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 ∆τp- r/μ', where ∆τp-r is the peak-to-residual stress drop at the front and μ' the effective elastic shear modulus. Rate-and-state simulations indicate that ∆τp-r is given reasonably accurately by bσ\ln(Vmaxθi/Dc), where σ is the effective normal stress, Vmax is the maximum slip speed behind the propagating front, θi is the the value of "state" ahead of the propagating front, and Dc is the characteristic slip distance for state evolution. Except for a coefficient of order unity, ∆τp-r is independent of the evolution law. This leads to Vprop/Vmax ~μ'/[bσ\ln(Vmaxθi/Dc)]. For slip speeds a few orders of magnitude above background, \ln(Vmaxθi/Dc) 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 Vprop/Vmax 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.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 8118 Dynamics and mechanics of faulting (8004);
- 8170 Subduction zone processes (1031;