While convergence of plates at a subduction zone produces shortening at the toe of the overriding plate, this deformation is localized to the frontal 100 km or so of the plate. There is increasing evidence that the state of strain in back-arcs, volcanic arcs, and possibly the entire forearc is extensional. The most likely cause of this extension is rollback of the subducting plate and trenchward advance of the overriding plate. Arcs are thermally weakened, and are activated repeatedly through time. The primary expression of the extensional state of strain is spreading of the weak arc material, either by splitting the arc or by back-arc spreading behind it. This study presents evidence for extension of volcanic arcs from an analysis of the moment tensors of shallow earthquakes (< 125 km) associated with active arcs._x000D_
Moment tensors are related to seismic strain tensors and thus are useful in tectonic problems. The primary conclusion is that most arcs are in a state of subhorizontal extension oriented normal to the volcanic line. Volcanic arcs are "decoupled" from convergence-related compression at the toe of the overriding plate. This state of extension evidently persist through the entire lithosphere to depths of 125 km. This suggests mantle drag forces are important in driving arc extension.