Origin and Vertical Extent of Damage Zones Around Continental Strike-slip Faults

University Of Maine, Orono ME

Investigators

Abstract

The objectives of this project are to: (1) scale-up a preliminary micromechanical model for grain-scale coseismic damage spanning the seismogenic zone that integrates analytical (electron backscatter diffraction and cathodoluminescence) and computational (asymptotic expansion homogenization) approaches and fully account for the grain-scale elastic interactions among the different minerals in the sample; (2) apply this integrated framework to quantify the effects of damage and other fault-zone microstructure on seismic anisotropy; (3) determine the spatial and temporal pattern of damage and related seismogenic structure around an ancient strike-slip fault (the Norumbega fault in Maine) exhumed from a depth of approximately 10-15 km; (4) develop a conceptual model linking this pattern to the damage structure of active faults observed at the surface; and (5) disseminate open-source software for damage analysis and calculation of bulk elastic stiffnesses and seismic properties of damaged rocks. This software will take input either from electron backscatter diffraction data files of natural microstructures, or synthetic (computer generated) microstructures that can be used in sensitivity analyses among other applications. 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. 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 they strongly affect the elastic properties of the rocks, and therefore the speed and preferred propagation direction of seismic waves. Although these so-called damage zones are of great importance for seismic hazards, a robust mechanical model for how they form is lacking, and there is almost no information on the vertical extent of this damage within active fault zones. Furthermore, there is currently no computational framework available for calculating how progressive damage evolution might affect seismic wave propagation. The combination of modeling and field-based results from this work will allow improved characterization of seismically active areas, including conceptual and wave speed models that can be used to better predict ground shaking directions and intensities. The open-source codes 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. 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. The project will contribute to the development of a diverse, globally competitive STEM workforce through training of graduate and undergraduate students and in participation of women in STEM.