of 0.1° down-dip on the midslab, and (4) application of smoothing kernel on the projected hypocenters. We also develop a model to characterize the focal mechanisms of the intraslab earthquakes using the regional moment tensor catalog. This model has median strike angles subparallel to subduction trenches and median dip angles ≥60° in both the subduction zones. The distribution of rake angles suggests that the Hikurangi slab has an extensional regime in the shallower parts but a compressional regime in the deeper parts, indicative of slab flexure. In contrast, the Puysegur slab predominantly exhibits a compressional regime.