Effects of Plate Coupling and Margin Topography on Forearc Stresses

The state of stress of forearc systems yields critical information on plate driving forces and the strength of subduction faults. In the outer forearc area within 100-150 km of trench, fold-and-thrust structure usually indicates horizontal, largely margin-normal, compression. However, in inner forearc regions, the state of stress is not well constrained. We infer inner forearc stresses at various subduction zones by examining earthquake focal mechanisms and fault slip data. Subduction zones examined include Northeast and Southwest Japan, Ryukyu, northern Cascadia, southern Mexico, Costa Rica, southern Peru, northern Chile, Hikurangi, and Hellenic. Stress inversion was carried out wherever appropriate. The results indicate that inner forearcs with low margin-normal compression outnumber those with high margin-normal compression. Most inner forearcs are under much less margin-normal compression than their outer forearcs, and many are under tension. Using a simple model of forearc force balance, we demonstrate that the landward stress decrease reflects an arcward decrease in the effect of plate coupling and an increase in the effect of margin topography. Plate coupling causes horizontal compression, but margin topography, with a tendency for gravitational collapse, induces horizontal tension. The abundance of low-stress inner forearcs indicates that the plate coupling force is generally too small to overcome the effect of margin topography. Finite element mechanical models show that the temporally and spatially averaged effective friction coefficient of subduction faults is usually no greater than 0.05.