The internal structure of fault zones in the upper continental crust exhibits considerable complexity. Mature faults consist of several basic structural elements including:

(i) A zone of concentrated shear, the fault core, which is often defined by the presence of extremely comminuted gouge; (ii) A damage zone, with the primary fault core centralized in or bordering that damage zone, in addition to a segmented network of several secondary cores within the damage zone. Damage zones display a greater intensity of deformation relative to the surrounding host rock, and contain features such as secondary faults and fractures, microfractures, folded strata, and comminuted grains; and (iii) host country rock with little or no damage. In general, the intensity of damage increases towards the fault core and the transition from undeformed host rock to damage zone rock is often gradual [8]–[10]. Overall, fault zones exhibit a combination of distributed damage as well as discrete anisotropic secondary fractures of different orientations and density [11] leading to coupling of volumetric and deviatoric deformations with important implications for source physics and seismic hazard.

Mechanisms of volumetric and shear processes in complex fault zones: A variety of mechanisms may contribute to the complexity of deformation in active fault zones as shown in the compilation image at top. These include: (i) co-seismic generation of secondary cracks and branches, (ii) topological interlocking, (iii) dilatation and compaction in sheared fault gouge with and without acoustic vibrations, (iv) fluid injection, (v) bi-material effects which induces a coupling between slip and normal stresses, (vi) quasi-brittle damage and distributed inelasticity associated with intense stress concentrations carried by the propagating rupture tip and accompanied with dynamically evolving bulk elastic moduli, and (vii) wave trapping in fault zones and wave reflection from the free surface.

Implications of Fault zone complexity: Fault zone damage and volumetric changes may have important implications on source physics as well as our interpretation of seismic observations. Evolution of fault strength, energy partitioning, high frequency generation, interpretation of moment tensors, and rupture directivity, are all influenced by the topology, geometry, and material heterogeneities within the fault zone. Next generation earthquake studies should thus consider the co-evolution of fault structure, bulk rheology, and stress heterogeneities at different spatio-temporal scales. 

Challenges and Opportunities: Understanding the physics of earthquake source processes has come a long way in the last 50 years. Progress towards incorporation of more realistic fault zone features will facilitate identification of dominant physical processes as well as development of predictive models for seismicity and next generation physics-based seismic hazard models. While challenges exist in resolving the range of scales and physics involved in this problem, there are multiple opportunities for breakthrough through leveraging advances in computations, observations, and experiments. Modeling sequence of earthquakes and aseismic slip with high resolution fault zone physics, using high performance computing, machine learning, and cutting-edge computational physics techniques, will transform our understanding of the earthquake processes across scales and will open new opportunities in predictive earthquake science and operational forecasting. Dense instrumentation of fault networks in the near field, using carefully design sensor network layouts, will enable picking up signals associated with small scale physics including constraining break down distances, rupture mode, and dynamic dilatation and compaction, and will shed new lights on connections between surface observations and processes at depth. Geological observations characterizing the hierarchical structures within fault zones and how they may have evolved over time through paleo-seismic studies will play a critical role in informing our understanding of the non-equilibrium nature of fault zones. Finally, analog experiments [12]–[14] with precisely engineered damage can shed light into the role of specific fault zone processes. Densely instrumented experiments [15] that enable imaging both volumetric and shear processes together with precursors, localization, and delocalization of deformation will provide a wealth of data that can be processed and carefully scaled to the field. Progress in big block experiments [16], [17] will enable imaging rupture nucleation, propagation, and arrest together with gouge generation and bulk damage. Through the fusion of these interdisciplinary efforts, we are embarking on a new era of discovery of our active Earth. The future of earthquake science is bright.

About the Authors

	Ahmed Elbanna is an associate professor of Civil and Environmental engineering at the University of Illinois Urbana Champaign , where he leads the mechanics of complex system lab. His research focuses on modeling friction, fracture, and wave propagation as they arise in problems in geophysics, biology, and engineering. He is a co-leader of the computational science group in the SCEC science planning committee and a member of SCEC committee on professional conduct.
	Mohamed Abdelmeguid is a graduate research assistant at University of Illinois at Urbana Champaign, working on developing physics-based computational frameworks to study earthquake mechanics. His research focuses on studying the implications of fault zone complexity on sequences of earthquakes and aseismic slip.
	Md Shumon Mia is a graduate research assistant at University of Illinois at Urbana-Champaign. He is working on modeling sequence of induced earthquakes and aseismic slip as well as the role of bulk inelasticity on fracture and frictional sliding.

Acknowledgements

This research was supported by the Southern California Earthquake Center, the National Science Foundation CAREER award, and the National Energy Technological Laboratory. SCEC is funded by NSF Cooperative Agreement EAR-1600087 and USGS Cooperative Agreement G17AC00047.

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