Three-dimensional attenuation structure of central and southern South Island, New Zealand, from local earthquakes

摘要

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.