Inferring mantle viscosity in Southern California from early Ridgecrest earthquake postseismic deformation

Jacob H. Dorsett, Kaj M. Johnson, Simone Puel, & Thorsten W. Becker

Submitted August 24, 2020, SCEC Contribution #10780

The 2019 Ridgecrest earthquake sequence provides a new opportunity to study postseismic deformation following the largest earthquake in southern California in two decades. Previous work shows the rheological structure of southern California can broadly be understood as a strong lower crust overlying a weaker upper mantle with steady-state viscosity of < 1019 Pa s which is effectively reduced by a factor of 3-30 during a transient phase following large earthquakes. Here, we utilize 110 days of GPS solutions following the M w 7.1 July 5 2019 Ridgecrest earthquake to investigate the early transient phase of postseismic deformation to better understand transient mantle flow mechanisms. We generate a suite of postseismically-reduced rheology models based on a 3D steady-state viscosity reference constructed using regional seismic tomography anomalies. We compute postseismic deformation due to coupled afterslip and mantle flow for each candidate viscosity model and find that viscoelastic relaxation signatures are already significant in the far field. Our results suggest that compared to our long-term reference, a postseismic viscosity reduction is required to fit the geodetic signal. A stress-dependent, power-law formulation does not appear to capture the reduction fully, and a transient Burger’s viscosity reduction mechanism is required. Best-fit models require a low effective postseismic viscosity of the order of 1- 5 × 1017 Pa s at 60 – 100 km depth, from which we estimate a permissible range of steady-state (pre-earthquake) effective viscosity depth profiles with viscosity of 1-5 × 1018 Pa s at 60-100 km depths, which tends towards the lower end of previous estimates.

Dorsett, J. H., Johnson, K. M., Puel, S., & Becker, T. W. (2020). Inferring mantle viscosity in Southern California from early Ridgecrest earthquake postseismic deformation. Journal of Geophysical Research--Solid Earth, (submitted).