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

View looking SW

De

pth

(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 (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)

Shear-wave Splitting Tomography

Questions ?