The effect of along-strike variable plate deflection on bending stress and seismicity at the southern Mariana Trench

Bending-induced faults are often recognized as sources of extensional or compressional earthquakes in the outer rise region, and have been extensively investigated in 2-D plate flexural models. However, such 2D models have difficulty in explaining observed earthquakes caused by reactivation of pre-exsiting fabrics inherited from mid -ridge, especially when the fabrics is oblique to the subduction. Here, we develop a 3-D flexural model to investigate the plate deflection at the southernmost Mariana subduction zone. The Particle Swarm Optimization (PSO) method was used to invert flexural parameters of the subducting plate. It shows that the boundary loading near the Challenger Deep is nearly twice that in the region further east. Under this deformation, the bending stresses within the outer rise vary significantly along the strike of the subduction zone. We find that the location of abrupt stress changes coincides with the outer rise earthquakes which were considered to be reactivation of remnant faults. We proposed a model to illustrate the influence of along-strike variation of plate deflection on bending stress and outer rise earthquakes: The variation of plate deflection along the trench may cause the change of the direction of the maximum extensional bending stress in the variable zone which provides greater possibilities for reactivation of preexisting weak faults, resulting in unevenly distributed outer rise earthquakes, reactivation of remnant fabric or earthquake.