Global slab deformation and centroid moment tensor constraints on viscosity

We analyze moment tensor solutions from deep subduction zone earthquakes to determine global slab deformation patterns. Inferred strain rates are compared to predicted deformation patterns from fluid models to help constrain the first-order radial and lateral viscosity structure of the Earth. While all slabs that reach the lower mantle are compressed at their tip, intermediate depth patterns are more complex. We compute 3-D spherical flow with various slab rheologies and compare the angular misfit between the compressive eigen-vectors of the resultant stress field and global centroid moment tensor (gCMT) solutions. We find that upper mantle slab viscosities of similar to 10-100 and lower mantle viscosities of similar to 30-100 times the upper mantle produce the best match to gCMTs. A 0.1 viscosity reduction in the asthenosphere seems preferred. Slab geometry and lower mantle viscosity exert significant control on deformation. Inclusion of the phase changes at 410 km and 660 km increases extensional deformation at intermediate depth and compressional deformation at the lower mantle, improving the match to gCMTs for strong slabs. Our conclusions are fairly insensitive to surface boundary conditions. However, models which include net rotations of the surface with respect to the lower mantle produce compression at intermediate depths for west directed slabs and extension for east directed slabs. Without allowing for regional variations, these models yield the best match to gCMTs. While significant deviations between model and seismicity remain, our results show that seismicity provides an under-utilized constraint for slab dynamics.