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Smith Lecture: Extreme Mechanics on the Surface of Our Planets

Ahmed Ettaf Elbanna, University of Illinois
Friday, October 13, 2017
3:30-4:30 PM
1528 C.C. Little Building Map
Our experience with earthquakes is that they are violent events that take a heavy toll on our societies through life and property losses. However, earthquakes present us with some of the most challenging questions in mechanics. By better understanding the nucleation and propagation dynamics of earthquakes, we may make progress towards minimizing their negative impact. Insights from mechanics may help in the development of better seismic hazard models as well as in the construction of more efficient earthquake early warning systems. In this presentation, I will give a brief overview of the multiscale nature of the earthquake mechanics problem and discuss some recent research efforts in my group to establish dynamic rupture models with high resolution fault zone physics,
As a starting point I will review evidence for fault zone complexity at different scales. I will introduce a thermodynamically consistent viscoplasticity theory, based on the shear transformation zone approach, that enables prediction of fault gouge rheology under a wide range of pressure and slip rate. By implementing this theory in a continuum mechanics framework, it is possible to model and resolve complex localization patterns observed in sheared fault zones as well as emergence of stick slip instabilities due to transitions in rate sensitivity. I will further show predictions of the theory for response of gouge to acoustic vibrations and implications for seismic triggering as well as slow slip generation.
Next, I will show that anisotropic damage features and material heterogeneities in fault zones, including small scale branches, fault-parallel joints, and soft inclusions, may play a significant role in modulating rupture dynamics which may be missed if standard plasticity models or bulk homogenization techniques are implemented. I will give two examples. First, I will show that a fault parallel soft inclusion may trigger supershear rupture transition under circumstances not possible in homogeneous materials. Second, I will show that small scale fault branches slow down rupture on main fault, reduce peak slip rate and lead to emergence of complex wave field in the bulk and enhancement of high frequency radiation due to destructive and constructive interferences.
I will close by describing some numerical challenges in modeling these complex systems and our progress in addressing them. I will briefly introduce a new hybrid numerical scheme that combines finite difference and spectral boundary integral methods for exact near field truncation of the wave field and efficient scale domain decomposition. By integrating the different mechanistic features of the problem, from multiphysics modeling of fault zone to multiscale representation of geometric and material complexities, we hope to establish a unique approach to the earthquake problem that will provide new opportunities in interpreting seismic observations and creating more accurate seismic hazard models.
Building: C.C. Little Building
Event Type: Lecture / Discussion
Tags: Lecture
Source: Happening @ Michigan from Earth and Environmental Sciences