Inner Forearc Stress State and Plate Coupling

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. Plain Language Summary The state of stress in the overriding plate between the trench and the volcanic arc of subduction zones depends on frictional coupling between the overriding and subducting plates and gravitational force, which causes lateral compression and tension, respectively. The trench‐ward portion of this so‐called “forearc” region is generally in compression due to the dominant effect of plate coupling, but for the arc‐ward portion, the relative importance of the two forces varies spatially. We constrain the state of stress in the forearc using earthquake data for five subduction zones and inferring results from previous studies for four other subduction zones. The results indicate that the forearc stress state seems to correlate with the downdip depth of plate coupling and the width of the forearc. A relatively shallow downdip extent of coupling in a wide forearc tends to have the arc‐ward portion in tension or neutral stress state as observed in parts of Nankai, Hikurangi, Mexico, and Cascadia althoug