SCEC Award Number 18108 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Towards a Southern California Community Rheology Model
Investigator(s)
Name Organization
Sylvain Barbot University of Southern California
Other Participants Walter Landry
SCEC Priorities 1c, 1e SCEC Groups CXM, Geodesy, CME
Report Due Date 03/15/2019 Date Report Submitted 04/26/2019
Project Abstract
 We studied the deformation that followed the 2010 Mw=7.2 El Mayor-Cucapah earthquake using GPS time series from the Plate Boundary Observatory. Our analysis revealed the transient nature of the lower-crust relaxation in the Salton Trough region, with a rapid evolution in the first few months after the mainshock followed by a more sluggish evolution in subsequent years. The stress relaxation is incompatible with a steady-state linear or power-law rheology for the lower-crust, if typical laboratory parameters are employed. The deformation is well explained by the response of a Burgers spring-slider assembly. This implies that diffusion creep governs deformation at steady-state, and that another mechanism accommodates deformation during transient creep. The constitutive parameters can be estimated without bias from an unknown pre-stress and background strain-rate. The apparent viscosity, i.e., the ratio of postseismic stress to post-seismic strain-rate (without the contributions from interseismic loading rates or pre-stress) shows a rapid evolution over the course of 1 year that is well explained by a Burgers rheology with a viscosity of 2.9x1019 Pa s in the Maxwell element and a viscosity of 3x1018 Pa s in the Kelvin element, and a stiffness ratio of GK/G=11.33, where GK and G are the rigidity of the Kelvin and Maxwell elements, respectively. Therefore, the available geodetic data provide hints at the constitutive properties for viscoelastic relaxation in the Salton Trough lower-crust and estimates for the rheological parameters. The analysis provides constraints for the nascent Community Rheology Model in the Salton Trough region.
Intellectual Merit We studied the deformation that followed the 2010 Mw=7.2 El Mayor-Cucapah earthquake using GPS time series from the Plate Boundary Observatory. The study helps identifying the constitutive properties for viscoelastic relaxation in the Salton Trough lower-crust and estimates for the rheological parameters. The analysis provides constraints for the nascent Community Rheology Model in the Salton Trough region.
Broader Impacts The project contributed to the training of a young scientist, Chi-Hsien Tang. The results will contribute to SCEC Community Rheology Model (CRM). The PI is contributing to the development of the CRM. The current study provides key elements to the Salton Trough region of the CRM. The methodology developed in this study may be applicable to other regions of Southern California, in particular the Mojave Desert, where the 1993 Landers and 1999 Hector Mine earthquakes took place. Analysis of data from these region will increase the degree of completeness of the CRM. Eventually, the CRM will be made available to the SCEC community.
Exemplary Figure Figure 1.
Caption: Top) Distribution of afterslip (left) and viscoelastic strain (right) eight years after the 2010 Mw=7.2 El Mayor-Cucapah earthquake from linear kinematic inversion of GPS time series from the Plate Boundary Observatory (blue arrows are the horizontal data, black arrows: the forward model). The forward model corresponds to a distribution of strain that exactly balances an unknown fraction of the deviatoric coseismic stress change. Bottom) The fraction of deviatoric coseismic stress change that is has been relaxed is varied between 0 and 1, and a best-fitting value is identified for each epoch (colored curves). The fraction of stress relaxed is a monotonically increasing function of time that is well explained by the relaxation of a Burgers spring-slider assembly (thick black profile) with a viscosity of 3x1018 Pa s in the Kelvin element, a viscosity of 2.9x1019 Pa s in the Maxwell element, and a rigidity ratio of GK/G=11.33, where GK and G are the rigidities in the Kelvin and Maxwell elements, respectively. (Credit: Sylvain Barbot)