Three-dimensional attenuation structure of the Hikurangi subduction zone in the central North Island, New Zealand

Abstract

We use waveform data from 2708 earthquakes recorded by dense seismograph deployments in the central North Island of New Zealand to image the 3-D attenuation structure of the Hikurangi subduction zone, down to ca. 300 km depth. Attenuation images are obtained by determining the quality factor of P-waves Qp, using a t* inversion with a previously determined 3-D seismic velocity model. We have included limited frequency dependence for Qp, with Qp being frequency independent above 10 Hz, and having a frequency dependence of (f/10)0.5 below 10 Hz. The Qp images provide further constraint on the large-scale features associated with subduction and magmatism beneath the central North Island, and serve to refine interpretations of crust and upper mantle structure from Vp and Vp/Vs. The subducted plate is the most prominent feature in the Qp images. Its high Qp (900–1200) is consistent with the ca. 120 Myr old slab being relatively cold. Qp is better at resolving the slab than Vp. The coincidence of a strong gradient in Qp with the upper plane of the dipping seismic zone indicates that even below 75 km depth the upper envelope of seismicity provides a good estimate of the location of the plate interface. The mantle wedge is generally imaged as a relatively low Qp (<400) feature below 50 km depth. However, there are significant changes evident in the wedge along the strike of the subduction zone. The most pronounced low Qp in the mantle wedge occurs from ca. 50 to 85 km depth beneath the productive, rhyolite-dominated central segment of the Taupo Volcanic Zone, suggesting a close link between volcanism and low Qp in the shallow mantle wedge. There is a strong correlation between low Qp, low Vp and high Vp/Vs in this part of the mantle wedge, suggesting that high temperature is the controlling influence on Qp there. Within 30 km of the surface of the slab, there is a large change in Qp but only a modest change in Vp. Our results are consistent with a fluid-rich, viscous blanket being entrained with the motion of the subducted slab down to ca. 140 km depth, where hydrous melting of mantle peridotite initiates. Trenchward of the volcanic front, a strong horizontal gradient to higher Qp is imaged in the mantle wedge. We interpret this as a stagnant mantle nose, not involved in the corner flow of the adjacent mantle. The distribution of Qp within both the slab and the overlying plate provides support for dehydration embrittlement being important in promoting seismicity in the mantle of the slab. Where Qp is high at shallow depth in the forearc, it is usually underlain by low Qp near the plate interface. This suggests that competent terranes in the forearc may act as aquicludes, increasing fluid content in the rocks below, and hence might control the distribution of coupling at the plate interface.