HR: 15:10h
AN: T13E-07    [Abstracts]
TI: 3-D Earthquake Nucleation on Rate-and-State Faults
AU: * Rubin, A M
EM: arubin@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AU: Ampuero, J
EM: ampuero@erdw.ethz.ch
AF: Inst. Geophysics, Seismology, and Geodynamics, ETH Honggergberg (HPP), Zurich, CH-8093 Switzerland
AB: Conditions for the instability of spring-block sliders obeying rate-and-state friction are well established, but translating this understanding into a prediction of the size of the nucleation zone on elastically-deformable faults has proven difficult: Does the zone scale as GD_c/bσ (≡ L_b^-1), as found by Dieterich (1992), or as GD_c/(b-a)σ (≡ L_(b-a)^-1), as suggested by stability analyses? We find two regimes of nucleation for both the ``aging'' and ``slip'' laws, distinguished by the behavior of Vθ/D_c: Well above steady-state everywhere, typical of low values of a/b, and near steady-state in the interior of the nucleation zone, typical (for slow loading conditions) of larger a/b. Analytic solutions are easier to come by for the aging law, and show that (1) the nucleation length scales as L_b^-1 in the first regime but (asymptotically at large slip speeds) as the potentially much larger L_[b/(b-a)]² in the second, (2) nucleation is essentially guaranteed to occur in the first regime only for a/b< ~0.4 in 2-D and a/b< ~0.2 in 3-D, below the lab range, and (3) the aging law should probably be abandoned as a constitutive description of nucleation, because the large nucleation lengths predicted for lab values of a/b are directly attributable to properties of that law that violate experimental observation. Despite point (3) above, the aging law results provide useful insight into the more complicated (and more faithful to relevant experiments) slip law. In the first regime (well above steady-state), nucleation proceeds on a patch of fixed size for the aging law but of slowly decreasing size for the slip law, smaller than the aging law case by roughly ln(Vθ/D_c). For the slip law the transition to the second regime occurs near a/b=0.5 in 3-D. For both laws, the nucleation zone in the second regime undergoes quasi-static expansion, during which regions at the margins are brought rapidly to large slip speeds. This allows us to estimate that the effective fracture energy of the nucleation zone increases as ~[ln(V/V_bg)]² for the aging law but only ~[ln(V/V_bg)] for the slip law. For the aging law this results in crack-like expansion (the crack energy release rate also increases as ~[ln(V/V_bg)]²), with the 3-D nucleation zone possibly reaching a radius of 100L_b for a typical lab value of a/b=0.9. It is the large increase in fracture energy with slip speed that appears to violate velocity-stepping experiments. For the more slowly-increasing fracture energy of the slip law such crack-like growth is not feasible in this regime. In 2-D the slip-law nucleation zone grows as a unidirectional pulse, with most of the moment rate coming from a region smaller than L_b at the edge of a zone >>L_b. Preliminary results indicate that similar behavior occurs in 3-D, preferentially at a mode-III margin of the nucleation zone.
DE: 7209 Earthquake dynamics (1242)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005