Tonga Slab Morphology and Stress Variations Controlled by a Relic Slab: Implications for Deep Earthquakes in the Tonga‐Fiji Region

Abstract

Deep focus earthquakes within the Tonga‐Fiji subduction system account for about two‐thirds of the global total and provide significant constraints on slab deformation. The factors controlling the intense deformation remain unclear. Here, we use two‐dimensional, time‐dependent geodynamic models to study the morphology, stress state, and thermal structure of the Tonga‐Fiji subduction zone. The results, consistent with tomographic images and focal mechanisms, demonstrate that collision between a relic slab from the Vanuatu Trench and the Tonga slab may control the steeper dip of the Tonga slab and earthquakes in the mantle transition zone. We suggest that the magnitude 8.2 and 7.9 earthquakes in 2018 mostly ruptured within the warm rim of the Tonga slab and occurred beneath the folding relic slab with high temperatures of at least ∼900°C and ∼1100°C, respectively. The findings support the hypothesis that local slab temperature likely controls rupture of deep earthquakes.