Strong interseismic coupling, fault afterslip, and viscoelastic flow before and after the Oct. 9, 1995 Colima‐Jalisco earthquake: Continuous GPS measurements from Colima, Mexico

Abstract

Continuous GPS measurements from Colima, Mexico during 4/93–6/01, bracketing the Oct. 9, 1995 M = 8.0 Colima‐Jalisco earthquake, provide new constraints on Rivera plate subduction mechanics. Modeling of margin‐normal strain accumulation before the earthquake suggests the Rivera‐North America subduction interface was fully locked. Transient postseismic motion from 10/95–6/97 is well fit by a model that includes logarithmically‐decaying fault afterslip, elastic strain from shallow fault relocking, and possibly a minor viscoelastic response, but is fit poorly by models that assume a dominant Maxwell viscoelastic response of the lower crust and upper mantle, independent of the assumed viscosities. Landward, margin‐normal motion since mid‐1997 is parallel to but ∼75% slower than the pre‐seismic velocity. Afterslip alone fails to account for this slowdown. The viscoelastic response predicted by a FEM correctly resolves the remaining velocity difference within the uncertainties. Both processes thus offset elastic strain accumulating from the relocked subduction interface.