I utilized inverse and forward dynamic modeling methods to investigate the forces responsible for driving the long-term deformation of the seismogenic crust of western North America. I have quantified depth integrated deviatoric stresses arising from internal buoyancy forces and the accommodation of relative plate motions for the upper crust of the diffuse plate boundary zone. The deviatoric stress field generated with the inverse method delineates nearly equal contributions from these driving forces. The deviatoric stress field generated with the forward dynamic method identifies the need for internal buoyancy forces that dominate over plate boundary forces for upper crust within regions east of the San Andreas system. For the inverse models, I quantified depth-integrated deviatoric stresses associated with differences in gravitational potential energy per unit area (GPE) for the upper crust using densities defined by seismic velocity data. I then accounted for sources of stress outside the model space by solving for a deviatoric stress field boundary condition that provides a best-fit to the tensor styles of the principal axes of the kinematic strain rates. The magnitudes of total deviatoric stress in the long-term seismogenic crust, from the surface to 20 km below the sea level, range between 0.05–0.75 × 1012 N ˙ m–1. The depth integrated total stress differences are used as a proxy for depth integrals of fault strength in moderate-to-high strain rate regions. I calculated low long-term fault friction coefficients (μ = 0.02–0.20), under hydrostatic pore pressure conditions, associated with these deforming regions. I assimilated a large set of highly detailed Quaternary fault observations into an existing data set in order to generate an updated long-term kinematic strain rate tensor field and model velocity field for western North America. I constructed forward dynamic models of the upper crust where the body force distributions, inferred lateral variations in effective viscosity, and the known far-field velocity boundary conditions are defined. The depth-integrated viscosities are proportional to the assumed long-term friction on faults and inversely proportional to the long-term strain rates. The velocity boundary conditions are defined using plate motion estimates. Self-consistent dynamic strain rate tensor solutions to the force-balance equations were solved and tested for best-fit match with the updated long-term kinematic strain rate and velocity fields of western North America. Models constructed with low fault friction coefficients (μ < 0.20) achieve a better fit to Quaternary fault observations than models with intermediate or high fault friction coefficients. I also delineate block model geometries for the crust of western North America. These block model geometries consist of weak shear zones and strong block-like interiors. The dynamic strain rate tensor styles associated with the forward dynamic models possessing block geometries yielded a poor match to the updated long-term kinematic strain rate and velocity fields of western North America. The scoring of self-consistent dynamic model output with detailed kinematic output show that models with a distributed fault fabric, defined with low uniform fault friction, provide a far better match to patterns of finite strain observed within the diffuse plate boundary zone of western North America than is achieved with the block model geometries explored to date.
|Advisor:||Holt, William E.|
|School:||State University of New York at Stony Brook|
|School Location:||United States -- New York|
|Source:||DAI-B 71/05, Dissertation Abstracts International|
|Keywords:||Deviatoric stress field, Gravitational potential energy, Plate boundary zone, Seismogenic layer|
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