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Abstract Using numerical models, we compute the evolution of the mantle flow field and the crystal preferred orientation (CPO) of mineral aggregates in the mantle wedge of generic subduction systems from their nascent to mature stage and investigate shear wave splitting (SWS) through the forearc mantle wedge corner and overriding crust. Upon subduction initiation, the maximum depth of slab‐mantle decoupling (MDD) is relatively shallow (∼20 km depth), resulting in mantle flow and CPO development in the wedge corner. As subduction continues, the MDD deepens, the wedge corner cools and stagnates, and the olivine CPO becomes frozen‐in. In the cool wedge corner, antigorite can form if water is available. In non‐deforming mantle, antigorite CPO develops relative to the host olivine CPO through topotactic growth. We calculate splitting parameters of synthetic local S waves based on the model‐predicted A‐ and B‐type olivine CPOs and topotactically grown antigorite CPO that replaces A‐type olivine CPO in the wedge corner. The fast direction is trench‐normal for A‐type olivine and antigorite CPOs and trench‐parallel for B‐type. When the delay times are long enough (>0.1 s), we find them positively correlated with the thickness of the mantle wedge corner. In NE Japan, where the results of detailed analyses on the spatial variation of the SWS parameters are available, such correlation is not observationally reported. However, the addition of an anisotropic overriding crust provides delay times (∼0.1 s) and trench‐normal fast directions that are consistent with the local SWS observations. 

Abstract The long‐term state of stress in the subduction forearc depends on the balance between margin‐normal compression due to the plate‐coupling force and the margin‐normal tension due to the gravitational force on the margin topography. In most subduction margins, the outer forearc is largely in margin‐normal compression due to the dominance of the plate‐coupling force. The inner forearc's state of stress varies within and among subduction zones, but what gives rise to this variation is unclear. We examine the state of stress in the forearc region of nine subduction zones by inverting focal mechanism solutions for shallow forearc crustal earthquakes for five zones and inferring the previous inversion results for the other four. The results indicate that the inner forearc stress state is characterized by margin‐normal horizontal deviatoric tension in parts of Nankai, Hikurangi, and southern Mexico. The vertical and margin‐normal horizontal stresses are similar in magnitudes in northern Cascadia as previously reported and are in a neutral stress state. The inner forearc stress state in the rest of the study regions is characterized by margin‐normal horizontal deviatoric compression. Tension in the inner forearc tends to occur where plate coupling is shallow. A larger width of the forearc also promotes inner‐forearc tension. However, regional tectonics may overshadow or accentuate the background stress state in the inner forearc, such as in Hikurangi.