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Strike-slip fault structure and fault-system evolution : a numerical study applying damage rheology

University of British Columbia

In seismically active regions, faults nucleate, propagate, and form networks that evolve over time. Progressive strain localization and periodic fault pattern re-configuration induce the accumulation and healing of fault zone damage. The damage zones are characterized by distributed fractures, veins, and secondary faults, and may act as barriers for propagating earthquake ruptures, or as nucleation sites for earthquakes. They interact with seismic waves, promoting strong surface motions during earthquakes, and can focus fluid flow and enhance mineralization. In spite of their great scientific, social, and economic significance, interactions between these evolving damage zones and crustal deformation remain unresolved. Indeed, geodynamic models generally treat active faults as surfaces embedded in a medium with non-evolving material properties. For my dissertation projects, I have simulated fault system evolution over thousands of years, applying a rheological model which incorporates concepts of damage mechanics. This model accounts for crack nucleation, growth and concentration (i.e., material degradation), macroscopic failure, and material healing. My Simulations show that strike-slip faults form as segmented structures before evolving into contiguous, simpler structures. Flower structures rapidly form along fault segments (before a total offset of 0.05 km), and stepovers display extensive, permanent damage and ongoing seismicity throughout the seismogenic crust. My models also indicate that the “effectiveness” of material healing strongly affects the spatial extent of damage zones and long-term fault complexity. Effective healing promotes rapid evolution of segmented faults to a simpler through-going fault, and ineffective healing preserves fault complexities, resulting in long-lasting, distributed deformation. I also find that lateral contrasts in lithosphere viscosity structure (or effective plate thickness) attract evolving faults and cause damage and strain to concentrate on the “weaker” side. Realistic contrasts in crustal rigidity, however, have only a minor effect on the symmetry of damage, deformation, or the propagation of faults. In addition, lower crust and mantle viscosity contrasts induce the formation of broad shear zones with relatively high strain-rate in the “weaker” side of the interface. These results demonstrate that reasonable, lateral contrasts in viscosity (rather than extreme, unrealistic contrasts in elasticity) can explain GPS observations of highly asymmetric, interseismic deformation around strike-slip faults.

Text

2010-01-29

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/19401

Geophysics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/