HR: 10:20h
AN: S12B-01 INVITED    [Abstracts]
TI: Slow-Slip Propagation Speeds
AU: * Rubin, A M
EM: arubin@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08540, United States
AU: Ampuero, J
EM: ampuero@erdw.ethz.ch
AF: ETH Zurich, Institute of Geophysics ETH Honggerberg HPP O13.1, Zurich, CH-8093, Switzerland
AB: Combined seismic and geodetic data from subduction zones and the Salton Trough have revealed slow slip events with reasonably well-defined propagation speeds. This in turn is suggestive of a more-or-less well- defined front separating nearly locked regions outside the slipping zone from interior regions that slide much more rapidly. Such crack-like nucleation fronts arise naturally in models of rate-and-state friction for lab-like values of a/b, where a and b are the coefficients of the velocity- and state-dependence of the frictional strength (with the surface being velocity-neutral for a/b=1). If the propagating front has a quasi-steady shape, the propagation and slip speeds are kinematically tied via the local slip gradient. Given a sufficiently sharp front, the slip gradient is given dimensionally by Δτ_{p- r}/μ^', where Δτ_p-r is the peak-to-residual stress drop at the front and μ^' the effective elastic shear modulus. Rate-and-state simulations indicate that Δτ_p-r is given reasonably accurately by bσ\ln(V_maxθ_i/D_c), where σ is the effective normal stress, V_max is the maximum slip speed behind the propagating front, θ_i is the the value of "state" ahead of the propagating front, and D_c is the characteristic slip distance for state evolution. Except for a coefficient of order unity, Δτ_p-r is independent of the evolution law. This leads to V_prop/V_max ~μ^'/[bσ\ln(V_maxθ_i/D_c)]. For slip speeds a few orders of magnitude above background, \ln(V_maxθ_i/D_c) can with reasonable accuracy be assigned some representative value (~4-5, for example). Subduction zone transients propagate on the order of 10 km/day or 10^-1 m/s. Geodetic data constrain the average slip speed to be a few times smaller than 1 cm/day or 10^-7 m/s. However, numerical models indicate that the maximum slip speed at the front may be several times larger than the average, over a length scale that is probably too small to resolve geodetically, so a representative value of V_prop/V_max may be ~10⁶. For μ^'=40 GPa and a lab value of b of ~10^-2, this implies a value of σ of order 1 MPa. While this is extremely low, it is broadly consistent with the observed periods of these events [Liu and Rice, JGR 2007], their very large dimensions (length scales are proportional to σ^-1), and their low stress drops (of order 10^-2 MPa). The 2005 Salton Trough event had a similar propagation speed but a stress drop and slip speed of order 100 times larger, broadly consistent with lab values of b and hydrostatic pore pressure. Another contrast, possibly related to the difference in effective stress, is that the subduction zone events are associated with tremor while the Salton Trough event was associated with more typical earthquakes.
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Seismology [S]
MN: 2007 Fall Meeting