Imaging P and S attenuation in the termination region of the Hikurangi subduction zone, New Zealand

Abstract

We examine seismic attenuation in the northern South Island, New Zealand, where subduction transitions to oblique collision, and plate motion occurs along multiple crustal faults overlying the subducted slab. The 3-D inversions derive Q (1/attenuation) using path attenuation t* observations for 334 distributed earthquakes recorded on permanent and temporary stations, including both velocity and acceleration records. A 2.5 s window was used for P spectra, but for S spectra longer varied lengths were selected around the predicted S arrival, using the 5–95per cent energy integral. The Q results highlight many aspects of the structure more clearly than previously derived seismic velocity models, including a high Q slab, low Q basins, moderately low Q active fault regions, and thick lithosphere. Qs tends to be greater than Qp, except in low Q shallow upper crust. The mantle above the slab does not exhibit low Q, unlike mantle to the north beneath the volcanic region of central North Island, and is inferred to be cool and stagnant with some vertical flux of slab dehydration fluid. In the brittle crust, low Q is imaged along those faults with most recent seismicity and may be related to distributed microfractures. In the ductile crust of the greywacke terranes, zones of low Q under the faults are attributed to localized ductile deformation with high strain-rate and grain size reduction, consistent with numerical models showing the development of enhanced strain-rate zones above the strong underlying slab. In contrast the Christchurch region has no ductile lower crust and instead has high Q indicative of strong mafic rocks at 12 km depth.