San Andreas Fault seismic-cycle models incorporating viscous shear zones and the SCEC Community Rheology Model (CRM)

Elizabeth H. Hearn

Submitted August 9, 2021, SCEC Contribution #11217, 2021 SCEC Annual Meeting Poster #112

I have developed models of northern and southern San Andreas Fault (SAF) seismic cycles to estimate how much viscoelastic effects perturb present-day surface velocities relative their mean interseismic values. These perturbations (or "ghost transients") will be subtracted from GPS velocities prior to some slip rate inversions for the 2023 USGS National Seismic Hazards Map project. Another goal is to assess how much rheology and uncertainties in the timing of large SAF earthquakes affect modeled seismic cycle deformation. Both model suites were developed using the CIG finite-element code PyLith (Aagaard et al., 2013). The northern SAF models incorporate a viscous shear zone extending the SAF through a layered, viscoelastic Earth. Most of the southern SAF models incorporate 3D viscosities computed from the CRM, using CTM temperatures and a range of assumed strain rates. Maxwell and/or Burgers viscoelastic rheologies, and various rheological parameter values and earthquake histories (based on Scharer and Yule [2020] and other studies) are explored.

For the southern SAF models, ghost transients of up about 5 mm/yr are obtained for a wide range of assumptions. For the northern SAF, the perturbations are up to about 3 mm/yr. In both cases, they display a left-lateral pattern across the SAF, so subtracting them from the GPS velocity field increases relative (right-lateral) velocities across the fault. This increases geodetic SAF slip rate estimates, and as previously noted for the southern SAF (e.g. Johnson et al., 2007 and Hearn et al., 2013), brings them closer to geologic estimates. Modeled ghost transients for the southern SAF are fairly insensitive to the timing of large SAF earthquakes prior to the 1857 Fort Tejon event, but they are highly sensitive to temperatures and strain rates assumed when computing CRM effective viscosities. Seismic cycle deformation is influenced mainly by material within about 100 km of the modeled ruptures, and in cases where a Burgers viscoelastic rheology is assumed, it is also sensitive to the Burgers compliance relaxation amplitude \bar{∆}.

Preliminary results from a steady-state lithosphere deformation model that incorporates the CRM will also be presented. Specifically, velocities and stresses from this model will be compared with results from models incorporating layered viscoelastic structure.

Key Words
rheology, seismic cycle, deformation model

Hearn, E. H. (2021, 08). San Andreas Fault seismic-cycle models incorporating viscous shear zones and the SCEC Community Rheology Model (CRM). Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)