HR: 15:10h
AN: T43C-06    [Abstracts]
TI: Submarine Wedges and Great Subduction Earthquakes: The Theory of Dynamic Coulomb Wedge
AU: * Wang, K
EM: kwang@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 W Saanich Rd, Sidney, BC V8L 4B2 Canada
AU: Hu, Y
EM: yhu@nrcan.gc.ca
AF: Institute of Earthquake Sciences, China Earthquake Administration, Beijing, 100036 China
AB: When applied to subduction zones, the classical theory of critically tapered Coulomb wedge describes a long-term, temporally averaged state of stress and pore fluid pressure. However, stress and fluid-pressure changes in subduction earthquake cycles are of fundamental importance to wedge mechanics. These changes hold keys to the explanation of many intriguing observations such as high outer-wedge slope, lower-than-predicted pore fluid pressure, normal faulting contemporaneous with state of extreme compression, coseismic activation of splay faults, and transient fluid and stress regimes. Building on the Coulomb-plasticity of the classical critical wedge theory, we assume an elastic - perfectly Coulomb-plastic rheology and develop the theory of dynamic Coulomb wedge. The new theory postulates that the actively deforming outer wedge overlies the updip velocity-strengthening part of the subduction fault, and the less deformed inner wedge overlies the megathrust seismogenic zone. To the first order, the outer wedge switches between critical and stable states in earthquake cycles. During a great earthquake, sudden increase in fault stress (due to velocity strengthening) and pore fluid pressure drives the outer wedge into a compressively critical state, causing accretion, thrust deformation, and even basal erosion. The outer-wedge geometry is thus controlled by the peak stress of the velocity-strengthening part of the subduction fault achieved in largest earthquakes. After an earthquake, the outer wedge returns to a stable state, with basal stress and fluid pressure decreasing with time. With a high enough surface slope such as at many margins dominated by basal erosion, interseismic stress relaxation may turn the outer wedge into an extensionally critical state, resulting in normal faulting. The inner wedge experiences stress changes opposite to the outer wedge but stays in the stable regime throughout the earthquake cycles, acting as an apparent backstop and providing a stable environment for the formation of forearc basins. The new theory explains a range of previously unexplained observations and provides many testable predictions regarding the dynamics of submarine wedges and the megathrust seismogenic zone.
DE: 7209 Earthquake dynamics (1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8020 Mechanics, theory, and modeling
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005