[S-CG43] Shallow and intermediate depth intraslab earthquakes: seismogenesis and rheology of the slab

[SCG43-08] Deep Slab Seismicity Limited by Rate of Slab Deformation in the Transition Zone

★Invited Papers

*Magali I Billen¹, Rebecca Fildes¹, Marcel Thielmann², Menno R.T. Fraters¹ (1.University of California Davis, 2.University of Bayreuth)

Keywords:deep earthquakes, subduction, rheology, shear instability

Deep earthquakes within subducting lithosphere (slabs) are puzzling because they have many similarities to shallow earthquakes, yet frictional failure of rocks is strongly inhibited at high pressure. There are several proposed triggering mechanisms for deep earthquakes including dehydration embrittlement, transformational faulting and thermal shear instability. However, it is not clear where in the slab each mechanism could be viable because multiple conditions (e.g., temperature, stored elastic energy, fluids) must be met simultaneously. It has also been proposed that multiple mechanisms work in tandem. First, I will discuss simulations of subduction with non-linear rheology and compositionally-dependent phase transitions that exhibit strongly variable strain-rate magnitude in space and time. 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. The spatial patterns of strain-rate are similar to observed seismicity versus depth profiles, while the changes in time suggest that present-day observations capture a single snap-shot in time of a constantly changing seismicity pattern. Based on this comparison, I propose that in addition to the temperature and stress requirements of deep earthquake mechanisms, 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, using both 2D subduction simulations and 1D shear instability models, that share the same visco-elasto-plastic rheology.