HR: 08:20h
AN: T51E-02    [Abstracts]
TI: Coseismic Strengthening of the Shallow Portion of the Subduction Fault and Effects on Frontal Prism Taper
AU: * Hu, Y
EM: yanhu@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, 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: 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: He, J
EM: jhe@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AB: Ample evidence suggests that the shallowest segment of subduction interface, down to a few km depth and updip of the megathrust seismogenic zone, exhibits a velocity-strengthening (aseismic) behavior. According to the dynamic Coulomb wedge model, it is mainly this coseismic strengthening that is responsible for the growth and permanent deformation of the overlying frontal prism (Wang and Hu, 2006). However, the degree of the strengthening as controlled by earthquake size and the location of the seismogenic zone has not been investigated. In this work, we use numerical models to study how the stress is coseismically transferred from the velocity-weakening seismogenic zone to the velocity-strengthening shallow segment. The model is a hybrid of the frictional contact model and the classic crack model. An earthquake is simulated using a decrease in the effective friction coefficient μ' (i.e., weakening) along the seismogenic zone, producing a few MPa stress drop. The simultaneous strengthening of the aseismic updip and downdip segments is simulated using an increase in their μ'. Only the net effect of the coseismic weakening/strengthening is modeled in this work; the dynamic evolution of the friction and post-seismic stress relaxation are not included. We demonstrate how the slip distribution along the subduction fault is controlled by the degree of coseismic strengthening of the updip segment. The minimum level of strengthening required to prevent the rupture from breaking the trench depends on the force drop along the seismogenic zone, defined as the product of the average shear stress drop along the seismogenic zone and its area. For a fault geometry similar to that of Nankai Trough with about 3 MPa stress drop along a seismogenic zone of 120 km downdip width, an increase of μ' by 0.055 along the 30 km wide updip segment will render the segment on the verge of breaking the trench. This level of stress increase may readily push the outer wedge into a critical state of failure. With a much higher degree of strengthening, the rupture will propagate into the shallow segment only slightly, causing localized compression in the area of slip termination. This may explain the formation of an outer ridge between the outer and inner wedges at some subduction zones. Model results also indicate that for the same force drop, an updip segment with narrower downdip width will require a higher degree of strengthening to prevent trench-breaking rupture. We measured the taper of frontal prisms at 24 subduction zones, and found that the surface slope is generally too high to be explained using the classical Coulomb wedge model but can be explained using the dynamic Coulomb wedge model including coseismic strengthening of the shallow portion of the megathrust. Wang, K., and Y. Hu (2006), Accretionary prisms in subduction earthquake cycles: The theory of dynamic Coulomb wedge, J. Geophys. Res., 111, doi:10.1029/2005JB004094.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8020 Mechanics, theory, and modeling
DE: 8045 Role of fluids
DE: 8163 Rheology and friction of fault zones (8034)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting