Major strike-slip faults, commonly found on the margin of overriding plates in obliquely convergent subduction zones, greatly influence the tectonics of these margins and represent sources of seismic hazard of historic proportions. The strike-slip faults, which are the locus of the arc parallel component of oblique convergence, are generally assumed to occur only if shear stresses in the overriding plate are large enough to activate strike-slip faulting. The magnitude of shear stress in the overriding plate is controlled in part by the plate convergence obliquity, but also by the degree of coupling of the two converging plates along the plate interface and the strength of the overriding lithosphere, as well as other factors, such as basal tractions and the slab-pull force. The role of these factors is not well understood, nor is the threshold of stress that must be achieved before partitioning will be initiated. Understanding the mechanics of strain partitioning has important implications for seismic hazard as it is often thought to be associated with increased frictional coupling between the converging plates, and therefore large earthquakes at the plate interface. Strain partitioning also leads to large earthquakes on strike-slip faults in the overriding plate, that often have large neighboring populations. This project is addressing the causes and consequences of strain partitioning at oblique subduction margins by combining Global Positioning System (GPS) measurements in the northeastern Caribbean with deformation models. The northeastern Caribbean is well suited to such a study. The region consists of neighboring zones of frontal subduction (Lesser Antilles), oblique subduction with no strain partitioning (Puerto Rico), and oblique subduction with strain partitioning (Hispaniola). The investigators are acquiring new GPS data in the Dominican Republic and Haiti and the generating a geodetically consistent velocity field for the entire northeastern Caribbean. The GPS velocities provide constraints for kinematic and dynamic deformation models aimed at understanding (1) how stress is imparted on an overriding plate in an oblique subduction setting and how lateral variations along the margin can trigger strain partitioning and forearc deformation, and (2) how strike-slip fault earthquakes in the forearc interact with the plate interface earthquakes through co- and postseismic stress transfer. This study is relevant to other areas sharing tectonic similarities with the northeastern Caribbean, such as the Mediterranean (e.g., southern Italy, western Greece), the western Pacific (e.g., Philippines, Indonesia, New Guinea), and the south Atlantic (Scotia arc). Historical records from the Dominica Republic reveal several magnitude 7 or greater earthquakes in the past 500 years. This project is providing basic geophysical information needed for seismic hazard analysis in the northeastern Caribbean.
