WHAT CAN GPS DATA TELL US ABOUT THE DYNAMICS OF POST-SEISMIC DEFORMATION?

Abstract

This paper describes differences in time-varying post-seismic deformation due to after-slip and viscoelastic relaxation following large strike-slip earthquakes, and how these differences may be exploited to characterize the configuration and rheology of aseismically deforming material in the subsurface. The analysis involves two steps. First, near-field, time-dependent post-seismic deformation characteristics of a typical Mw= 7.4 strike-slip earthquake is defined based on analysis of GPS data from three recent earthquakes. Secondly, this earthquake is modelled (assuming uniform slip along a rectangular surface), and several classes of after-slip and viscoelastic relaxation models that can reproduce the evolution of early post-seismic displacements with time at a near-field reference point are developed. Postseismic displacements and velocities away from the reference point, where the differences are greatest (and thus most likely to be distinguished with GPS) are compared. I find that displacements from a judiciously designed network of continuous or frequently occupied campaign-mode GPS sites are sufficiently precise to distinguish linear viscoelastic relaxation from after-slip on a vertical surface extending the coseismic rupture. Furthermore, both the thickness and viscosity of a relaxing, linearly viscoelastic layer may be identified. To maximize what post-seismic GPS surveys can tell us, particularly concerning potential relaxation of low-viscosity layers in the crust and/or upper mantle, some GPS sites should be located along strike beyond the rupture tip. Also, far-field GPS sites should be occupied as frequently as sites close to the rupture.