Shear wave attenuation structure and anisotropy in the Hikurangi subduction zone, central North Island, New Zealand

The seismic attenuation structure of a subduction zone can constrain variations in temperature, composition and fluids. Amplitude spectra of 2206 local earthquakes, recorded by a dense network of 116 seismic stations in central North Island (NI), New Zealand, are modelled to image the three-dimensional (3D) shear attenuation structure of the Hikurangi subduction zone, down to 100km. Shear attenuation images are obtained by inverting 22260 t* observations for Qs (the quality factor of S waves) using a previously determined seismic velocity model. A frequency dependence of Q s is applied below 10 Hz, by parameterising t* as tol-ex where to is t* at 1 Hz, I is frequency and a = 0.3. Qs is frequency-independent above 10Hz. The 3D Qs images are interpreted alongside previously determined 3D Qp and Vp/Vs images, providing further constraints on features associated with subduction and magmatism in the Hikurangi subduction zone. The subducted slab is a prominent feature, exhibiting Qs = 1000, consistent with a 120Myr old slab. The upper surface of the slab is lined with hydrous fluids from 10km to > 100 km depth, derived from the dehydration of subducted sediments and hydrated oceanic crust. Between 50 and 75 km depth, the hydrous blanket lining the slab extends 20 km into the mantle wedge, and is imaged as having moderately low Qs ~ 300. The mantle wedge beneath the rhyolite-dominated, central segment of the Taupo Volcanic Zone (TVZ) is highly attenuating, with Qs 1400øC) and is responsible for the large extent of melting and high rate of melt production observed in the TVZ. Along-strike to the southwest, the hydrous blanket lining the slab ceases abruptly at a trench-perpendicular plane coincident with the southernmost point of the TVZ, as does a very low Qs (~ 50) region in the forearc crust (0-25 km depth). The cessation correlates with geodetic data. These fluid-rich regions reduce friction at the interface of the two plates and permit low-resistance subduction. Southwest of the trench-perpendicular plane, hydrous fluids are absent and the plates are locked. The seismic anisotropic structure of a subduction zone is a consequence of strain and flow in the mantle, and stress and deformation in the crust. In order to refine interpretations of crustal and upper mantle structure in the Hikurangi subduction zone, anisotropy is mapped by analysing shear wave splitting (SWS) of 773 local earthquakes recorded at 29 seismic stations (3371 station-event pairs). SWS results are interpreted in the context of previous anisotropy studies that used local, regional and teleseismic events, alongside 3D Qs, Qp and Vp/Vs structures. In eastern NI, fast directions are trench-parallel, following the trends of major faults in the axial ranges. The observed 3.7% anisotropy is crustal and results from the alignment of cracks and rock fabric in NI crust, 'perpendicular to the direction of maximum horizontal stress. In southern TVZ, fast directions are NE-SW, NW-SE and N-S, consistent with the alignment of highly-fractured crustal structures associated with rifting in discrete rift segments and intruding dikes. Anisotropy beneath the TVZ is larger (>5%) and is entirely crustal. In western NI, fast directions are predominantly N-S, following the trend of major tectonic features west of the TVZ (e.g., the Hauraki Rift and Coromandel Peninsula). There, anisotropy is small ®2%), crustal and results from the alignment of crustal fabric with trends in regional deformation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo