HR: 1340h
AN: T53B-1422    [Abstracts]
TI: A Classic Test of the Hubbert-Rubey Weakening Mechanism: M7.6 Thrust-Belt Earthquake Taiwan
AU: * Yue, L
EM: lyue@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AU: Suppe, J
EM: suppe@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AB: The Hubbert-Rubey (1959) fluid-pressure hypothesis has long been accepted as a classic solution to the problem of the apparent weakness of long thin thrust sheets. This hypothesis, in its classic form argues that ambient high pore-fluid pressures, which are common in sedimentary basins, reduce the normalized shear traction on the fault τ_b/ρ g H = μ_b(1-λ_b) where λ_b=P_f/ρ g H is the normalized pore-fluid pressure and μ_b is the coefficient of friction. Remarkably, there have been few large-scale tests of this classic hypothesis. Here we document ambient pore-fluid pressures surrounding the active frontal thrusts of western Taiwan, including the Chulungpu thrust that slipped in the 1999 M_w7.6 Chi-Chi earthquake. We show from 3-D mapping of these thrusts that they flatten to a shallow detachment at about 5 km depth in the Pliocene Chinshui Shale. Using critical-taper wedge theory and the dip of the detachment and surface slope we constrain the basal shear traction τ_b/ρ g H ≈ 0.1 which is substantially weaker than common lab friction values of of Byerlee's law (μ_b= 0.85-0.6). We have determined the pore-fluid pressures as a function of depth in 76 wells, based on in-situ formation tests, sonic logs and mud densities. Fluid pressures are regionally controlled stratigraphically by sedimentary facies. The top of overpressures is everywhere below the base of the Chinshui Shale, therefore the entire Chinshui thrust system is at ambient hydrostatic pore-fluid pressures (λ_b ≈ 0.4). According to the classic Hubbert-Rubey hypothesis the required basal coefficient of friction is therefore μ_b ≈ 0.1-0.2. Therefore the classic Hubbert & Rubey mechanism involving static ambient excess fluid pressures is not the cause of extreme fault weakening in this western Taiwan example. We must look to other mechanisms of large-scale fault weakening, many of which are difficult to test.
DE: 8108 Continental tectonics: compressional
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: Fall Meeting 2005