Dynamically Self-Consistent Contstraints on the Long-Term Strength of Faults in Western North America

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This work is advancing the understanding of the long-term strength of faults, lower crust, and upper mantle within the plate boundary zone of Western North America. Constraints on the long-term frictional resistance to steady sliding on active faults holds information on processes that affect earthquake rupture and the seismic cycle in general, such as dynamic weakening mechanisms, the role of pore pressure, and the role of intrinsic weakness, such as fault gauge. Observationally constrained kinematic and dynamic models of western North America are being refined using the Quaternary Fault database and GPS velocities as a constraint. The long-wavelength components of the GPS data are being used to help constrain the long-term deformation field, while the transient signal is filtered out. Moreover, the no-length-change directions in the Quaternary Fault database, along with fault style and slip vector directions, are being used to help constrain the long-term deformation field for latest Quaternary in western North America. The long-term estimates of the directions of principal axes and relative magnitudes of principal axes (style of faulting) for the rates of strain provide a proxy for orientation and style of deviatoric stress field responsible for such deformation. Dynamic models are constrained by topography, gravity, seismically defined crustal thicknesses and densities, heat flow, as well as the observations important for global mantle circulation models, such as tomography and history of subduction. Given these dynamic models, significant advances are being made on the constraints of long-term frictional behavior of faults because (1) the absolute magnitudes of vertically integrated deviatoric stresses acting within the brittle crust and the lithosphere are quantified and (2) a general approach is applied that accounts for the dramatic change in fault mechanisms observed across the plate boundary zone. Furthermore, hypotheses are being tested using a general three-dimensional model that properly deals with lateral strength anisotropy, lateral variations in strength profile characteristics, and non-uniform layer thicknesses resulting from topography on the surface and the Moho. The grant will be used to support the Ph.D. research of Stony Brook graduate student Elliot Klein. This work is providing further support for the mentoring of REU undergraduates who participate in development of visualization tools for understanding the plate boundary zone in western North America. These visualization tools will be displayed on the interactive web site maintained at UNAVCO (http://jules.unavco.org/). Moreover, this research is further supporting activities to updateexisting models on the ILP's Global Strain Rate Map (GSRM) interactive web site, used for both teaching and research (www.world-strain-map.org). In addition, this work is providing infrastructural support for broadening and enriching the activities of a graduate student, Daniel Hernandez, from an underrepresented group. The proposed work is basic research. It willadvance fundamental knowledge about the long-term strength of faults, and it holds implications for fault mechanics and even the earthquake rupture energy budget and earthquake cycle. However, the contributions we make to the younger generation are of societal benefit in itself.