Iain W. Bailey

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 eigenvectors of the resultant stress field and global centroid moment tensor (gCMT) solutions. We find that upper mantle slab viscosities of ∼10–100 and lower mantle viscosities of ∼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 underutilized constraint for slab dynamics.

Statistics of Earthquake Stress Drops on a Heterogeneous Fault in an Elastic Half-Space

We investigate properties of earthquake stress drops in simulations of evolving seismicity and stress field on a heterogeneous fault. The model consists of an inherently discrete strike-slip fault surrounded by a 3D elastic half-space. We consider various spatial distributions of frictional properties and analyze results generated by 150–300 model years. In all cases, the self-organized heterogeneous initial stress distributions at the times of earthquake failure lead to stress drops that are systematically lower than those predicted for a homogeneous process. In particular, the large system-sized events have stress drops that are consistently ∼25% of predictions based on the average fault strength. The type and amount of assumed spatial heterogeneity on the fault affect the stress-drop statistics of small earthquakes ( M L <5) more than those of the larger events. This produces a decrease in the range of stress drops as the earthquake magnitudes increase. The results can resolve the discrepancy between traditional estimates of stress drops and seismological observations. The general tendency for low stress drops of large events provides a rationale for reducing the statistical estimates of potential ground motion associated with large earthquakes.