Viscoelastic shear zone model of a strike-slip earthquake cycle

I examine the behavior of a two-dimensional (2-D) strike-slip fault system embedded in a I-D elastic layer (schizosphere) overlying a uniform viscoela stic half-space (plastosphere) and within the boundaries of a finite width shear zone. The viscoelastic coupling model of Savage and Prescott [1978] c onsiders the viscoelastic response of this system, in the absence of the sh ear zone boundaries, to an earthquake occurring within the upper elastic la yer, steady slip beneath a prescribed depth, and the superposition of the r esponses of multiple earthquakes with characteristic slip occurring at regu lar intervals. So formulated, the viscoelastic coupling model predicts that sufficiently long after initiation of the system, (1) average fault-parall el velocity at any point is the average slip rate of that side of the fault and (2) far-field velocities equal the same constant rate. Because of the sensitivity to the mechanical properties of the schizosphere-plastosphere s ystem (i.e., elastic layer thickness, plastosphere viscosity), this model h as been used to infer such properties from measurements of interseismic vel ocity. Such inferences exploit the predicted behavior at a known time withi n the earthquake cycle. By modifying the viscoelastic coupling model to sat isfy the additional constraint that the absolute velocity at prescribed she ar zone boundaries is constant, I find that even though the time-averaged b ehavior remains the same, the spatiotemporal pattern of surface deformation (particularly its temporal variation within an earthquake cycle) is marked ly different from that predicted by the conventional viscoelastic coupling model. These differences are magnified as plastosphere viscosity is reduced or as the recurrence interval of periodic earthquakes is lengthened. Appli cation to the interseismic velocity field along the Mojave section of the S an Andreas fault suggests that the region behaves mechanically like a simil ar to 600-km-wide shear zone accommodating 50 mm/yr fault-parallel motion d istributed between the San Andreas fault system and Eastern California Shea r Zone.