Collaborative Research: An Integrated GPS and Structural Study of the Cocos-Caribbean Boundary in Nicaragua and El Salvador

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In this four year geodetic and structural study of the Cocos-Caribbean convergent margin along the Pacific coasts of El Salvador and Nicaragua, where a combination of oblique convergence, a prominent bend in the trench, and weak or no locking of the subduction interface, University of Wisconsin and University of Arkansas researchers are studying how a volcanic arc and forearc sliver deform in response to oblique convergence. Through Digital Elevation Model analysis, structural field work, GPS observations, and forward and inverse modeling using a three-dimensional, geometrically realistic subduction/forearc finite element mesh, the team is addressing the following important problems: (1) whether changes in convergence obliquity due to bends in trenches are efficiently transferred into changes in forearc motion or deformation and whether escape of a forearc sliver toward the trench (a free surface) is favored over crustal thickening at the leading edge of a forearc sliver; (2) why the bend between the two forearc slivers, where crustal thickening is expected, is instead occupied by a topographic depression (the Gulf of Fonseca); (3) whether the motions of the two forearcs are accommodated by arc-parallel strike-slip faulting, bookshelf faulting, or some combination of the two; (4) whether the east-west stretching characteristic of the crust inboard from the Salvadoran volcanic arc also extends into the forearc and if so, how this stretching interacts with and influences sliver transport. Finally, a series of modeling experiments are being conducted to better understand the kinematics and dynamics of the subduction/forearc wedge system, using a three-dimensional finite element model and a series of plausible boundary conditions, including dip-slip tractions at the base of the wedge, variable resistance to slip along the subduction interface and forearc faults, and varying degrees of buttressing at the leading edge of the wedge. Through this integrated approach to studying active deformation in western Central America with GPS and field-based structural work, the conceptual and kinematic framework for understanding the earthquake cycle in this region is being established. , Tens of thousands of deaths have occurred since 1900 during numerous moderate and large magnitude earthquakes in this region. Knowledge and technology transfer to El Salvadoran and Nicaraguan collaborator significantly benefit future earthquake hazard studies in both countries. Graduate students are being trained in the use of structural geology and high-precision geodesy for studying the earthquake cycle.