Stress Variations in Southern Tonga Slab Derived From Deep‐Focus Earthquakes.

Tonga is a convergent plate boundary between the Pacific and Australian plates and is the fastest and the most seismically active deep subduction system in the world. We focused on southern Tonga (south of latitude 22°S) and the mantle transition zone (depths of 410–670 km), where seismic activity forms two subparallel bands of events in the east and west. We performed stress analysis by inverting focal mechanisms of earthquakes available in the Global Centroid Moment Tensor catalog and revealed two distinct stress regimes in the slab. While the stress orientation in the eastern slab segment conforms to the down‐dip compressional stress along the entire slab, the stress orientation in the western slab segment is different, having the maximum compression in the vertical direction. This suggests that the western segment can represent a stagnant slab with flattening and bending, as proposed by modeling studies. Its connection with the younger actively subducting slab is supported by the horizontal westward shift at 520 km depth. The stress analysis also indicates substantially different fault orientations in both segments. In the actively dipping slab, the majority of activated faults are predominantly sub‐horizontal. However, they are significantly inclined from vertical in the stagnant slab segment. A higher scatter in fault orientations in the stagnant slab suggests deformation, fragmentation and rheological complexity resulting from bending and flattening. Plain Language Summary: Tonga is a part of Tonga‐Kermadec, the 2,550‐km long subduction system in SW Pacific. It represents a convergent plate boundary between the Pacific and Australian plates. It is the fastest converging and the most seismically active deep subduction in the world. In the mantle transition zone, especially at depths greater than 500 km, the geometry of the slab becomes complex, forming separated slab segments. We performed stress analysis in the mantle transition zone (depths 410–670 km) with a focus on southern Tonga (south of latitude 22°S) where seismicity forms two subparallel bands of events in the east and west. We inverted publicly available data, focal mechanisms of earthquakes from the Global Centroid Moment Tensor catalog, and revealed two distinct stress regimes. While the stress orientation in the eastern segment matches the down‐dip compression of the subducting slab, the stress orientation of the western segment is different with the maximum compression in the vertical direction. This suggests that the western segment represents a stagnant slab with flattening and bending. Such findings are supported by the horizontal westward shift at 520 km depth and by substantially different fault orientations in both segments. Key Points: We performed stress analysis in southern Tonga mantle transition zone from focal mechanisms of earthquakes listed in Global Centroid Moment Tensor catalogWe revealed two distinct stress regimes: down‐dip compression matching subducting slab in the east and vertical compression in the westWestern segment suggests a stagnant slab with flattening and bending supported by horizontal shift and different fault orientations [ABSTRACT FROM AUTHOR]