Shear-wave Splitting Tomography in the Central American Mantle Wedge Geoffrey A. Abers Boston University J. Marino Protti, Victor Gonzalez OVSICORI Wilfried Strauch, Pedro Perez, Allan Morales INETER David L. Abt Karen M. Fischer, Laura Martin Brown University AGU 2005 Fall Meeting Session: T31D 12/7/ TUCAN T omography U nder C osta Rica A nd N icaragua NSF-Margins

TUCAN Seismic Array 48 IRIS/PASSCAL Broadband Seismic Stations 20 Month Deployment (August March 2006) Close Station Spacing ( km) Excellent Data Recovery Rate (>95%) Shear-wave Splitting Tomography

Why Central America? Large along arc geochemical variations Different melting environments? Compare geochemical models with seismic imaging results (e.g., V P, V S, V P /V S, Q, Anisotropy) STUDY AREA e.g., Carr (1984); Carr et al. (2003); DeMets (2001); Roggensack (2001); Walker et al. (2001) NOAA National Geophysical Data Center

Shear-wave Splitting Tomography Objective: Method: Preliminary Results & Implications… Help constrain flow, melt and volatile distribution by modeling anisotropic structure in the wedge Shear-wave splitting analysis Forward model: synthetic waveforms Linearized inversion SHEAR-WAVE SPLITTING TOMOGRAPHY

Shear-wave Splitting Tomography Alignment of anisotropic minerals (LPO) e.g., Olivine and OPX * Effects of H 2 O, Pressure, Stress * Alignment of melt bands or fractures (SPO) Causes of Anisotropy Jung and Karato (2001) J&K (2001) Mehl et al. (2003) Typical Bystricky et al. (2000) ~350 MPa o C Holtzman et al. (2003) Actual Apparent Fast Axis Not Necessarily Parallel to Shear Direction After Jung and Karato (2001)

Data Set (so far…): Local S waves Magnitude > 2.6 In shear-wave window SKS waves Magnitude > 5.8 Local S SKS ~ Phase Waveforms Analyzed High Quality Splits Local S Ray Path Arc Volcano Seismic Station Shear-wave Splitting Tomography Caribbean Sea Pacific Ocean Laura Martin Trench

Shear-wave Splitting Tomography Pacific Ocean Caribbean Sea Shear-wave Splitting Measurements  Local S splits plotted at ray path midpoints  SKS splits plotted at stations

Shear-wave Splitting Tomography Inversion Method Block parameterized model FORWARD MODELING - Synthetic waveform propagation through multiple anisotropic blocks Weighted, damped, least- squares inversion Fischer et al. (2000) Apply starting crystallographic orientation and strength of anisotropy to each model block Use forward modeling to calculate partial derivatives for each inversion iteration Perturb a-axis or anisotropic strength in each block

Shear-wave Splitting Tomography Preferred Model: km thick blocks 0.5 x 0.5 degree blocks Minor damping No covariance Starting Parameters:  = -30 Strength of Anisotropy = 24% 25 Iterations Preliminary Results Tested: Block size, starting model, damping, and block covariance

Shear-wave Splitting Tomography Preliminary Results Horizontal slices through preferred model Arc-Parallel Arc-Normal Arc-Parallel Arc-Normal

Shear-wave Splitting Tomography View looking SW Depth (km) Preliminary Results

Shear-wave Splitting Tomography Summary of Results Local S Inversion SKS Measurements Arc-parallel fast directions dominate, except for two arc- normal columns Additional anisotropy with arc- parallel fast direction beneath region sampled by local S waves and farther into the back-arc ( s of splitting)

Shear-wave Splitting Tomography InterpretationFurther Work Full data set 3-D rotations of fast direction Optimize model uniqueness and resolution ~5 months 127 Local Splits ~20 months ~500 Splits? Regional arc- parallel flow Oblique convergence or far-field effects? Localized arc- normal zones affected by melt and possibly volatiles? e.g., Clint Conrad (personal communication); Behn et al. (2004)

Shear-wave Splitting Tomography Questions ?