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Abstract The rheology of the crust and mantle and the interaction of viscoelastic flow with seismic/aseismic slip on faults control the state of stress in the lithosphere over multiple seismic cycles. The rheological behavior of rocks is well constrained in a laboratory setting, but thein situproperties of the lithosphere and its lateral variations remain poorly known. Here, we access the lower‐crustal rheology in Southern California by exploiting 8 years of geodetic postseismic deformation following the 2010 El Mayor‐Cucapah earthquake. The data illuminate viscoelastic flow in the lower crust with lateral variations of effective viscosity correlated with the geological province. We show that a Burgers assembly with dashpots following a nonlinear constitutive law can approximate the temporal evolution of stress and strain rate, indicating the activation of nonlinear transient creep before steady‐state dislocation creep. The transient and background viscosities in the lower crust of the Salton Trough are on the order of ~1018and ~1019 Pa s, respectively, about an order of magnitude lower than those in the surrounding regions. We highlight the importance of transient creep, nonlinear flow laws, and lateral variations of rheological properties to capture the entire history of postseismic relaxation following the El Mayor‐Cucapah earthquake.
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This content will become publicly available on August 1, 2026
Kinematic Representations of Viscoelastic Postseismic Deformation
Abstract Following large earthquakes, viscoelastic stress relaxation may contribute to postseismic deformation observed at Earth's surface. Mechanical representations of viscoelastic deformation require a constitutive relationship for the lower crust/upper mantle material where stresses are diffused and, for non‐linear rheologies, knowledge of absolute stress level. Here, we describe a kinematic approach to representing geodetically observed postseismic motions that does not require an assumed viscoelastic rheology. The core idea is to use observed surface motions to constrain time‐dependent displacement boundary conditions applied at the base of the elastic upper crust by viscoelastic motions in the lower crust/upper mantle, approximating these displacements as slip on a set of dislocation elements. Using three‐dimensional forward models of viscoelastically modulated postseismic deformation in a thrust fault setting, we show how this approach can accurately represent surface motions and recover predicted displacements at the base of the elastic layer. Applied to the 1999 Chi‐Chi (Taiwan) earthquake, this kinematic approach can reproduce geodetically observed displacements and estimates of the partitioning between correlated postseismic deformation mechanisms. Specifically, we simultaneously estimate afterslip on the earthquake source fault that is similar to previous estimates, along with slip on dislocations at the base of the elastic layer that mimic predictions from viscous stress dissipation models in which viscosity is inferred to vary three‐dimensionally. A use case for the dislocation approach to modeling viscoelastic deformation is the estimation of spatiotemporally variable fault slip processes, including across sequential interseismic phases of the earthquake cycle, without assuming a lower crust/upper mantle rheology.
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- Award ID(s):
- 2343233
- PAR ID:
- 10639126
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Earth and Space Science
- Volume:
- 12
- Issue:
- 8
- ISSN:
- 2333-5084
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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