SlowSlip Propagation Speeds
Abstract
Combined seismic and geodetic data from subduction zones and the Salton Trough have revealed slow slip events with reasonably welldefined propagation speeds. This in turn is suggestive of a moreorless well defined front separating nearly locked regions outside the slipping zone from interior regions that slide much more rapidly. Such cracklike nucleation fronts arise naturally in models of rateandstate friction for lablike values of a/b, where a and b are the coefficients of the velocity and statedependence of the frictional strength (with the surface being velocityneutral for a/b=1). If the propagating front has a quasisteady 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 ∆τpr is the peaktoresidual stress drop at the front and μ' the effective elastic shear modulus. Rateandstate simulations indicate that ∆τpr 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, ∆τpr 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 (~45, for example). Subduction zone transients propagate on the order of 10 km/day or 101 m/s. Geodetic data constrain the average slip speed to be a few times smaller than 1 cm/day or 107 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 ~102, 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 σ1), and their low stress drops (of order 102 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.
 Publication:

AGU Fall Meeting Abstracts
 Pub Date:
 December 2007
 Bibcode:
 2007AGUFM.S12B..01R
 Keywords:

 8118 Dynamics and mechanics of faulting (8004);
 8170 Subduction zone processes (1031;
 3060;
 3613;
 8413)