HR: 1340h
AN: S23B-1371    [Abstracts]
TI: Resolving Three-Dimensional Anisotropic Structure with Shear-wave Splitting Tomography
AU: * Abt, D L
EM: David_Abt@brown.edu
AF: Brown University, 324 Brook St. Box 1846, Providence, RI 02912, United States
AU: Fischer, K M
EM: Karen_Fischer@brown.edu
AF: Brown University, 324 Brook St. Box 1846, Providence, RI 02912, United States
AB: Shear-wave splitting observations are a commonly used tool for inferring seismic anisotropy and deformation within the Earth's interior. We have developed a method for tomographically inverting shear-wave splitting observations for three-dimensional anisotropic structure, and we have tested it with synthetic splitting measurements from local events in subduction zone settings. The mantle is parameterized as a three- dimensional block model of crystallographic orientations with the elastic properties of olivine and orthopyroxene. To efficiently forward calculate splitting, the Christoffel equation is used to progressively split the horizontal components of a synthetic wavelet to account for the anisotropy in each model block, and predicted shear-wave splitting parameters are obtained with an eigenvalue minimization technique. Predictions from this approximate method, in general, compare well with splitting measurements from full waveform, pseudospectral synthetic seismograms. We solve for a best-fitting model of crystallographic orientations using a linearized, damped least- squares inversion that employs numerically calculated partial derivatives. To account for the non-linear behavior of shear-wave splitting, the inversion is applied iteratively and partial derivatives are recalculated after each iteration. We tested the capabilities and limitations of this method with inversions of synthetic data from target structures including lateral and vertical variations in anisotropy in both idealized and real (Nicaragua-Costa Rica) subduction zones. Convergence to accurate and stable solutions is found with widely varying starting models. However, in general the target structures are best retrieved using a starting model that is based on spatial averaging of predicted fast directions and splitting times. With a station spacing of 25 km in an idealized subduction zone containing uniformly spaced events down to 225 km and a slab dip of roughly 60вк, both the azimuth and dip of crystallographic axes are resolvable to a depth of 100-150 km, and lateral heterogeneities in anisotropy on a scale of 50 km at arc and fore-arc distances from the trench are easily retrieved. Spatial resolution of anisotropy at scales of 75 km is possible further into the back-arc above 150 km depth. The geometry of stations and observed seismicity in the Nicaragua-Costa Rica subduction zone yields partial resolution at scales of 50-75 km beneath the fore-arc, arc, and limited regions of the back-arc down to 100 km, and resolution at coarser scales is possible in wider regions beneath the back-arc. Given the distributions of seismic sources within many subduction zones and the advances in broadband seismic array deployments, this new method offers a powerful means with which to accurately constrain the orientation of anisotropic fabric in the mantle wedge.
UR: http://www.geo.brown.edu/geopeople/grads/abt/CR.htm
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8180 Tomography (6982, 7270)
SC: Seismology [S]
MN: 2007 Fall Meeting