Asthenosphere readjustment and the earthquake cycle

Abstract

A simple two-dimensional modi:l of the earthquake cycle (preearthquake strain accumulation, coseismic strain release, and postseismic readjustment) has been constructed from the Nur-Mavko solution for a screw dislocation in an elastic plate (lithosphere) overlying a viscoelastic substrate (asthenosphere). The deformation at the free surface is calculated for an earthquake cycle imposed by prescribed slip on a transform fault. This deformation is compared to that produced by a similar cycle in an elastic half space so that the effects of viscoelastic relaxation in the asthenosphere may be isolated. The following conclusions are drawn: (I) The surface deformation produced by viscoelastic relaxation in the asthenosphere can be dupli,c, ated identically by a reasonable distribution of slip at depth on a vertical fault in an elastic half space. Thus differentiation of two possible modes of postearthquake readjustment will be difficult. (2) The effect of asthenosphere relaxation is important only if the depth of the seismic zone is comparable to the thickness of the lithosphere. If the seismic zone is 15 km deep and the lithosphere is 75 km thick, as commonly estimated for the San Andreas fault zone, asthenosphere relaxation is not particularly significant in determining surface deformation. (3) In a periodic sequence of earthquakes the principal observable effects of viscoelasticity in the asthenosphere are to produce a rapid postearthquake deformation and to concentrate strain accumulation and relaxation even closer to the fault than in the elastic half-space model. Paper number 8B0385. 3369

The earthquake deformation cycle

“…The simplest model of the full earthquake deformation cycle for strike-slip faults consists of regularly repeating earthquakes that rupture an infinitely-long fault in an elastic lid overlying a viscoelastic substrate (Savage, 2000, Savage andPrescott, 1978). In this viscoelastic coupling model, the upper blocks slide past each other in the long term, but are locked together at the fault between events.…”

Inference of Multiple Earthquake-Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation

“…Models for time-dependent earthquake-cycle deformation have generally focused on either the diffuse deformation of a linear viscoelastic subseismogenic layer (e.g., Thatcher, 1983;Savage, 1990;Dixon et al, 2002Dixon et al, , 2003Ergintav et al, 2002;Segall, 2002;Hilley et al, 2005, Motagh et al, 2007 or localized shear on a down-dip extension of the coseismic fault zone (e.g., Marone et al, 1991;Bürgmann et al, 2002), whereas recently more postseismic models have incorporated both (e.g., Freed et al, 2006;Johnson et al, 2009). Both deformation mechanism models have been extended to predict time-variable surface deformation throughout the entire earthquake cycle under the assumption of periodic earthquake occurrence (Savage and Prescott, 1978;Cohen, 1982;Johnson and Segall, 2004;Hetland and Hager, 2005) and to clustered and nonperiodic earthquake occurrence (Meade and Hager, 2004;Hetland and Hager, 2006). However, the most common tools for interpreting nominally interseismic geodetic velocity fields are, by far, still CEH models.…”

“…Idealized earthquake-cycle models fall into two general classes: (1) quasistatic classical elastic half-space (CEH) models, and (2) dynamic earthquake-cycle (DEC) models. In the former, deformation between earthquakes is assumed to be time invariant (Savage and Burford, 1973;Savage, 1983), whereas in the latter, deformation rates vary continuously throughout the earthquake cycle due to the relaxation of coseismic stresses (e.g., Nur and Mavko, 1974;Savage and Prescott, 1978;Cohen, 1982;Li and Rice, 1987;Savage, 2000;Johnson and Segall, 2004;Hetland and Hager, 2005). CEH models are a frequently used method for interpreting geodetic velocity gradients near faults and estimating fault slip rates and locking depths, mainly due to their simplicity and low computational cost.…”

Influence of anelastic surface layers on postseismic thrust fault deformation

“…However, in this work we recognize that the actual rheology of shallow crustal materials is likely to be complex, and the distinction between true afterslip and true bulk relaxation is likely to be gradational. Savage and Prescott [1978] and Savage [1990] discussed the equivalence between vertical strike-slip postseismic viscoelastic relaxation and strain in an elastic half-space due to proxy faults. In a related vein, Savage [1987] found distributions of elastic half-space faulting that are equivalent to vertical strike-slip faulting in layered elastic structures.…”