EFFECT OF PLATE COUPLING AND GRAVITATIONAL FORCE ON FOREARC STRESSES

The state of stress in the subduction forearc depends largely on the balance between plate coupling force and gravitational force on the margin topography, respectively. The plate coupling force causes margin-normal compression in the forearcs whereas the margin topography due to gravitational force leads to margin-normal tension. In most subduction margins around the world, the outer forearc stresses are assumed to be dominated by the plate coupling force (i.e., in margin-normal compression) as indicated by the presence of folds and thrust faults in the region. However, in the inner forearc, due to presence of margin topography, the difference between the plate coupling compression and gravitational tension could be small. In this work, we invert focal mechanism solutions for shallow forearc crustal earthquakes in six subduction zones around the world to study the competing effects of the plate coupling and margin topography on forearc stresses. These subduction zones are northern Cascadia, Hikurangi, southern Mexico, Costa Rica, southern Peru, and Sumatra. In the stress analyses, we also include published results of focal mechanism inversion for Northeast and Southwest Japan, Northern Chile, and Hellenic. The results indicate that Nankai is in tension, in some parts of northern Cascadia, Hikurangi, southern Mexico are in tension, and the rest are in compression. In regions under tension, the effect of plate coupling force is likely small relative to that of the gravitational collapse force on the inner forearc stresses due to a weak subduction fault and/or a narrow coupling width.