Crustal heterogeneity and subduction processes:: 3-D Vp, Vp/Vs and Q in the southern North Island, New Zealand

Along 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.