Rheologic constraints on the upper mantle from 5 years of postseismic deformation following the El Mayor‐Cucapah earthquake

Abstract

AbstractWe analyze five years of Southern California GPS data following the Mw=7.2 El Mayor‐Cucapah earthquake. We observe transient postseismic deformation which persists for 3 years at epicentral distances greater than ∼200 km. In the near field, rapid postseismic transience decays to a sustained rate which exceeds its preseismic trend. We attempt to determine the mechanisms driving this deformation, where we consider afterslip at seismogenic depths and viscoelastic relaxation in the lower crust and upper mantle as candidate mechanisms. We find that early, rapid, near‐field deformation can be explained with afterslip on the fault that ruptured coseismically. The later, sustained, near‐field deformation can be explained with viscoelastic relaxation in the lower crust with a steady‐state viscosity of ∼1019 Pa s and possibly continued afterslip. The later postseismic deformation in the far field is best explained with a transient viscosity of ∼1018 Pa s in the upper mantle. We argue that a transient rheology in the mantle is preferable over a Maxwell rheology because it better predicts the decay in postseismic deformation and also because it does not conflict with the generally higher, steady‐state viscosities inferred from studies of geophysical processes occurring over longer timescales.