New Zealand-Wide 3-D Qp Attenuation Model: Low Qp North Island Crust Contrasts with High Qp South Island Terranes

Abstract:

We have obtained a 3-D model of seismic attenuation covering all of New Zealand, where regional structure ranges from strong, relatively undeformed crust to thin rift zones with active volcanism. This New Zealand-wide model will be useful for modeling tectonic deformation and coseismic motion at regional distances. Using local earthquakes, Qp (1/attenuation) is derived from path attenuation t* observations. The New Zealand model is primarily based on merged results from existing studies. These are supplemented with data in areas that were not previously analyzed for Qp, then using additional iterations with the merged model to ensure consistency. There is large heterogeneity in Qp both within some regions and along the length of the country. This suggests potential for greater variation in modeled ground motion compared to that estimated with uniform attenuation relations.

 Along the Hikurangi subduction zone, the subducted slab forms a prominent high Qp feature in the North Island and northern South Island. In the forearc, zones of low Qp above the plate interface suggest thick subducted sediment, and high Qp features represent blocks of Cretaceous volcanics in the overlying plate. The Taupo Volcanic Zone, with extension and geothermal activity, is delineated by low Qp in the upper crust. The underlying mantle wedge has very low Qp from 50-85 km depth below the rhyolite section. In contrast, the Fiordland subduction zone, southwestern South Island, lacks a mantle wedge and instead has the young slab steeply bending against a broad high Qp feature in the Pacific mantle which may represent mantle shortening or relic oceanic slab material.

 The South Island crust exhibits relatively high Qp, which is related to the Otago schist, igneous terranes, and mafic lower crust. Between the subduction zones, the Alpine fault and Marlborough fault zones accommodate plate motion. These fault zones show low Qp in their high-strain ductile roots, related to grain-size reduction and metamorphic fluid release. In the brittle crust, low Qp is imaged along fault zones which have the most recent large earthquakes and may be related to distributed microfractures.