[1] The material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the Raukumara Peninsula. Attenuation images are obtained using t* values measured from 2848 spectra of P wave arrivals. The t* values are inverted for frequencyindependent three-dimensional (3-D) Q p , using a 3-D velocity model. Linked nodes in a preliminary 2-D inversion are used to obtain the most reliable initial model for 3-D inversions. The results show the benefits of obtaining 3-D Q in addition to 3-D V p and V p /V s . Q is related to localized permeability and heterogeneity, and thus 3-D Q is an important tool for interpreting active tectonic regions. Q p in the shallow overlying plate generally corresponds to the mapped geology. Two high Q p features (>600) represent subducted seamounts or Cretaceous volcanics within the overlying plate. The gradient above the subducting plate is the strongest feature in the Q p model, with the slab having high Q p (600 -900). Below 20-km depth and above the plate interface, there is a zone of 50% decrease in Q p and high V p /V s about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating.
S U M M A R YAlong the southern Hikurangi subduction zone in the southern North Island, New Zealand, the subduction zone changes from volcanic with moderately weak interseismic coupling, to non-volcanic with strong coupling in Wellington. To investigate how material heterogeneity relates to subduction processes, we compute 3-D seismic properties using 342 earthquakes and 2 shots recorded by a 165-station seismic array. The time base for some telemetered temporary stations was not stable, which required revising the inversion code to allow an additional origin time related to the telemetered stations. Progressive inversions from 2-D to fine 3-D were carried out for Vp, Vp/Vs and Qp. Combined interpretation of these three seismic properties highlights features of the subduction zone. The subducted slab is imaged as a high-velocity, high-Q feature. In the overlying crust, from southeast to northwest, there is low velocity in the accretionary wedge, high velocity in the axial ranges and low velocity in the Wanganui basin. High Vp/Vs and low Qp characterize the crust east of the North Island dextral fault belt. High Vp/Vs is imaged above the subducted slab to 40 km depth in the northern half of the study area. Low Qp is also prominent above the slab, particularly trenchwards of the volcanic region. In marked contrast to the high Vp/Vs, the low Qp above the slab extends as a continuous feature up to the low Qp in the shallow crust underlying the volcanic region. The low Qp and high Vp/Vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release. Vp/Vs is particularly sensitive to overpressured fluid in the compressional forearc, whereas Qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region. The rheology of the lower crust of the overlying plate is a factor in large-scale subduction coupling as evidenced by the terrane boundaries and Haast schist in the section near Wellington. The changing rheology of the lower crust of the overlying plate in the southern North Island also provides a possible explanation for the southward cessation of volcanism.
S U M M A R YWe 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.
[1] The three-dimensional attenuation properties of the South Island, New Zealand, are obtained from local earthquake t* (spectral decay) data. We extend the spectral fitting method used to obtain t* by enabling limited frequency dependence for Q. Observed Qp heterogeneity relates to both active geological processes and to constituent tectonic blocks, with high Qp for the Haast schist. The most pronounced low Qp features in the upper crust correspond to zones of recent seismicity, rather than the most significant faults. The central Alpine fault, a dipping, oblique-slip fault, forms the western boundary to a low Qp volume in the ductile crust. Particularly low Qp is imaged at the base of the Alpine fault at 30-km depth and may be related to metamorphic fluid release in the crustal root, a region of predicted high strain. Conversely, the southern Alpine fault, which is a vertical strike-slip fault, forms the western boundary to the high Qp schist region. In the southern South Island, the plate boundary becomes a subduction zone, exhibiting unusual Qp character. Moderately high Qp is associated with the subducted Australian plate. There is also a high Qp, high Vp feature in the Pacific mantle to 160-km depth that appears to cause the subducting slab to bend to near vertical. This feature may represent mantle shortening since the Miocene. Regional attenuation rates for Fiordland earthquakes, calculated from the 3-D Qp model, show large variations over the South Island; the low Qp in the crustal root has a significant effect on passing raypaths.Citation: Eberhart-Phillips, D., M. Chadwick, and S. Bannister (2008), Three-dimensional attenuation structure of central and southern South Island, New Zealand, from local earthquakes,
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