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[SCG50-11] Factors controlling the stress state in the subducting slabs
Keywords:subduction, stress in slab, intermediate-depth earthquake
Many studies have highlighted a marked asymmetry in subduction zones: W-directed subduction zones are characterized by steep slab dip, occurrence of back-arc basins, low topographic elevation, extensional stress of the arc-back arc area, and down-dip compression stress state of slabs, whereas E- or NE-directed subduction zones are characterized by shallow slab dip, lack of back-arc basins, high topographic elevation, compressional stress of the arc-back arc area, and down-dip tensional stress state of slabs with a seismic gap at 300–550 km depth. The cause of this asymmetry of subduction zones has generally been discussed in relation to a postulated westward drift of the lithosphere or an eastward sub-lithospheric mantle flow. Numerical modelling of the subduction zones has revealed that models with the free-moving overriding plate result in trench retreat, shallow slab dip and compressive stress state of the overriding plate. In contrast, models with the overriding plate fixed to the model’s side wall result in steep slab dip and extensional stress state of the overriding plate. Therefore, the condition for movement of the overriding plate can explain many of the asymmetric features of the subduction zones. However, there are few studies on the relation between the stress state in the subducting slabs and geodynamic condition of the subduction zones.
In this study we investigate the relations between the stress state of the subducting slab and the geodynamic conditions, such as the condition for movement of the overriding plate and thickness and viscosity of the subducting slab, by two-dimensional numerical modelling. The simulation results indicate that the geometry of subducting slab is largely controlled by the condition for movement of the overriding plate. Especially, models with the free-moving overriding plate result in flat-slab subduction after deep slab penetration into the lower mantle regardless of thickness and viscosity of the subducting slab. The stress state of the subducting slab is considered to be combination of two components associated with bending-unbending and uniform stretching-shortening and is largely controlled by thickness and viscosity of the subducting slab. We compare the simulation results with stress state estimated from focal mechanism of the intermediate-depth earthquake and discuss the implications for subduction dynamics.
In this study we investigate the relations between the stress state of the subducting slab and the geodynamic conditions, such as the condition for movement of the overriding plate and thickness and viscosity of the subducting slab, by two-dimensional numerical modelling. The simulation results indicate that the geometry of subducting slab is largely controlled by the condition for movement of the overriding plate. Especially, models with the free-moving overriding plate result in flat-slab subduction after deep slab penetration into the lower mantle regardless of thickness and viscosity of the subducting slab. The stress state of the subducting slab is considered to be combination of two components associated with bending-unbending and uniform stretching-shortening and is largely controlled by thickness and viscosity of the subducting slab. We compare the simulation results with stress state estimated from focal mechanism of the intermediate-depth earthquake and discuss the implications for subduction dynamics.