Dynamic fragmentation and inelastic energy partitioning at the base of the seismogenic zone

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Strike-slip faults like the San Andreas Fault in California represent major threats to life and property owing to the repeated generation of large earthquakes. Less than 20% of the total energy released during an earthquake radiates away from the source as seismic waves that cause ground shaking. Thus, a large portion of the energy is dissipated around the fault as frictional heat and by a variety of processes that lead to rock damage. During each earthquake, the rocks surrounding the ruptured segment of the fault are fragmented by cracking of individual mineral grains. Over many earthquake cycles, these rocks become so damaged that their elastic properties change, and this causes changes in the speed and preferred propagation direction of seismic waves. These so-called damage zones are therefore of great importance for understanding seismic hazards. Despite their importance, there is almost no direct information on the vertical extent of damage zones within active faults like the San Andreas, but this information is critical for our understanding of the energy budget through the entire seismogenic zone. The combination of modeling and field-based results from this work will allow improved characterization of seismically active faults in the deeper reaches of the seismogenic zone, including conceptual and wave speed models that can be used to better predict ground shaking directions and intensities. The open-source codes and analytical protocols developed in this project will be made available through existing public portals, and will have many community applications including the relationships between microcrack orientation and state of stress, interpretation of seismological data, and analysis of brittle damage and fragmentation of ceramics and advanced composite materials. Additional desired societal outcomes include full participation of women in STEM and development of a globally competitive STEM workforce through graduate and undergraduate student training.The objectives of this project are to: (1) comprehensively characterize the microfracture density and fragment size distributions for garnet and feldspar grains along three transects across a seismogenic fault/shear zone exhumed from near the base of the seismogenic zone (the Norumbega fault system, Maine), (2) apply physics-based fragmentation modeling to estimate fracture energies associated with the garnet fragmentation, (3) determine whether fragment size distribution analysis can be used to indicate that an exhumed fault or shear zone had a seismogenic history, (4) pursue the implications of the results for inelastic energy partitioning in the seismic source, (5) document asymmetrical damage around the shear zone core and quantitatively characterize the elastic and seismic properties of the rocks to characterize the bi-material effect, and (6) develop and disseminate protocols for using electron backscatter diffraction techniques to analyze mineral and rock fragmentation. Addressing the above objectives provides an opportunity to test hypotheses related to the source energy budget, styles of rupture propagation, and the relations between loading conditions and resulting rock microstructures in the deeper seismogenic zone. The research team will also assess whether intense fragmentation of brittle minerals can be used as a seismogenic signature at depth, as it is at the surface. Recognition of the importance of microfracturing for macroscopic behavior and the ability to treat it quantitatively is growing in both Earth sciences and materials engineering. The results of this project will provide a framework for future efforts in both fields, and for collaborations between them.