This project is investigating the active deformation of the western United States by representing the region as a series of elastic, rotating, interacting blocks. The goal is to see if such a block representation satisfies the constraints as well as or better than continuum, viscous-flow models. Understanding the relative importance of the two possible representations is vital in order to understand what drives the deformation. This work is designed to lead to better understanding of the tectonics of the western US, how continents deform in general, and earthquake potential in this region. A regional, surface velocity field is being developed using Global Positioning System (GPS) measurements and applying an inversion technique that simultaneously estimates block motions and elastic strain accumulation near block-bounding faults. The concept of describing the regional tectonics by a finite number of rotating blocks on a sphere is similar to plate tectonics and is in contrast to continuum models of continental deformation. The study region contains the important transition from Pacific-North America shear to Juan de Fuca - North America convergence. This work will elucidate how this transition is manifested in deformation within the continent. While block models have been proposed and evaluated before, this approach fundamentally differs in that it explicitly models the short-term elastic strains. Hence it is able to incorporate a geodetic data set that includes tilt rates and strain rates in addition to the primary constraints on the regional deformation from GPS. Furthermore, it can use geologic data such as fault slip directions and rates, spreading rates, rotation rates, and fault orientations. Hence, geodetic data from the short-term that are subject to elastic strains are combined with longer-term geologic data that reflect permanent deformation - the combination will improve our understanding of the active tectonics of the western United States.
