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Shear-wave Splitting Tomography in the Central American Mantle Wedge
Geoffrey A. AbersBoston UniversityJ. Marino Protti, Victor GonzalezOVSICORIWilfried Strauch, Pedro Perez, Allan MoralesINETER
David L. AbtKaren M. Fischer, Laura MartinBrown University
AGU 2005 Fall MeetingSession: T31D12/7/05 0900
TUCAN
Tomography
Under
Costa Rica
And
Nicaragua
NSF-Margins
TUCAN Seismic Array
• 48 IRIS/PASSCAL Broadband Seismic Stations
• 20 Month Deployment (August 2004 - March 2006)
• Close Station Spacing (10 - 50 km)
• Excellent Data Recovery Rate (>95%)
Shear-wave Splitting Tomography
Shear-wave Splitting Tomography
Why Central America?
• Large along arc geochemical variations
• Different melting environments?
• Compare geochemical models with seismic imaging results (e.g., VP, VS, VP/VS, 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 H2O, 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
1200-1300oC
Holtzman et al. (2003)
Actual
App
aren
t
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 817 127
SKS ~200 64
PhaseWaveforms 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:
• 25-30 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
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 (1 - 1.5 s of splitting)
Shear-wave Splitting Tomography
Interpretation Further 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)
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