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Subduction zones are expressed topographically by long linear oceanic trenches flanked by a low outer rise on the seaward side and an island arc on the landward side. This topographic structure is reflected in free air gravity anomalies, suggesting that much of the topography originates from dynamical forces applied at the base of the crust. We have successfully reproduced the general topographic features of subduction zones by supposing that the stresses generated by the bending of the viscous lower lithosphere as it subducts are transmitted through the thin elastic upper portion of the lithosphere. The trench is due to a zone of extensional flow (associated with low pressure) in the upper part of the viscous lithosphere.The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90") were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 x lo2* P. This low value is probably due to the stress dependence of the effective viscosity. However, these models also show that the topography of the subduction zone depends primarily upon the geometry of the subducting slab (dip, radius of curvature of the bend) rather than upon' its rheology. Shear stresses beneath the trench reach maxima of approximately 50 MPa. An interesting feature of some solutions is a dynamically supported bench or platform between the trench and island arc.

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Numerical models of the strike‐slip earthquake cycle, assuming a viscoelastic asthenosphere coupling model, are examined. The time‐dependent simulations incorporate a stress‐driven fault, which leads to tectonic stress fields and earthquake recurrence histories that are mutually consistent. Single‐fault simulations with constant far‐field plate motion lead to a nearly periodic earthquake cycle and a distinctive spatial distribution of crustal shear stress. The predicted stress distribution includes a local minimum in stress at depths less than typical seismogenic depths. The width of this stress “trough” depends on the magnitude of crustal stress relative to asthenospheric drag stresses. The models further predict a local near‐fault stress maximum at greater depths, sustained by the cyclic transfer of strain from the elastic crust to the ductile asthenosphere. Models incorporating both low‐stress and high‐stress fault strength assumptions are examined, under Newtonian and non‐Newtonian rheology assumptions. Model results suggest a preference for low‐stress (a shear stress level of ∼;10 MPa) fault models, in agreement with previous estimates based on heat flow measurements and other stress indicators.

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