Colloquium Series


Monday, November 4, 2019, 12:00pm


Haller Hall Geo Museum 102
Speaker: Professor Magali Billen
University of California, Davis

Title: "Deep Slab Seismicity Limited by Rate of Slab Deformation in the Transition Zone"

Abstract:  Proposed mechanisms for deep earthquakes (100–680 km), such as dehydration embrittlement, shear instability, or transformation faulting, have attempted to explain the global seismicity depth profile while accounting for the effects of the thermal structure of the subducting lithosphere. However, such analysis fails to explain the variability in seismicity both within and among the world’s subduction zones. Here, I propose that the seismicity pattern in subducted lithosphere is determined by the spatially variable strain-rate in deforming slabs. Numerical simulations of subduction with non-linear rheology and compositionally-dependent phase transitions, exhibit strong variability in the strain-rate magnitude both in space and time. High strain-rates occur in bending regions and migrate as the slab buckles and folds in the transition zone. However, in between these strongly-deforming regions the strain-rate is low due to the strong temperature-dependence of viscosity and high yield strength of the slab. The similarity between the modeled and observed strain-rate depth profiles suggests that the seismicity pattern of deep earthquakes is determined by the same first-order control as seismicity at the Earth’s surface: the strain-rate. Considering strain-rate as a first-order control on deep earthquake seismicity explains why there are large gaps in seismicity (i.e., low strain-rate regions), variable peaks in seismicity (bending regions), higher rates of seismicity where slabs exhibit a more complex shape (i.e., are undergoing more deformation), and why there is an abrupt cessation of seismicity below 660 km (due to a sharp drop-off in the strain-rate.

Short Bio: I am a geodynamicist who works on subduction dynamics from the outer rise through the transition zone and into the lower mantle. I am particularly interested in understanding and constraining the complex rock rheology in subduction zones, and linking processes at different time-scale and length-scales through rheology. I’ve been a professor at UC Davis since 2003, with post-docs at the University of Leeds and Woods Hole Oceanographic Institution following my PhD at Caltech. I started out as a physics major at the University of Puget Sound.