Finite element modeling of deformations of the lithosphere at an arc-arc junction: The Hokkaido corner, Japan.

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Abstract
The stress-field near an arc-arc junction shows an anomalous state against the expectation from the relative motion of converging plates. The Hokkaido corner is a typical example of such anomalous regions, where the compressional axes (P-axes) derived from focal mechanism of earthquakes in the continental lithosphere lie parallel to the strike of trench axes. In order to explain this anomalous state of stress, a three-dimensional finite element method is employed to model the configuration of the Pacific plate subducting beneath the Hokkaido corner. Several types of plate driving forces are taken into consideration: (1) a negative buoyancy due to the density contrast between the subducted lithosphere and the surrounding asthenosphere, (2) a negative buoyancy which might act on the lid of the Sea of Japan to simulate a presumed subduction of the "Japan Sea plate, " (3) a northwestward compressive force generated by the movement of the Pacific plate, (4) a northwestward compressive force due to the flow in the asthenosphere, (5) an eastward compressive force expected from the movement of Eurasia or the assumed "Japan Sea plate, " (6) a drag force exerted at the edge of the subducted slab in the model space, (7) thermal stresses arising in the subducted slab which are caused by the contact with a hot asthenosphere. We calculate the displacements and stresses in the crust and upper mantle around this region, due to these forces under the assumption of viscoelastic rheology, compare them with the observations, and evaluate their relative importance. It is concluded that a negative buoyancy exerted on the descending slab could yield the observed anomalous stress field and that all of the compressive forces under consideration might not play an important role. However, downwarping of crustal vertical movements is predominant over the whole region under consideration in the case of a negative buoyancy acting on the subducted Pacific slab. Other sources such as thermal stresses might be required, which yield upward motions and preserve the trend of P-axes. An anomalous wedge zone which might exist above the subducted slab on the landward side of the junction would have some effects on the generation of the complex stress field.

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