Geodynamic Modeling of Long-term Fault Interactions During the Evolution of the Pacific-North American Plate Boundary Zone

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The San Andreas Fault is the tectonic boundary between the Pacific and the North American plates, yet only a portion of the relative plate motion is absorbed by slip on the San Andreas Fault proper. The rest, up to 25 per cent, is accommodated by a complex system of faults over a broad plate boundary zone, by the Eastern California shear zone. Such diffuse deformation in the plate boundary zone is responsible for the widely scattered earthquakes in California and Nevada, but it is not clear how the Eastern California shear zone and other faults in the San Andreas Fault system formed, and what controls the strain partitioning among these faults. This research project will attempt to address the following questions: 1) what caused the inception and development of the Eastern California shear zone? 2) how has the development of the Eastern California shear zone, the Garlock Fault, and the San Jacinto Fault in the past few million years affected the crustal dynamics in the San Andreas Fault plate boundary zone? The study employs numerical modeling of long-term mechanical coupling and fault interactions in which the research team will develop a finite element model for three-dimensional, regional-scale lithospheric deformation. The viscoelastoplastic formulation in this model allows simulation of strain localization in the crust outside the faults, hence the inception of new faults.Plate boundaries are major fault zones, across which the relative plate motion produces concentrated crustal deformation, volcanoes, and earthquakes. However, the Pacific- North American plate boundary is a major exception. Here the plate boundary, the San Andreas fault absorbs only part of the relative plate motion and the rest is accommodated by slip on many faults in a complex system that extents into central Nevada. The proposed research seeks to understand why these fault initiated and what controls strain partitioning among these faults. The results of this study will help us to better understand the geodynamic implications of the observed crustal motion, hence provide insights into earthquakes in California and Nevada.