HR: 13:40h
AN: T33E-01 INVITED     [Abstracts]
TI: Modeling Seismic Anisotropy at Strike-slip Boundaries
AU: * Savage, M K
EM: Martha.savage@vuw.ac.nz
AF: Victoria University of Wellington, Box 600, Wellington, 6001 New Zealand
AU: Fischer, K M
AF: Brown University, Providence, RI, 02912
AU: Hall, C E
AF: California Institute of Technology, Pasadena, CA, 91125
AU: Tommasi, A
AF: University of Montpellier, II, Montpellier, 34095 France
AU: Chery, J
AF: University of Montpellier, II, Montpellier, 34095 France
AU: Ellis, S
AF: Institute of Geological and Nuclear Sciences, Gracefield, Lower Hutt, 6004 New Zealand
AB: Despite similar surface transform faulting behavior, shear-wave splitting in the California (Ca) and New Zealand (NZ) plate boundary regions is markedly different. To better understand the origin of the anisotropy we model mantle flow and strain for a variety of strike-slip plate boundary scenarios. In our first models, simple relations between the flow or strain and elastic anisotropy are assumed to determine the integrated splitting in shear particle motion along teleseismic paths. Of these, strain-controlled models fit the observations in NZ and Ca better than simplified flow-controlled models. Fast shear polarizations are progressively rotated toward the shear plane over time, and even a constant viscosity model provides a good fit to the fast directions in NZ and southern Ca. The constant viscosity implies strong coupling between the surface and the deeper mantle. To fit the lack of decrease in delay times with distance from the fault, the relationship between delay time and strain must saturate at small strains. If we consider only directions, then southern NZ and southern Ca both fit models in which strain is smaller than in nearby regions, equivalent to that achieved along an infinite fault by about 3-10 My of their present motion. Stratified viscosity allows more rapid rotation of fast directions toward fault-parallel than occurs in isoviscous models, and can explain the nearly fault-parallel fast directions in the central South Island. Different aspects of the northern Ca results are fit with different models, but a rapid change in viscosity with depth is needed to produce the full effects of the behaviour previously modelled as two layers of anisotropy noted in the area, suggesting vertical decoupling. We are testing these conclusions with new models for NZ that allow for the evolution of temperature-dependent viscosity and for anisotropy to be determined using polycrystal plasticity theory. Preliminary modelling shows that increased thickness of the crust in the continental compared to oceanic regions localises the strain under the continental landmass, which may help to explain the large width of the shear zone in NZ.
DE: 7260 Theory and modeling
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 7218 Lithosphere and upper mantle
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting