HR: 13:40h
AN: S43C-01    [Abstracts]
TI: Three-Dimensional Attenuation Structure of the South Island, New Zealand, from Local Earthquakes
AU: * Eberhart-Phillips, D
EM: d.eberhart-phillips@gns.cri.nz
AF: GNS Science, Private Bag 1930, Dunedin, 1 New Zealand
AU: Chadwick, M
EM: m.chadwick@gns.cri.nz
AF: GNS Science, P.O. Box 30368, Lower Hutt, 1 New Zealand
AB: The three-dimensional attenuation properties of the South Island, New Zealand, are obtained from local earthquake t* data. Observed heterogeneity relates to both active processes and to constituent tectonic blocks. We improve the spectral fitting method used to obtain t* by implementing mixed frequency dependence for Q. Q is allowed to be frequency independent at high frequencies, while being frequency dependent at low frequencies. The imaged variations in attenuation are consistent with observed regional variations in strong motion data from large earthquakes. The patterns of 3-D Q are broadly related to the constituent rocks, with high Q for the Haast schist. There are numerous faults throughout the region with the Alpine fault being the major plate boundary fault. The most pronounced low Q features in the upper crust correspond to zones of recent seismicity, rather than the most significant faults. The central Alpine fault, which is a dipping, oblique-slip fault, forms the western boundary to a low Q volume. Particularly low Q 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 Q schist region. In the southern South Island, the plate boundary becomes a subduction zone. High Q is associated with the subducted Australian plate. There is also a high Q, high Vp feature in the Pacific mantle at 100-km depth that causes the subducting slab to bend to near vertical as it transitions to the Alpine fault.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 8038 Regional crustal structure
DE: 8104 Continental margins: convergent
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Seismology [S]
MN: Fall Meeting 2005