1. Slab deformation and seismicity Thorsten W. Becker Lisa A. Alpert Iain W. Bailey Melanie Gerault Meghan S. Miller University of Southern California Los Angeles EGU meeting, Vienna April 26, 2012

2. Harvard gCMT catalog, Engdahl et al. hypocenters, and P-wave tomography (Li et al. 2008) Objective: Use global subduction zoo to infer mantle dynamics (e.g. slab strength)

3. cf. Isacks & Molnar (1969); Chen et al. (2004) Bailey et al. (2012) We use the Harvard/Lamont GCMT catalog up to 2010 down slab along slab Global CMT analysis depth All CMTs are rotated into a slab-local coordinate system based on Benioff seismicity contours New coordinate system shows CMTs looking at slab from side Compressional-Oblique-Extensional Largest earthquake in depth bin strain in down-dip direction, f 

4. Bailey et al. (2012) Global CMT analysis Kostrov- summed moment tensors Compressional-Oblique-Extensional Largest earthquake in depth bin Kostrov summation normalized summation strain in down-dip direction, f 

5. down dip angle Bailey et al. (2012) P axis T axis Non-double couple components: Compensated Linear Vector Dipole (CLVD,  ) Extensional – Oblique - Compressional Extensional Compressional pure double couple strain in down-dip direction, f 

6. Kuge and Kawakatsu (1993); Bailey et al. (2012) P axis T axis Extensional, shallow Compressional, deep Trends in actual CMTs

7. Alisic et al. (2010) Alpert et al. (2010)cf. Vassiliou & Hager (1988) Global geodynamic models

8. 8/18 Circulation modeling Incompressible, laminar (Stokes) flow Boundary conditions: weak zones with plate motions driven by density anomalies, or prescribed plate motions Layered viscosity structure, with lateral viscosity variations, Newtonian Density anomalies assigned to Wadati-Benioff zones for slabs Solve with MILAMIN (Dabrowski et al., 2008) or CitcomS (Zhong et al., 2000), with modifications as in Becker & Faccenna (2011) and Gerault et al. (2012) Resolution ~5 – 20 km (global 3D), 1 km (cylindrical) (Alpert et al., 2010; Gerault et al., 2012)

9. Effect of lower mantle viscosity on in slab stress orientations η slab /η mantle = 100 Alpert et al. (2010) P axes: blue = model predictions, green = at centroid, red = data  lm /  um = 1

10. Effect of lower mantle viscosity on in slab stress orientations η slab /η mantle = 100 Lower mantle viscosity increase is required to generate significant compression Alpert et al. (2010) P axes: blue = model predictions, green = at centroid, red = data  lm /  um = 1  lm /  um = 100 cf. Vassiliou & Hager (1988)

11. Alpert et al. (2010) Regional misfits of depth averaged P/T axes for different rheologies Moderately strong (viscosity <~ 100 upper mantle) slabs preferred, lower mantle viscosity increase required

12. Bailey et al. (2012) Regional CMT summation cf. Isacks & Molnar (1969); Chen et al. (2004) Compressional-Oblique-Extensional strain in down-dip direction, f 

13. Compressional-Oblique-Extensional Alpert et al. (2010); Bailey et al. (2012) Geodynamic modeling results strain in down-dip direction, f 

14. Geodynamic model predictions Compressional-Oblique-Extensional strain in down-dip direction, f  Regional selection of intermediate depth extension- deep compression type subduction Box size scales with number of CMTs in summation Bailey et al. (2012)

15. CLVD component Compressional-Oblique-Extensional Model predictions for stress state and CLVD component strain in down-dip direction, f 

16. Bailey et al. (2012) Intermediate depth extension- deep compression slabs Model predictions Model misfit

17. Bailey et al. (2012) Intermediate depth compression- deep compression slabs Model predictions Model misfit

18. Conclusions Global circulation models provide good (“reference”?) fit to deep, co-seismic slab deformation style world wide (Newtonian, isotropic flow works) Moderately strong slabs (viscosity ~100 upper mantle) and ~50 viscosity increase in lower mantle preferred Non double couple (CLVD) components of CMTs, and dependency of CLVD style on major stress axis, are newly predicted by fluid models