Present‐Day Tectonics in Four Active Island Arcs Based on Gps Observations and Forearc Stress Fields

The present-day tectonics of active island arcs is discussed based on the velocity fields derived from Global Positioning System (GPS) observations and the state of stress derived from focal mechanism solutions. The Tonga, Mariana, Ryukyu, and Hellenic arcs are discussed. Extensional forearc deformation and forearc stress are found in most of these arcs, except for Tonga, where no arc-parallel extension was observed. This deformation may represent arc stretching related to arc evolution over geologic time. Strain rates within the forearcs are on the order of 10 nanostrain/ yr and are fairly uniform among the different arcs, especially in their upper bounds. This strain rate being generally higher than those for oceanic crust suggests that the strength of forearcs may be controlled as continental structure. The observed forearc stresses where an arc-parallel extensional component exists are important to understand the dynamics of backarc opening because they have implications for the mechanical interaction of the backarc and the arc. Seismological data suggest that interplate seismic coupling, which is an important parameter influencing arc-parallel extension due to oblique subduction, is generally weak in subduction zones accompanied by active backarc opening.