Asthenosphere readjustment and the earthquake cycle

Abstract

A simple two‐dimensional model 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: (1) The surface deformation produced by viscoelastic relaxation in the asthenosphere can be duplicated 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.