HR: 14:40h
AN: T13G-05 INVITED    [Abstracts]
TI: Role of Plate Coupling and Mantle Wedge Flow in Affecting Stresses in Subduction Zone Upper Plate
AU: * Wang, K
EM: kwang@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AU: * Wang, K
EM: kwang@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, Canada
AU: Wada, I
EM: ikukow@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, Canada
AB: We have studied the state of stress in forearc - back arc systems of many subduction zones by examining and, where possible, inverting focal mechanisms of upper plate earthquakes. The study led to two general observations. (1) The margin-normal compressive stress in the forearc region of most subduction zones, excluding the frontal sedimentary prism, is no greater than lithostatic. (2) Wherever data are available, the stress states of the forearc and back arc regions are similar. Because the magnitude of the margin-normal stress as compared to lithostatic stress is controlled by the gravitational force and frictional coupling of the converging plates, the first observation indicates that the shear stress along the plate interface is generally very low. The strength of the subduction fault can be represented by an effective coefficient of friction μ'. Using a model of two converging elastic plates in frictional contact, we estimate the value of μ' to be of the order of 0.03 - 0.05. These μ' values give an average shear stress of about 10-20 MPa along the seismogenic part of the subduction fault. The observed stress drop of a few MPa in subduction earthquakes is therefore a significant fraction of the absolute stress along the fault. However, although subduction faults appear to be weak in general, a large coupling area or unusual roughness of the surface of the subducting plate may induce large compressive stresses in the upper plate. The second observation, that is, the lack of a large stress gradient across the forearc - back arc system, indicates that the basal drag force due to mantle wedge flow is very small. Using a model of slab-driven viscous mantle wedge flow with dislocation-creep olivine rheology, we find that the shear stress in most of the flowing mantle wedge is less than 2 MPa, mainly because of the high temperature. The mantle wedge is thus incapable of exerting significant drag on the base of the overriding lithosphere. If mantle drag is insignificant, the coupling force along the subduction interface must be balanced by far flied forces, most likely at other boundaries of the plate.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8164 Stresses: crust and lithosphere
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting