SPEAKER: Magali Billen (UC Davis)
DATE: 4:00pm, Friday, February 26th, 2021

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

ABSTRACT:
Deep earthquakes within subducting lithosphere (slabs) have remained enigmatic because they have many similarities to shallow earthquakes, but frictional failure of rocks is strongly inhibited at high pressure. While there are several proposed triggering mechanisms for deep earthquakes, it is not clear where in the slab each mechanism would be viable, because multiple conditions (e.g., temperature, stored elastic energy) must be met simultaneously. I will show simulations of subduction with non-linear rheology and compositionally-dependent phase transitions, which exhibit strongly variable strain-rate magnitude in space and time with similarities to observed seismicity versus depth profiles. High strain-rates occur in bending regions of the slab and migrate as the slab buckles and folds at the base of 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. I argue that in addition to temperature and stress requirements of deep earthquake mechanisms, spatial variations in strain-rate determine the spatially-variable distribution of deep earthquakes (gaps, peaks, rate of seismicity)*. Finally, I will describe how we are further testing this hypothesis for the mechanism of thermal shear instability, and show preliminary results for visco-elastic-plastic models of subduction.